Organic Chemistry

23

Amino Acids, Peptides,

and Proteins

959

oxidized glutathione

T

he three kinds of polymers that are prevalent in nature are

polysaccharides, proteins, and nucleic acids. You have al-

ready learned about polysaccharides, which are naturally

occurring polymers of sugar subunits (Section 22.18), and nucle-

ic acids are covered in Chapter 27. We will now look at proteins

and the structurally similar, but shorter, peptides. Peptidesand

proteinsare polymers of amino acidslinked together by amide

bonds. The repeating units are called amino acid residues.

Amino acid polymers can be composed of any number of monomers.

A dipeptidecontains two amino acid residues, a tripeptidecontains three,

an oligopeptidecontains three to 10, and a polypeptidecontains many amino

acid residues. Proteins are naturally occurring polypeptides that are made up of 40 to

4000 amino acid residues.

From the structure of an amino acid, we can see that the name is not very precise.

The compounds commonly called amino acids are more precisely called amino-

carboxylic acids.

Proteins and peptides serve many functions in biological systems. Some protect or-

ganisms from their environment or impart strength to certain biological structures.

Hair, horns, hooves, feathers, fur, and the tough outer layer of skin are all composed

largely of a structural proteincalled keratin. Collagen, another structural protein, is a

major component of bones, muscles, and tendons. Some proteins have other protective

functions. Snake venoms and plant toxins, for example, protect their owners from

other species, blood-clotting proteins protect the vascular system when it is injured,

OOOO

CH

-aminocarboxylic acid

an amino acid

R C OH NHCHC

R

NHCHC

R′

NHCHC

R′′

amino acids are linked together by amide bonds

amide bonds

+NH

3

a-

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