Chem 30B Dr. R. Rinehart
Chapter 19: Amino Acids and Proteins
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Amino Acids: grouped in families based on side chain polarity
showing charges at pH 7

Name

3-letter abbreviation

1-letter abbreviation

structure drawn
showing ionization at pH 7

comments

Nonpolar or Hydrophobic Amino Acids

glycine

gly

G

“aliphatic”
the simplest amino acid and the only one not of the L- configuration; R is nonpolar, but not very

alanine

ala

A

“aliphatic”
R is small, nonpolar

valine

val

V

“aliphatic”
”branched-chain”
R is bulky, very nonpolar
diet-essential

isoleucine

ile

I

“aliphatic”
”branched-chain”
R is bulky, very nonpolar
diet-essential

leucine

leu

L

“aliphatic”
”branched-chain”
R is bulky, very nonpolar
diet-essential

proline

pro

P

an “imino acid”;
 introduces kinks in protein chains; the N is unable to H-bond when in a protein chain

methionine

met

M

“sulfur-containing”
R is somewhat bulky, very nonpolar
diet-essential

phenylalanine

phe

F

“aromatic”
R is bulky but planar, very nonpolar;
diet-essential

tryptophan

trp

W

“aromatic”
”heterocyclic”
R is extremely bulky, nonpolar; the NH of the indole ring can H-bond but this by itself is not enough to make the side chain polar, although some texts may list it as such;
diet-essential

Polar, neutral amino acids

serine

ser

S

“hydroxy”
R is polar but neutral
found at active site of many enzymes

threonine

thr

T

“hydroxy”
R is polar but neutral; diet essential

cysteine

cys

C

sulfur-containing; its thiol group is frequently found at enzyme active sites; oxidation of the thiol to a disulfide is important in 3^o and 4^o structure of proteins

asparagine

asn

N

“half-amide”
R is polar but neutral; the amide N can serve as attachment point for sugars

glutamine

gln

Q

“half-amide”
R is polar but neutral

tyrosine

tyr

Y

“aromatic”
”phenolic”
“hydroxy”
R is bulky, nonpolar except for the –OH group, which can ionize at pH>10; some texts classify TYR as nonpolar.

Negatively-charged R at pH 7 [“Acidic”]

aspartic acid or
aspartate

asp

D

R is ⊝ at pH 7; often found at active site of enzymes

glutamic acid or
 glutamate

glu

E

R is ⊝ at pH 7
familiar as the food additive MSG;
also a neurotransmitter

Positively-charged R at pH 7 [“basic”]

arginine

arg

R

R is ⊕ at pH 7
contains the guanidino group, which is still 50% + charged even at pH 12.5
semi-essential

lysine

lys

K

R is ⊕ at pH 7
at pH 10, only about half the R are still +
diet-essential

histidine

his

H

R is ⊕ at pH 7 [~40%]
contains the imidazole group, the only R-group with a pK_a close to 7;
found very often at enzyme active sites;
semiessential

© Ronald W. Rinehart, 2002   Structures drawn with MDL IsisDraw^®

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