Only LAmino Acids Occur in Proteins

With the sole exception of glycine, the a-carbon of amino acids is chiral. Although some protein amino acids are dextrorotatory and some levorotatory, all share the absolute configuration of L-glyceraldehyde and thus are L-a-amino acids. Several free L-a-amino acids fulfill important roles in metabolic processes. Examples include ornithine, citrulline, and argininosuccinate that participate in urea synthesis; tyrosine in formation of thyroid hormones; and glutamate in neurotransmitter biosynthesis. D-Amino acids that occur naturally include free D-serine and D-aspartate in brain tissue, D-alanine and D-glutamate in the cell walls of grampositive bacteria, and D-amino acids in some nonmam-malian peptides and certain antibiotics.

Table 3-1. l- a-Amino acids present in proteins.

Name

Symbol

Structural Formula pKi pK2

With Aliphatic Side Chains

Glycine

Alanine

Valine

Leucine

Isoleucine

CH, cH2

CH CH COO-

NH, a-COOH

R Group

With Side Chains Containing Hydroxylic (OH) Groups

Serine

Threonine

Tyrosine

See below.

about 13

about 13

With Side Chains Containing Sulfur Atoms

Cysteine

Methionine

CH2 CH COO-

CH2 CH2 CH COO-I I

10.8

With Side Chains Containing Acidic Groups or Their Amides

Asparagine

Glutamic acid

Glutamine

H2N C CH2 CH2 CH COO

(continued)

Table 3-1. L-a-Amino acids present in proteins. (continued)

Name

Symbol

Structural Formula p*1

With Side Chains Containing Basic Groups

Arginine

Lysine

Histidine

2 CH2 CH2 CH2

HN N

2 CH2 CH2 CH2

CH

COO-

nh3+

CH

COO-

nh3+

CH

COO-

nh3+

R Group

12.5

Containing Aromatic Rings

Histidine j His [H]

Phenylalanine Tyrosine

Tryptophan

See above.

— CH —COO-

2.2

9.2

nh3+

2.2

9.1

— CH —COO-

nh3+

2.4

9.4

— CH —COO-

nh3+

Proline

1 1

k +

N

^COO

H2

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