Amino acids all have a similar structure, with carboxyl

and amino functional groups. What makes each amino acid

unique, is the presence of a variable, or R group.

The R group gives each amino acid its chemical properties.

Amino acids are linked together by peptide bonds, forming long

polypeptide chains. As these chains form,

the polypeptide develops multiple levels of structure, which contributes to its

overall shape.

These levels are called the primary, secondary, tertiary

and quaternary structures.

The primary structure is the order of amino acids in the polypeptide

chain. Amino acids can be likened to letters in the alphabet,

Just as thousands of words can be constructed by varying

the 26 letters in our alphabet, so too

can different proteins be created by varying the number and sequence of just

20 amino acids.

The secondary structure refers to the pleated sheet or helix

that a protein chain can form due to hydrogen bonding and other

chemical attractions between the R groups of nearby amino acids.

The overall shape of a polypeptide is called its tertiary structure.

This level of structure is created when the secondary structure folds

and twists upon itself. The tertiary structure is

held in place by a variety of bonds and interactions that form between the R groups

of the amino acids on the chain. Once polypeptides

have formed a tertiary structure, they are commonly referred to as proteins.

In some cases, proteins can form

quaternary structures, which consist of interactions between multiple proteins.

These interactions often result in the formation of

large protein complexes. Examples of proteins that have

quaternary structures are hemoglobin and the sodium channels in the cell

membranes.