Page 92 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

7.5 What Mutations Tell Us About Gene Function 251

(a) Generic amino acid structure (b) Amino acids with nonpolar R groups
R groups Backbone R groups Backbone
Amino (–NH ) group
2
Glycine (Gly) (G) H H Proline (Pro) (P)
Glycine (Gly) (G)
H R H 2
CHR group H 2 C C
H H C C COOH C COOH
COOH
H N C C OH H 2 C N
NH 22
H O Carboxyl (–COOH) group NH H H
Alanine (Ala) (A) H H Phenylalanine (Phe) (F) H
Alanine (Ala) (A)
COOH
CH C C COOH CH 2 C COOH
CH 33
NH NH 2
NH 22
V
Valine (Val) (V)aline (Val) (V) H H Tryptophan (Trp) (W) H
(c) Rare amino acids CH
CH 33
C C COOH
CH
COOH
CH C C COOH CH 2
Selenocysteine (Sec) (U)
H CH
CH
CH 33
NH N NH 2
NH 22
SeH CH 2 C COOH H
Leucine (Leu) (L) H H Methionine (Met) (M) H
NH 2 CH
CH 33
COOH
CH
Pyrrolysine (Pyl) (O) H CH CH C C COOH CH 3 S CH 2 CH 2 C COOH
CH 22
HC N CH
CH 33
NH NH 2
NH 22
H 2 C CH C NH CH 2 CH 2 CH 2 CH 2 C COOH
CH Isoleucine (Ile) (I)
O H
CH 3 NH 2
CH 3 CH 2 CH C COOH
CH 3 NH 2
Amino acids with uncharged polar R groups
Serine (Ser) (S)
Serine (Ser) (S) Tyrosine (Tyr) (Y)
(d) Peptide bond formation H H H
AA1 AA2 AA3 HO CH C C COOH HO CH 2 C COOH
HO
COOH
CH 22
H R + H R + H R
NH
H N C C OH H N C C OH H N C C OH NH 22 NH 2
Threonine (Thr) (T)
Threonine (Thr) (T)
H O H O H O H H Asparagine (Asn) (N) H
NH 2
H 2 O H 2 O CH CH C C COOH
COOH
CH 22
CH 33
C CH 2 C COOH
OH
OH NH 2 O
H R H R H R NH 2
N terminus H N C C N C C N C C OH C terminus Cysteine (Cys) (C) H Glutamine (Gln) (Q) H
H O H O H O NH 2
HS CH 2 C COOH C CH 2 CH 2 C COOH
O
Peptide bonds NH 2 NH 2
Amino acids with basic R groups
Lysine (Lys) (K) H Histidine (His) (H) H
H 2 N CH 2 CH 2 CH 2 CH 2 C COOH HC C CH 2 C COOH
Figure 7.28 Proteins are chains of amino acids N C NH
linked by peptide bonds. (a) Amino acids contain a NH 2 H NH 2
basic amino group (–NH 2 ), an acidic carboxylic acid group Arginine (Arg) (R) H
(–COOH), and one of 22 different side chains (R). (b) The
20 amino acids commonly found in proteins, arranged H 2 N C NH CH 2 CH 2 CH 2 C COOH
according to the properties of their R groups. NH
(c) Selonocysteine and pyrrolysine are amino acids found NH 2
only in a few proteins or in specific organisms. (d) One
molecule of water is lost when a covalent amide linkage (a Amino acids with acidic R groups
peptide bond) is formed between the –COOH of one amino Aspartic acid (Asp) (D) H Glutamic acid (Glu) (E) H
acid and the –NH 2 of the next amino acid. Polypeptides HO HO
such as the tripeptide shown here have polarity; they C CH 2 C COOH C CH 2 CH 2 C COOH
extend from an N terminus (with a free amino group) to O O
a C terminus (with a free carboxylic acid group). NH 2 NH 2