Standard Amino Acids Twenty standard a-amino acids. Differ in side-chain characteristics: >-H or alkyl >contains an -OH >contains sulfur >contains a nonbasic nitrogen >has -COOH >has a basic nitrogen > Chapter 24 6
Chapter 24 6 Standard Amino Acids • Twenty standard -amino acids. • Differ in side-chain characteristics: ➢ -H or alkyl ➢contains an -OH ➢contains sulfur ➢contains a nonbasic nitrogen ➢has -COOH ➢has a basic nitrogen =>
Essential Amino Acids ·Arginine(Arg) ·Tryptophan(Trp) ·Threonine(Thr) ·Methionine(Met) ·Lysine(Lys) 。Histidine(His) ·Valine (Val) ·Leucine(Leu) ·Phenylalanine(Phe)·Isoleucine(le) > Chapter 24 7
Chapter 24 7 Essential Amino Acids • Arginine (Arg) • Threonine (Thr) • Lysine (Lys) • Valine (Val) • Phenylalanine (Phe) • Tryptophan (Trp) • Methionine (Met) • Histidine (His) • Leucine (Leu) • Isoleucine (Ile) =>
Complete Proteins Provide all the essential amino acids. Examples:those in meat,fish,milk,eggs. Plant proteins are generally incomplete. Vegetarians should eat many different kinds of plants,or supplement diet with milk or eggs. => Chapter 24
Chapter 24 8 Complete Proteins • Provide all the essential amino acids. • Examples: those in meat, fish, milk, eggs. • Plant proteins are generally incomplete. • Vegetarians should eat many different kinds of plants, or supplement diet with milk or eggs. =>
Rare Amino Acids 4-Hydroxyproline,5-hydroxylysine found in collagen. D-Glutamic acid in cell walls of bacteria. D-Serine in earthworms. y-Aminobutyric acid,a neurotransmitter. B-Alanine,constituent of the vitamin pantothenic acid. > Chapter 24 0
Chapter 24 9 Rare Amino Acids • 4-Hydroxyproline, 5-hydroxylysine found in collagen. • D-Glutamic acid in cell walls of bacteria. • D-Serine in earthworms. • -Aminobutyric acid, a neurotransmitter. • -Alanine, constituent of the vitamin pantothenic acid. =>
Zwitterion Amino acid exists as a dipolar ion. .-COOH loses H+,-NH2 gains H+. H,N一CH一C-OH HN-CH-C- R R uncharged structure dipolar ion,or zwitterion (minor component) (major component) Copyright2005 Pearson Prentice Hall,Inc. Actual structure depends on pH. Chapter 24 =>10
Chapter 24 10 Zwitterion • Amino acid exists as a dipolar ion. • -COOH loses H+ , -NH2 gains H+ . • Actual structure depends on pH. =>