Placeholder Image

字幕列表 影片播放

  • Captioning provided by Disability Access Services

  • at Oregon State University.

  • Kevin Ahern: Okay, folks, let's get started!

  • I can't hear, there I go!

  • You guys are the quickest-to-quiet-down class

  • I've ever had, and that's good.

  • Just think how much more biochemistry we can squeeze in

  • now when you quiet down quickly.

  • I hope everybody's doing well and ready for a big weekend.

  • We will have an exam in here a week from Monday, not today.

  • For those of you who thought we were having it today,

  • I know you'll be disappointed.

  • I will announce later where that stopping point will be.

  • I sort of decide on, it depends on where I get in my lectures,

  • and it varies a little bit from term to term.

  • Today I'm going to finish up hemoglobin

  • and start talking about enzymes.

  • I hope I gave you the impression

  • or the understanding that hemoglobin is a remarkable protein.

  • It has a remarkable number of functions built into it,

  • and those functions are directly related

  • to the structure of the protein.

  • We saw structural things about proteins,

  • in general, when I talked about primary,

  • secondary, tertiary, et cetera.

  • But here's a protein where you get a real live,

  • up close and personal look

  • at how those structures that we see in proteins

  • give proteins specific functions.

  • We'll see more of that when we talk about enzymes.

  • I want to emphasize, speaking of enzymes,

  • that hemoglobin is not an enzyme.

  • People commonly think that, but it's not.

  • Enzymes catalyze reactions and hemoglobin

  • isn't catalyzing anything.

  • So it's an oxygen-carrying protein.

  • That's really its only function.

  • We'll see that the way that it binds oxygen

  • is not unlike the way that enzymes bind their substrates,

  • but hemoglobin is not an enzyme.

  • Last time I finished by talking about fetal hemoglobin

  • and fetal hemoglobin has the very interesting property

  • of having the slightly different subunits.

  • It has the two gammas instead of the two betas,

  • and that sort of makes a structure

  • that doesn't have that doughnut hole

  • that fits the 2,3BPG in the same way.

  • As a result of that, as I had noted,

  • the fetal hemoglobin stays in the R state almost all the time,

  • and that's why the fetal hemoglobin

  • has greater affinity for oxygen than adult hemoglobin.

  • There are yet other things

  • that we need to understand about hemoglobin.

  • I always like to think about hemoglobin as having,

  • obviously, structures that give it the functions that it need,

  • that those structures correlate very well

  • with the needs of the body.

  • They correlate very, very well with the needs of the body.

  • You saw 2,3BPG was being produced by cells

  • that were actively metabolizing,

  • and it was providing a signal that,

  • "Hey, here's the place where I need the oxygen.

  • Let go of the oxygen."

  • And they cause hemoglobin to let go of the oxygen.

  • There are other signals that hemoglobin can respond to

  • with respect to actively respiring cells.

  • One of these is pH.

  • When we talk later about,

  • it'll actually be next term,

  • but when we talk about actively respiring cells next term,

  • one of the things we'll discover

  • is that actively respiring cells

  • have a higher concentration of protons around them

  • than non-actively respiring cells,

  • which means that the pH around an actively respiring cell

  • is lower than that of a non-actively respiring cell.

  • Again, when I think of an actively respiring cell,

  • you can almost always think of muscle.

  • Muscles really change a lot.

  • When muscles are contracting,

  • they're really needing energy a lot,

  • and things that need energy need oxygen.

  • They go hand in hand.

  • A scientist named Bohróand no, that's not B-O-R-E,

  • that's B-O-H-Rómade a very interesting observation

  • about hemoglobin many, many years ago.

  • The observation was, if he took hemoglobin

  • and he did this oxygen-binding curve that we did before,

  • where we see the percent of the hemoglobin

  • saturated with oxygen and the concentration

  • of the oxygen on the x-axis,

  • what he saw was, he did the plot

  • and he saw that nice sigmoidal plot that we did before

  • and if he dropped the pH the curve actually shifted downwards.

  • Well, that shift downwards corresponds to less affinity,

  • meaning that the hemoglobin is releasing oxygen,

  • and it's releasing oxygen as a function

  • of the pH environment in which it finds itself.

  • Now, again, this is a functionality

  • built into hemoglobin that is directly responding

  • to the body's needs: pH drops around actively respiring cells,

  • hemoglobin will tend to give up more oxygen

  • around actively respiring cells.

  • It's a very cool phenomenon.

  • Now, the chemical basis of the effect is not too surprising.

  • When we look inside of the hemoglobin molecule,

  • we see that there are various amino acid side chains

  • that can be charged.

  • In this case, we see a lysine up here

  • that is attracted to a portion of a histidine,

  • and here's the functional part of the histidine

  • where it can gain or lose a proton.

  • If it gains a proton, it is positively charged

  • and it will attract a negatively charged side chain,

  • in this case, of an aspartic acid.

  • If that proton is off of there,

  • then it will not attract that,

  • and we could imagine there would be

  • some slight shape changes that would happen,

  • whether it's being attracted or not being attracted.

  • And that, actually, that very subtle difference there,

  • is the molecular basis for the change

  • in affinity of hemoglobin for oxygen.

  • So a slight shape change happening

  • according to whether or not we put a proton onto a histidine,

  • and that histidine changes its interaction

  • with another side chain of aspartic acid,

  • and, a result of that,

  • causes the protein to actually change its configuration

  • and its affinity for oxygen.

  • Another thing that we see around actively respiring cells,

  • and it's actually one of the causes of the drop in pH,

  • is carbon dioxide.

  • Carbon dioxide is the final oxidative product of metabolism.

  • When we go and we burn sugar,

  • or we go and we burn fatóI'm on a diet right now,

  • so that burning fat is really on my mind.

  • If you see me exhaling a lot of carbon dioxide, that's good.

  • I wish I could do that.

  • Carbon dioxide is an end product of those kinds of processes.

  • So, not surprisingly, if we examine the environment

  • around actively respiring cells,

  • we discover there's more carbon dioxide there.

  • It turns out that carbon dioxide also affects hemoglobin.

  • I thought I had a graph there.

  • That's not the thing.

  • If we look at this now, what we see is,

  • now we're going back to these plots that we did before,

  • the first plot was the, this is not getting my voice.

  • The first plot was the pH 7.4, no CO2.

  • Then if we take no CO2 and that same hemoglobin

  • and we drop the pH, this is what we saw before,

  • the affinity for oxygen drops.

  • But now look at the bottom line.

  • If we have a pH 7.2 and we add carbon dioxide,

  • the affinity drops even more.

  • That means, therefore, that hemoglobin is releasing oxygen

  • in response to both protons and to carbon dioxide.

  • Again, these are things that are both present

  • in higher concentrations around cells

  • that are actively respiring.

  • Questions about that?

  • Question over here?

  • Student: Could you go back to the figure real quick with the,

  • where you had the histidine structure?

  • Kevin Ahern: Okay.

  • Student: I had a question about that.

  • Kevin Ahern: Okay.

  • Here, yeah.

  • Student: When you say "add a proton,"

  • Kevin Ahern: So, as the pH changes,

  • protons will come on or come off.

  • So that's the variable that's there.

  • That's always true, yeah.

  • Student: So it's the pH drop, then?

  • Kevin Ahern: A pH drop, so a pH drop would be more likely

  • to put a proton on there.

  • Exactly.

  • Okay?

  • Yes, sir?

  • Student: When we're talking about CO2

  • contributing in the same way

  • that a more acidic environment

  • around the respiring cells contributes,

  • is that actually in the form of CO2 or as carbonic acid?

  • Kevin Ahern: Good question.

  • His question is, "How does CO2 manifest its effect?"

  • I'm going to show you that in just a second.

  • Is that your question?

  • Student: My question was similar.

  • Kevin Ahern: Okay, so CO2 exerts its effect.

  • How does CO2 exert its effect?

  • Well, one of the ways it exerts its effect

  • is by forming a covalent bond with amine side chains.

  • CO2 can be carried in the blood in two ways.

  • One is it can actually be dissolved in the blood,

  • and we'll talk about that later.

  • The other way it can be carried

  • is by this covalent bond to hemoglobin.

  • We could imagine, looking at this structure here,

  • that we have a carbon dioxide.

  • We've got an amine,

  • in this case that is shown with no charge on it.

  • We put a CO2 on it and we develop something

  • that has a negative charge.

  • Again, we're introducing a charge

  • where there wasn't one before.

  • We could expect that we would, in fact,

  • see some changes that would happen in structure of hemoglobin.

  • Again, that's the basis for the change in affinity.

  • Very, very subtle changes that are happening to the protein,

  • but they're having big effects on its affinity for oxygen.

  • In this case, it actually, you'll notice it releases a proton,

  • and that actually enhances the effect that we saw before,

  • because, more protons, of course,

  • now we're going to affect hemoglobin in its own way, as well.

  • So these really work together

  • to make hemoglobin give up oxygen

  • at the places where it's needed.

  • So that's what I want to say about the Bohr effect.

  • The last thing I want to talk about

  • are some genetic considerations.

  • It's a disease that we hear a lot about,

  • and there are some interesting,

  • or at least one very interesting aspect of it,

  • and that disease is known as sickle cell anemia.

  • Sickle cell anemia is a disease.

  • It's a genetic disease where there are mutations

  • in one or more of the subunits of hemoglobin,

  • and there are different forms of sickle cell anemia

  • that correspond to, of course, different mutations.

  • Some may affect alpha, some may affect beta, et cetera.

  • So sickle cell anemia is not just one genetic mutation.

  • What happens when an individual has sickle cell anemia

  • is that the hemoglobin,

  • and by the way,

  • the changes that can happen,

  • the mutations that happen,

  • can change a single amino acid,

  • and if that single amino acid is in the appropriate place,

  • it will cause the disease.

  • So why do the cells get sickle shaped?

  • The reason that the cells get sickle shaped is,

  • under low concentrations of oxygen,

  • the hemoglobin inside the cells will actually form a polymer.

  • Multiple subunits will start joining,

  • joining, joining, joining together.

  • Now, normally hemoglobin doesn't do that,

  • and, in fact, regular hemoglobin,

  • that is, unmutated hemoglobin,

  • does not form polymers like that.

  • But sickle cell, people who have sickle cell anemia

  • will have their hemoglobin do that,

  • and the polymers actually cause

  • the shape of a blood cell to change.

  • Normal blood cells look like this.

  • Sickled cells look like this.

  • Now, I want to emphasize,

  • if you have sickle cell anemia,

  • all of your blood cells don't look like this.

  • They only look like this when the cell encounters

  • low concentrations of oxygen.

  • So if you are, for example,

  • exercising heavily and you have sickle cell anemia,

  • you will find, people who do that find

  • that their muscles will get excruciatingly sore.

  • In some cases, it can be life threatening,

  • because what's happening is,

  • the regular blood cells are starting to sickle.

  • They get in the muscle, in the tissues,

  • where they're dumping all their oxygen.

  • They get in the muscles and when these are dumping their oxygen

  • they're actually in little, tiny capillaries.

  • Most of the exchange of oxygen occurs

  • in capillaries of the body.

  • Normally the rounded blood cells

  • go through those capillaries very smoothly

  • like Teflon and don't have any problem.

  • But when they form sickle cells,

  • they don't, and they get stuck there.

  • Not only do they get stuck there,

  • but they stop the blood flow in that capillary,

  • which is one of the reasons

  • that people get this very intense pain,

  • because their muscle cells are starving for oxygen

  • and they can't get any because it's all blocked there.

  • You might wonder why it's called "anemia."

  • The reason it's called anemia is because our body

  • has a way of recognizing damaged blood cells,

  • and when it sees misshapen blood cells

  • it takes them out of action.

  • So even though we might be able to get this to revert

  • to some extent,

  • and by the way,

  • I can't tell you that that happens,

  • but if we might get this to revert to this form,

  • before that happens our body

  • can take this out of action and say,

  • "That's a damaged blood cell.

  • "I don't want to have that floating around.

  • "It's going to cause a problem."

  • So the more your blood sickles,

  • the more blood cells you lose,

  • and of course that's exactly what anemia is,

  • a lowered concentration of blood cells in your body.

  • People have studied sickle cell anemia for a long time.

  • Sickle cell anemia actually

  • has a very interesting historical component.

  • It was the first disease

  • that was proposed as a genetic disease,

  • the first disease proposed as a genetic disease.

  • Does anybody know who made that proposal?

  • Student: Linus Pauling.

  • Kevin Ahern: Linus Pauling made that proposal,

  • a very cool thing.

  • So there's an Oregon State connection there, again.

  • One of the questions people ask

  • when they see a genetic disease

  • that persists in the population for a long time,

  • and there's been evidence that this

  • has been around for a long time,

  • is that we think that there must be a reason why it persists.

  • Why doesn't it just die out?

  • Why don't people who have sickle cell anemia

  • eventually have trouble reproducing or don't reproduce as well,

  • and it would die out of the population?

  • But sickle cell anemia persists

  • and it has persisted over human evolution for a long time.

  • There's a very interesting observation

  • that people made about the distribution

  • of the genes in sickle cell anemia.

  • If you overlay the incidence of sickle cell type

  • with the prevalence of malaria in the world,

  • you'll see a disproportionate amount

  • of sickle cell genes present in locations

  • where there's very high incidence of malaria.

  • People have done epidemiological studies and found that,

  • in fact, there is an advantage for survival

  • for people in malarial infected areas

  • to have the heterozygous form of sickle cell anemia,

  • that is one normal and one mutant.

  • They have an increased incidence of survival

  • compared to people, for example,

  • who have both wild type or both mutant.

  • So there is, apparently, a genetic basis

  • for why sickle cell anemia persists in the population.

  • That's the next-to-last thing I want to leave you with.

  • The last thing I want to leave you with is,

  • one of the things that we're interested in

  • with sickle cell anemia

  • is what kind of treatments can we offer.

  • It's a disease that is being investigated very intently.

  • I've had actually several of my own students

  • who've gone off to summer internships

  • working on sickle cell anemia.

  • One of the interesting treatments

  • that has been experimented with

  • and I think is still being experimented with,

  • is actually trying to get around

  • the mutant component of hemoglobin,

  • whether it's the alpha or the beta.

  • What they do,

  • this thing keeps popping out on me,

  • what they do is they treat patients with a drug

  • that will induce the fetal hemoglobin to start being expressed.

  • Now, the fetal hemoglobin, of course,

  • didn't have that mutation.

  • The fetal hemoglobin is perfectly good,

  • and so by doing this, they flood the blood

  • or the blood cells with a normal hemoglobin

  • and in some cases it appears to help alleviate the disease.

  • That fetal hemoglobin, of course,

  • normally stops being made around the time

  • we're one or two years old.

  • But with proper drug treatment you can actually induce

  • it to be made again and for some people that provides relief.

  • So it's, again, another connection that

  • we have to one of the hemoglobin genes.

  • That said, I will take any questions that you might have.

  • Yes, Shannon?

  • Student: So I'm not sure if I understand

  • the correlation between someone having one allele

  • for sickle cell and their survival in malaria areas.

  • Is that implying that someone with sickle cell

  • survives malaria better, or...

  • Kevin Ahern: So her question is,

  • does the person who's heterozygous for sickle cell anemia,

  • are they more resistant to malaria?

  • And the answer is exactly that.

  • They are.

  • Yes.

  • Yes, sir?

  • Student: Does the increased affinity of fetal

  • hemoglobin affect the person?

  • Kevin Ahern: That's a really good exam question.

  • Did you hear what he said?

  • He said, "Does the presence of that fetal hemoglobin

  • "change anything for that person?"

  • What do you guys think?

  • Describe to me what you think might happen.

  • Student: They will have a lower net oxygen capacity,

  • as far as the ability to dump it off.

  • Kevin Ahern: Yeah.

  • Student: But their overall capacity,

  • because the functional gamma units will probably

  • be increased as compared to anemia.

  • Kevin Ahern: Yeah.

  • So they'll have less capacity, basically, is what'll happen,

  • because, if we think about it,

  • the more of that fetal we have,

  • the more hemoglobin is going to be in the R state.

  • And the R state's really good for binding oxygen,

  • but it's not so good for giving it up.

  • But it's probably better than not being able to get any at all.

  • But you're exactly right.

  • Maybe we should sing a song to summarize all of this.

  • Okay?

  • Let's do that.

  • Lyrics: Oh, isn't it great what proteins can do,

  • especially ones that bind to O2, hemoglobin's moving around.

  • Inside of the lungs, it picks up the bait,

  • and changes itself from T to R state.

  • Hemoglobin's moving around.

  • The proto-porphyrin system, its iron makes such a scene,

  • arising when an O2 binds, pulling up on histidine.

  • The binding occurs cooperatively,

  • thanks to changes qua-ter-nar-y.

  • Hemoglobin's moving around.

  • It exits the lungs, engorged with O2,

  • in search of a working body tissue.

  • Hemoglobin's moving around.

  • The proton concentration is high and has a role,

  • between the alpha betas it finds imidazole.

  • Kevin Ahern: That's histidine.

  • Lyrics: To empty their loads, the globins decree,

  • "We need to bind 2,3BPG".

  • Hemoglobin's moving around.

  • The stage is thus set for grabbing a few cellular dumps of CO2

  • Hemoglobin's moving around.

  • And then inside the lungs it discovers oxygen,

  • and dumps the CO2 off to start all o'er again.

  • So see how this works,

  • you better expect to have to describe the Bohr effect.

  • Hemoglobin's moving around.

  • [applause]

  • Kevin Ahern: Thank you.

  • Okay, So you better expect to have to describe the Bohr effect.

  • That's a hint there, right?

  • We turn our attention from hemoglobin now to enzymes.

  • Enzymes, of course, are proteins that catalyze reactions.

  • Hemoglobin, as I said, didn't catalyze any reaction,

  • but enzymes do.

  • We're going to spend a fair amount of time thinking

  • about how enzymes act as catalysts and what they do.

  • Enzymes are remarkable, and they are remarkable

  • especially when we compare them to chemical

  • or other chemical catalysts.

  • If I have a chemical catalyst

  • that I use to catalyze a reaction,

  • it might not be unreasonable

  • for me to expect a hundred or a thousandfold

  • enhancement using a chemical catalyst.

  • When I use an enzyme,

  • an enzyme can provide

  • up to 10 to the 17th enhancement.

  • I believe that's 170 quadrillion.

  • Now we start to see that enzymes are catalysts,

  • but they're really incredible catalysts,

  • absolutely incredible catalysts.

  • How in the world can something work like that?

  • Well, let me just give you some,

  • maybe some things that you can think about

  • or feel the magnitude of this.

  • If we look at this top enzyme

  • we'll actually talk about this next term,

  • the enzyme has a half-life,

  • meaning if...

  • not the enzyme, the reaction that this enzyme

  • catalyzes has a half-life of 78 million years,

  • meaning that if we took a mixture of it

  • and we let it sit in a test tube,

  • it would take 78 million years for half of it to react.

  • If I treat this with an enzyme,

  • I make this enzyme catalyze the conversion of

  • 39 molecules per second per molecule of enzyme.

  • Now, that's pretty incredible.

  • The rate enhancement, if you do the math,

  • corresponds to this 140 quadrillion that I told you about here.

  • Now, that's mind boggling, okay?

  • That may sound very rapid, and that is, in fact, very rapid.

  • But there are other enzymes that are even more

  • incredible in terms of what they do.

  • We're going to spend a fair amount of time

  • talking about this enzyme, right here, carbonic anhydrase.

  • Carbonic anhydrase does something that, to me,

  • I can't get my head around.

  • It only does things by about 7.7 millionfold greater.

  • That's nowhere near the 140 quadrillion.

  • But when I look at how many molecules

  • of product each molecule of enzyme makes per second,

  • it's mind boggling.

  • Each molecule, I take one enzyme

  • one, a single enzyme,

  • and I put it with its substrate

  • the substrate's what an enzyme acts on,

  • and I discover it makes

  • one million molecules of product per second!

  • Now, I don't know about you,

  • but I can't think of something happening that rapidly.

  • One million molecules of product per second

  • every enzyme is making.

  • Imagine that I was running a factory,

  • and a factory has an assembly line,

  • and the assembly line is putting products out the end.

  • I don't care how fast or how many people

  • you have working in that factory,

  • you are not going to make a million products per second.

  • This tells us that the nanoscale world,

  • "nanoscale" being the level of molecules,

  • the world that exists at the level of molecules

  • is very different than the world we know out here.

  • The nano world is very different than the macro world.

  • There's no way I can do a million things a second.

  • No matter how hard I try, I'm not going to do that.

  • Yes, sir?

  • Student: As long as it's not high enough to denature them,

  • would a higher temperature increase these reaction rates,

  • as in inorganic chemistry?

  • Kevin Ahern: His question is,

  • will temperature affect enzymatic rate?

  • And the answer is, yes, it will, to a point.

  • You could imagine that if we raise the temperature

  • we may favor the reaction,

  • and then we'll actually see it fall off rapidly.

  • Any ideas why it falls off rapidly?

  • We denature the enzyme.

  • Yeah.

  • Yes, sir?

  • Student: Is this one million per second only

  • when the substrate is in excess?

  • Kevin Ahern: So, yes, and it's a good question.

  • His question is, does the substrate have to be in excess?

  • And the answer is, yes, it does.

  • So you're actually getting a little ahead of me,

  • but I will address that directly in what

  • I'm going to say in just a little bit,

  • but it's a very good question.

  • So, pretty cool stuff.

  • This sort of sets the stage for enzymes.

  • Enzymes are proteins, and what you've seen in a protein

  • so far is its structure is critical.

  • Proteins have very, very specific structures,

  • and, consequently, they have at least fairly specific

  • molecules that they will bind to and catalyze reactions on.

  • Notice I said "fairly specific."

  • Some enzymes are more specific than others.

  • Some are really rigid,

  • they only want one thing and that's it.

  • But enzymes have a specificity.

  • They don't catalyze a reaction on everything

  • because they can't bind to everything,

  • and the reactions that would be catalyzed

  • would differ from one molecule to another.

  • This shows a reaction that we're going to spend

  • a fair amount of time talking on,

  • and it's a reaction that involves the cleavage

  • of a peptide bond.

  • To cleave a peptide bond,

  • you have to add water across it,

  • and that adding water causes the bond to split.

  • It's called proteolysis,

  • P-R-O-T-E-O-L-Y-S-I-S.

  • Proteolysis breaks peptide bonds,

  • and enzymes that catalyze proteolysis are called proteases,

  • P-R-O-T-E-A-S-E-S, proteases.

  • We're going to spend some time talking about those,

  • but before I talk too much about those,

  • I'll come back to that later,

  • I want to say a few words about energy.

  • I'm going to talk about delta G.

  • You guys have heard the change in Gibbs free energy

  • in your basic chemistry classes.

  • I'm going to introduce it here only in very general terms,

  • and I will tell you right now that you're not,

  • underline "not,"

  • going to do delta G calculations on this exam.

  • Student: Yay!

  • Kevin Ahern: You will, later.

  • [laughing]

  • Students: Ohhh.

  • Kevin Ahern: But I'm not,

  • I figure you've got enough for this exam.

  • They're actually not very relevant for us right now,

  • except for to understand the beginnings of enzymes.

  • But later we'll see that they actually will be important.

  • But on this exam you will not have to do delta G calculations.

  • But I'm going to say a few words about delta G

  • because it's relevant for understanding

  • how enzymes do what they do.

  • We know that there's a standard Gibbs free energy.

  • The change in the standard Gibbs free energy

  • is known as "delta G."

  • Delta G tells us the direction of a reaction.

  • If the delta G is negative,

  • the reaction proceeds forward as it's written.

  • If the delta G is positive,

  • the reaction proceeds backward as it's written.

  • If delta G is equal to zero,

  • the reaction is at equilibrium.

  • Equilibrium does not mean equal concentrations

  • of products and reactants.

  • Get that in your head, okay?

  • That's the number one mistake that students make.

  • They didn't learn what equilibrium meant, back when.

  • It does not mean equal concentrations

  • of products and reactants.

  • It means that they're unchanging over time.

  • You might have ten times as much of one or than the other.

  • But it doesn't mean equal concentrations.

  • There's a related quantity called "delta G zero prime."

  • Delta G zero prime is called the "standard Gibbs free energy."

  • The prime is on there for biologists,

  • like biochemists, because,

  • for a regular delta G zero that one would calculate,

  • that would correspond to everything being present

  • at a concentration of 1 molar.

  • Well, if we have a reaction that involves protons,

  • we don't want that being at 1 molar because

  • it'll kill our enzyme.

  • Right?

  • It would have a pH of zero.

  • That would not be good.

  • So the delta G, the prime on there indicates

  • that everything is at 1 molar,

  • except for the protons.

  • So we've got a pH 7, basically, that we're doing this at.

  • So what is the standard Gibbs free energy?

  • The standard Gibbs free energy is the standard

  • Gibbs free energy change under standard conditions.

  • That's all it is.

  • Under standard conditions, that's what it is.

  • So delta G tells us the Gibbs free energy

  • change under any conditions.

  • The delta G zero prime is what corresponds

  • to standard conditions.

  • There's a calculation that we're not going to do.

  • We will do it later.

  • But just to remind you from your delta G equation,

  • delta G equals delta G zero prime plus the gas constant R,

  • times the temperature in Kelvin,

  • times the natural log of the concentration

  • of products over the concentration of reactants.

  • That's a simplification of the actual equation,

  • but for our purposes that's fine.

  • At equilibrium, delta G equals zero,

  • so the delta G zero prime must equal

  • these two things must be the opposite sign of each other,

  • so they cancel out.

  • That's not really essential for us to understand

  • to understand to understand enzymes.

  • But we do understand and recognize that delta G

  • is a very important parameter to understand

  • for directions of reactions.

  • It tells us some very important things.

  • So I want you to keep that in mind

  • and I'll show you a couple of things here.

  • Enzymesóno surprise from that first table

  • I showed youóspeed reactions and

  • they can speed reactions immensely.

  • They're very, very important for speeding reactions.

  • In this case, we're calculating, we're determining

  • the concentration of product versus time on the x-axis.

  • We see there's more product being made with time.

  • When we measure velocities of reactions,

  • we measure the concentration of product per time.

  • That's what velocity is.

  • When we measure the velocity of a car,

  • we measure distance per time.

  • When we measure a reaction rate,

  • we calculate concentration of product per time.

  • Okay, everybody got that?

  • So this should be concentration of products going up,

  • and that's concentration of product over time.

  • This schematic introduces a sort of unusual delta G,

  • and we can think of it as an activation energy.

  • It doesn't really relate to the equation that we had before,

  • but we can think of the importance of this activation energy.

  • Activation energy is an energy that has to be put

  • into a reaction before the reaction will proceed.

  • It has to be put into the reaction before

  • the reaction will proceed.

  • If we look at an uncatalyzed reaction,

  • we see here is the free energy of the starting materials

  • and here is the free energy of the product.

  • The change in the Gibbs free energy is the difference

  • between this and this,

  • and we see that this will give us a negative delta G.

  • This reaction is favorable and it will go forwards.

  • Now, on the x-axis, we're plotting what's called

  • "reaction progress," and we're just sort of seeing

  • how this thing is going, what's happening

  • to the energy of this reaction over time.

  • Okay, well, the top line corresponds

  • to an uncatalyzed reaction.

  • For an uncatalyzed reaction,

  • I don't have an enzyme that's helping me out.

  • I've got a molecule A over here,

  • and I've got a molecule B over here.

  • They're in solution, and they're bouncing around

  • and they're bouncing around.

  • All of a sudden, they bounce and, if they hit the right way,

  • they will, in fact, react and give a product.

  • Alright?

  • Uncatalyzed.

  • We could imagine that these two things

  • bouncing around in here,

  • if there's only one here and one over here,

  • the likelihood they'd bounce and hit each other

  • in the right orientation is low.

  • If we increase the concentration,

  • we've got a bunch of these.

  • The more concentrated it is,

  • the more likely it's going to go.

  • Right?

  • If we measure the energy it takes to put these guys together,

  • when they do work right,

  • that's what's being plotted here.

  • It's called the "transition state."

  • It's also called the "activation energy."

  • I'll take either one on an exam.

  • Once we get to that hump,

  • that reaction can either fall backwards

  • to where it started from,

  • or it can fall forwards and go down this hill.

  • There's our delta G for the reaction.

  • Now, what happens if I put an enzyme in there?

  • Well, I'll tell you something very important to remember.

  • Enzymes do not change the delta G for a reaction.

  • They do not change.

  • Notice, the enzymatic reaction starts with

  • a substrate at the same energy

  • and it creates a product with the same energy as

  • the uncatalyzed reaction.

  • The delta G is exactly the same.

  • So what did the enzyme do?

  • Well, the enzyme lowered the activation energy.

  • It lowered the activation energy,

  • and it made it much more likely that when two molecules

  • hit each other they would have enough energy

  • to make this thing go forward.

  • Now, we'll see enzymes do some other tricks,

  • as well, but the number one thing that enzymes

  • do to speed a reaction is they lower the activation energy.

  • Now the analogy I like to give for this is,

  • if I took the class and I said,"Okay.

  • "I'm going to give you guys extra credit.

  • "We're going to go out here and we're going to go up

  • "to this giant steel ball-bearing."

  • And we're going to go out the door

  • and Corvallis is at about 250 feet above sea level,

  • and we're going to push it towards the ocean.

  • In theory, it should go right over to the ocean,

  • because 250 feet higher, there's Corvallis, there's the ocean.

  • Duh, right?

  • Well, of course, it's not going to go.

  • Why isn't it going to go?

  • There is a coastal mountain range between us and them,

  • between us and the ocean.

  • There's our activation energy.

  • So we say,

  • "Okay, well, we've got a whole bunch of us.

  • "Let's take, and we want to make sure it gets there,

  • "so let's go take this ball-bearing

  • "and we'll push it to Marys Peak."

  • Marys Peak, of course, is the tallest peak

  • in the Coastal Range.

  • It's just to the southwest of Corvallis.

  • We work and we struggle very hard to get that up there.

  • I'm assuming there's no trees in the way,

  • and, given clear cutting, that's not an unreasonable

  • thing to think, anymore.

  • [class laughing]

  • Bad joke, huh?

  • Bad professor!

  • We push this ball-bearing to the top of Marys Peak.

  • And we say, "Well, it's going to have some ups

  • "and downs along the way, but it's going to have enough

  • "energy to make it to the ocean."

  • And it will.

  • Again, assuming we do enough clear cutting, right?

  • Okay, well, then the smart person says,

  • "Wait a minute, this is the dumb way to go.

  • "We really don't have to go to Marys Peak to make it go.

  • "All we have to do is make sure we get over

  • "the highest pass."

  • Right?

  • As long as we get it to the point of the highest pass,

  • then it's still going to have enough energy

  • to get down because all of the other passes will be lower.

  • The enzyme is helping you to find the pass.

  • That's what it's doing.

  • The enzyme has found the pass.

  • It's found that.

  • You're not going to have to put as much energy

  • into getting it all the way up to Marys Peak.

  • You've only got to get it up to this pass to get it across,

  • over to the ocean.

  • So that is my metaphor of the day.

  • Questions on that?

  • You guys are looking tired.

  • Student: It's Friday.

  • Student: Yeah, it's Friday.

  • Kevin Ahern: You're still looking tired.

  • Yes, sir?

  • Student: Are there enzymes that can bring

  • that down to where there is no positive activation energy

  • and just make it spontaneous?

  • Kevin Ahern: Enzymes will help this reaction go.

  • The spontaneity of a reaction is really

  • determined by the delta G.

  • But the enzymes can lower that significantly.

  • They can lower the activation energy significantly, yes.

  • Question over here?

  • Student: You said earlier that there's molecule A

  • and molecule B and they bump into each other?

  • Kevin Ahern: Bang!

  • Student: Does the enzyme grab A and B?

  • Kevin Ahern: Oh, good question!

  • Does the enzyme grab A and B?

  • In fact, that's one of the other tricks the enzyme does.

  • It has a specific binding site for A,

  • it has a specific binding site for B,

  • and the enzyme is positioning them in exactly

  • the right way so we don't have to worry about

  • them hitting the right way and bouncing off.

  • It positions them in exactly the right way so they bond.

  • Kevin Ahern: So, what?

  • Student: [unintelligible] lowering the activation energy?

  • Kevin Ahern: That would also contribute

  • to lowering the activation energy.

  • That's correct.

  • Yes, Connie?

  • Student: What usually contributes to that activation energy?

  • Like, is it the heat of the general surroundings?

  • Kevin Ahern: What contributes to activation energy?

  • Well, activation energy is a function of the temperature,

  • certainly the heat of the surroundings.

  • It's a function of the concentration.

  • So those are two variables that can really affect what's there.

  • There are other things that can play into it, as well.

  • Concentration is a very important one.

  • Concentration is very, very important,

  • because the more concentrated something is

  • and this was the question that he had over here earlier today

  • when I want to see a million molecules of product per second,

  • do I have to have the enzymes saturated with substrate?

  • You betcha!

  • That really works well.

  • So I'm going to get to that in just a second.

  • But before I do that, why don't we stand up and stretch?

  • Just stand up and stretch.

  • You guys will get some oxygen in your system.

  • [indistinct talking]

  • Now you look alive, alert, refreshed, and ready for more,

  • yes, sir, more biochemistry.

  • Kevin Ahern: What's that?

  • Student: [unintelligible]

  • Kevin Ahern: Oh, yeah, it is distracting.

  • I wish I could just make this thing go away,

  • but it's part of the security system

  • and they will not let us fire that thing.

  • But whenever you see it,

  • let me know and I'll be happy to turn it off, yes.

  • About every hour it starts up.

  • Let's think about concentration.

  • This is a plot that you're going to see a lot of.

  • It's called a "velocity-versus-substrate concentration plot."

  • It's also called a V-versus-S plot.

  • So I want to introduce it to you and describe

  • to you what it tells us.

  • What does this plot tell us?

  • This plot tells usóI think the batteries

  • in this thing are just going badólet's imagine that,

  • instead of having an enzymatic reaction,

  • I have this factory that I'd talked about.

  • The factory is full of workers and the factory is try to make,

  • let's say, automobiles.

  • To make an automobile, you have to have a lot of materials,

  • they have to be assembled,

  • they have to be stuck together.

  • This group of workers in the factory assemble

  • the automobile from the parts, and that's their job every day.

  • If they have very, very few parts, or the parts come in very,

  • very slowly, what happens to their ability to make automobiles?

  • Well, it's going to go down.

  • They're not going to make automobiles so fast.

  • They're going to be spending a fair amount

  • of their time waiting on parts, waiting on parts,

  • waiting on parts, right?

  • Their velocity is going to be low when their amount

  • of raw materials is low.

  • As they get more and more raw materials,

  • they start making more and more cars,

  • and we see that go up fairly rapidly, at least at first.

  • And now, all of a sudden, the factory is starting

  • to get more and more raw materials,

  • and the employees are going,

  • "Oh, wow, I'm going as fast as I can, going as fast as I can.

  • "Whoa, there's more!

  • "I'm going to go as fast as I can!"

  • Eventually, we get to a point where the employees,

  • no matter how hard they work,

  • they can't work any faster.

  • They reach a maximum velocity of making cars.

  • In this case, the enzyme is reaching

  • a maximum velocity of making product, exactly the same thing.

  • Not surprisingly, enzymes have a maximum.

  • Now, if I take my automobiles, I take my factory, and I say,

  • "Whoa, this factory is working at maximum output

  • "of cars that it can get."

  • This is 400 cars a day.

  • But I can sell 1,000 cars a day.

  • Well, it doesn't matter how much more I pour into

  • that factory in terms of raw materials,

  • the employees can only do so much, right?

  • So what do I decide to do?

  • Well, the smart thing to do would be

  • to build another factory, right?

  • If I build another factory identical to the first one,

  • with the identical abilities of the workers to work,

  • instead of having 400 cars per day,

  • my two factories can make 800 cars per day.

  • You with me?

  • Now, I'm illustrating a point about factories

  • to you that's important to understand when

  • understanding how enzymes work.

  • The maximum velocity an enzyme can work at is called Vmax,

  • velocity maximum, Vmax.

  • Vmax is reached when an enzyme is saturated with substrate.

  • Substrate is the stuff that the enzyme works on.

  • It doesn't matter if I keep increasing the substrate anymore.

  • I'm not going to get any more product made per time.

  • Just like the factory, I'm working as hard as I can do.

  • I can't put out more.

  • But if I add twice as much enzyme,

  • what's going to happen to Vmax?

  • It's going to double, right?

  • I double the enzyme, I'm going to double the velocity.

  • That tells us something very important.

  • Vmax is an interesting quantity,

  • but it's not a characteristic of an enzyme,

  • because Vmax depends on how much enzyme I use in my reaction.

  • If I use twice as much enzyme,

  • I'm going to get twice as much Vmax.

  • That make sense, no, yes?

  • Student: Well, so, in theory, proportionally speaking,

  • it still could be a characteristic quantity, then, right?

  • It could be unique to an enzyme in terms of...

  • Kevin Ahern: Vmax is not a characteristic of an enzyme.

  • No, okay?

  • Vmax is not a characteristic.

  • And you will talk to many people who don't know that.

  • They'll go, "Oh, yeah, the Vmax of this enzyme is

  • blah, blah, blah."

  • And I'd say,

  • "And how much enzyme did you use?"

  • [mumbling]

  • Now, Shannon is sort of thinking ahead.

  • There's gotta be something characteristic

  • about the enzyme that's here.

  • What could it be?

  • Well, I told you that the Vmax depended upon

  • the concentration of the enzyme.

  • I double the enzyme, I double the velocity.

  • What if I took Vmax and I divided it

  • by the concentration of the enzyme?

  • Now I've taken the concentration of the enzyme

  • out of the equation.

  • What do I get?

  • I get something called "Kcat,"

  • K, with a lowercase c-a-t subscript.

  • Kcat is equal to Vmax divided by the concentration of the enzyme.

  • Now I've gotten rid of the concentration out of the equation,

  • and I get something that's really interesting.

  • It's called Kcat, "Cat," c-a-t, yes.

  • It's also called "turnover number."

  • And you've already seen Kcat.

  • That very first table I showed you that had those big numbers?

  • When I said that carbonic anhydrase had made a million

  • molecules of product per molecule of enzyme per time?

  • That's Kcat.

  • So Kcat is a measure of the number of molecules

  • of product an enzyme makes per time.

  • A Kcat of one million means it makes a million molecules

  • of product per molecule of enzyme, per second, in this case.

  • Remarkable.

  • That is a characteristic of an enzyme.

  • We can compare Kcats between two enzymes.

  • We can't compare Vmaxes between two enzymes.

  • Everybody got that?

  • If you get that, you will know as much as many

  • people know about enzymes, and I'm very pleased,

  • in this class, that students usually take

  • that message away with them,

  • and I think it's a very important message.

  • Kcat is a characteristic of an enzyme.

  • Vmax is not.

  • It depends on how much enzyme I use.

  • So if I want to compare two enzymes,

  • I've got to compare their Kcats, not their Vmaxes.

  • That's a lot of stuff for one day.

  • Let's finish a few minutes early.

  • I have a new clock to celebrate, over here.

  • It tells me I'm finishing early.

  • So let's do that and I will see you guys on Monday.

  • So, is that where you were headed?

  • Student: Yeah.

  • Kevin Ahern: I figured it was.

  • Student: I guess it must remind me of something.

  • So on Monday I have to miss class.

  • Kevin Ahern: Okay.

  • I'll give a pop quiz.

  • Student: Oh, okay.

  • Well, of course, I'll stay on top of the reading and so forth.

  • Is there anything else I ought to do?

  • Kevin Ahern: You'll be fine, you'll be fine.

  • Kevin Ahern: Take care, Shannon.

  • I've have nobody to pick on in the front row.

  • Hi, how're you doing?

  • Student: I have a question for you.

  • Kevin Ahern: Yes, sir.

  • Come on back.

  • [END]

Captioning provided by Disability Access Services

字幕與單字

單字即點即查 點擊單字可以查詢單字解釋

B1 中級

#09生物化學血紅蛋白II/酶I講座Kevin Ahern的BB 450/550。 (#09 Biochemistry Hemoglobin II/Enzymes I Lecture for Kevin Ahern's BB 450/550)

  • 111 11
    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
影片單字