Amino Acids

Amino acids all possess a carboxylic acid group and an amino group, both linked to a single carbon atom called the a-carbon (Figure 1.1). The differences between amino acids result from the side chain attached to the a-carbon atom, which can be

The Amino Acid

The general formula of an amino acid is

a-carbon Atom

Carboxyl Group

Side-chain Group a-carbon Atom

Carboxyl Group

Side-chain Group

R is commonly one of 20 different side chains. At pH 7 both the amino and carboxyl groups are ionized.

Optical Isomers

The a-carbon atom is asymmetric, which allows for two mirror image (or stereo-) isomers, L and D.

Proteins consist exclusively or L-amino acids.

Families of Amino Acids

The common amino acids are grouped according to whether their side chains

Acidic Basic

Uncharged polar Nonpolar

These 20 amino acids are given both three-letter and one-letter abbreviations.

Basic Side Chains

H CH

I 2 CH2 CH

Arginine

This group is very basic because its positive charge is stabilized by . resonance. t

NH NH

Histi

H IH

HN HI

-NH+ / \ These nitrogens have a relatively weak affinity for an H+ and are only partly positive at neutral pH.

Peptide Bonds

Amino acids are commonly joined together by an amide linkage, called a peptide bond.

Peptide bond: The four atoms in each gray box form a rigid planar unit. There is no rotation around the C-N bond.

Proteins are long polymers of amino acids linked by peptide bonds, and they are always written with the N-terminus toward the left. The sequence of this tripeptide is histidine-cysteine-valine.

Amino- or N-terminus

O

CH2

H

CN

C

CN

H

\

Carboxyl- or C-terminus

HN HC

These two single bonds allow rotation, so that long chains of amino acids are very flexible.

Lysine

FIGURE 1.1 The 20 amino acids found in proteins.

Acidic Side Chains

Nonpolar Side Chains

Aspartic Acid

I II

H CH2

Glutamic Acid

H CH H CH2

CH2 C

I II

NC C

I II

I II

NC C

I II

The -OH group is polar.

I II

H CH3

I II

H CH

H CH2 CH

CH3 CH

I II

(actually an CH2 imino acid)

H CH

Glycine

I II

I II

H CH

I II

H CH

Phenylalanine

I II

H CH2

Tryptophan

I II

H CH2

Cysteine

I II

Disulfide bonds can form between two cysteine side chains in proteins.

S CH

H CH

FIGURE 1.1 Continued aliphatic or aromatic in nature and can include extra amino, imino, or carboxylic acid functional groups (Figure 1.1). All amino acids except glycine can exist as optical isomers in D- and L-forms (Figure 1.1), but only L-forms are found in living organisms (with the exception of D amino acids in certain bacterial cell wall proteins).6 The chemical versatility provided by the 20 common amino acids is critically important to the function of proteins. Five of the 20 amino acids have side chains that can form ions in solution and impart polar and hydrophilic properties to the protein.

Other amino acids have aliphatic side chains that are nonpolar and are therefore hydrophobic. Structures for the various amino acids are presented in Figure 1.1. The collective properties of the amino acid side chains underlie the diverse and sophisticated functions that proteins perform.1

Amino acids are connected together via covalent peptide bonds formed between the amino functional group on the a-carbon of one amino acid and the carboxyl functional group attached to the a-carbon on the adjacent amino acid. The formation of a covalent "peptide bond" occurs through the action of enzymes resulting in the loss of water (dehydration reaction) and the formation of an amide bond between adjacent amino acids (Figure 1.1). Proteins are polymers of amino acids joined head-to-tail in a long chain that is then folded into a three-dimensional structure unique to each protein. When several amino acids (less than 50) are linked together covalently in a chain, the resulting molecule is called a "polypeptide" or peptide.1 When the amino acid chains are composed of more than 50 amino acids connected together, the polymer is considered a protein.1 All polypeptides and proteins have an amino (NH) group at one end (N-terminus) and a carboxyl (COOH) group at the other end (C-terminus). This gives it a definite directionality — a structural (as opposed to an electrical) polarity.

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Responses

  • sofia
    Are amino acids good after.food poisoning?
    1 year ago

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