Amino Acids and Peptides

A protein is a polymer of amino acids. An amino acid has a central carbon atom with an amino (—NH2) and a car-boxyl (—COOH) group bound to it (fig. 2.23a). The 20 amino acids used to make proteins are identical except for a third functional group called the radical (R group) attached to the central carbon. In the simplest amino acid, glycine, R is merely a hydrogen atom, while in the largest amino acids it includes rings of carbon. Some R groups are hydrophilic and some are hydrophobic. Being composed of many amino acids, proteins as a whole are therefore often amphiphilic. The 20 amino acids involved in proteins are listed in table 2.8 along with their abbreviations.

A peptide is any molecule composed of two or more amino acids joined by peptide bonds. A peptide bond, formed by dehydration synthesis, joins the amino group of one amino acid to the carboxyl group of the next (fig. 2.23b). Peptides are named for the number of amino acids they have—for example, dipeptides have two and tripeptides have three. Chains of fewer than 10 or 15 amino acids are

Chapter 2 The Chemistry of Life 79

called oligopeptides,23 and chains larger than that are called polypeptides. An example of an oligopeptide is the childbirth-inducing hormone oxytocin, composed of 9 amino acids. A representative polypeptide is adrenocorti-cotropic hormone (ACTH), which is 39 amino acids long. A protein is a polypeptide of 50 amino acids or more. A typ-

ical amino acid has a

molecular weight of about 80

amu,

23 oligo = a few

Table 2.8

The 20 Amino Acids and

Their Abbreviations

Alanine

Ala

Leucine

Leu

Arginine

Arg

Lysine

Lys

Asparagine

Asn

Methionine

Met

Aspartic acid

Asp

Phenylalanine

Phe

Cysteine

Cys

Proline

Pro

Glutamine

Gln

Serine

Ser

Glutamic acid

Glu

Threonine

Thr

Glycine

Gly

Tryptophan

Trp

Histidine

His

Tyrosine

Tyr

Isoleucine

Ile

Valine

Val

Some nonpolar amino acids

Some polar amino acids

Amino acid 1

Amino acid 2

C OH

Peptide bond

A dipeptide

Figure 2.23 Amino Acids and Peptides. (a) Four representative amino acids. Note that they differ only in the R group, shaded in pink. (b) The joining of two amino acids by a peptide bond, forming a dipeptide. Side groups R, and R2 could be the groups indicated in pink in figure a, among other possibilities.

Saladin: Anatomy & I 2. The Chemistry of Life I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

80 Part One Organization of the Body and the molecular weights of the smallest proteins are around 4,000 to 8,000 amu. The average protein weighs in at about 30,000 amu, and some of them have molecular weights in the hundreds of thousands.

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