Download l8.l The omino ocids

l8.t The Amino Acids
555
l8.l Theomino ocids
AIM: To clossifythe 20 commonomino acids occordingto their
side-choin structures.
Focus
There are 20 common natural
amino acids.
Anarinoacid,is any carboxylicacid (RCO2H)that contains an amino group
(-NH, in the same molecule.Tomost chemists and biochemists, however,
the term is usually reservedfor the 20 amino acids that are found and used
in living organisms.All 2Oamino acids common in nature have a skeleton
consisting of a carboxylic acid group and an amino group covalently
bonded to a central atom. The central atom of amino acids is called the
alpha carbon. The remaining two groups on the alpha carbon are hydrogen
and an R group-the amino acid side chain.
Alpha
carbon-----._
Side chain
\ +*\l
HrN-C-COOH
1;1p;nal
./H\
\Alphacarboxylic
acidgroup
The chemical nature of the side chains accounts for differences in amino
acid properties. The 20 common naturally occurring alpha amino acids are
divided into seven categories according to their side-chain structures.
Glycine
glycos (Greek): sweet
(The amino acid glycine has a
sweet taste.)
Aliphatic side chains This group consists of amino acids having hydrocarbon (aliphatic) side chains. Glycine, the simplest amino acid, has a
hydrogen rather than an aliphatic side chain, but it is still placed in this category.Alanine, with a methyl side chain, is the smallest true member of this
group. other aliphatic R groups are those of the amino acids valine (isopropyl), leucine (isobutyl), and isoleucine (sec-butyl)' Leucine and
isoleucine have the same molecular formulas and are stluctural isomers.
Proline is the only amino acid with its alpha amino group incorporated into
a ring. The amino group of proline is a secondary amine.
H.C\
H
I
H2N-C-CO2H
lll
HHH
Glycine
H:C.,
CH,
I
H2N-C-CO2H
/CH.
CH
I
HZN-C-CO2H
Valine
Alanine
CH.
,CH:
CH
I
CHr
I
H,N-C-CO,H
"tl-
I
CH,
I
H-C-CH3
I
H,N-C-COUH
l
| ,co,u
H,C
C.
\_-/ \H
t\
I
H
HH
Leucine
H,C-CH,
-
Isoleucine
Proline
5!t4
CHAPTERIS Arnino Acids, Peptides;and proteins
lrydrorylic side chains The amino acids in this group are serine and
threonine. Both have aliphatic side chains containing hydroxy funciional
groups.
OH
OH
I
I
CH,
H-C-CH3
I
I{rN-C-COrH
H2N-C-CO2H
rt
H
Serine
Eight of the amino acids,
isoleucine, leucine, lysine, methionine, phenylalanine, threonine,
tryptophan, andvaline, are essential amino acids. These amino
acids cannot be synthesized by the
bodybut are essential to its proper
functioning. They must be supplied in our diet.
Threonine
Aromatic side chains There are three amino acids that have aromatic
rings in their side chains. Phenylalanine is an amino acid in which one of
$e methyl hydrogens of alanine is replaced by a phenyl group. In tyrosine"
the phenyl ring of phenylalanine is substitute dintt'e. paraposition with a
phenolic hydroxyl group. The aromatic ring system of tryptophan is derived
from indole (seeSec.l5.l).
OH
I
<-\
ttl
H
Y
CH"
Cy-c-cH,-l-co,n
NH,
CH,
I-
t-
I{2N-C-CO2H
I
H
V*-tt
H2N-C-CO2H
I
H
Phenylalanine
H
T\nosine
Tryptoph-an
Acidic side chains Aspartic and glutamic acids have side chains that are
terminated by carbo;rylic acid groups. At the usual biorogical pH, slightly
abovepH 7, these carbo4ylic acid groups are ionized. For this reison, aspartic acid and glutamic acid are often referred to'as-their carboxylate i-ons,
aspartate and glutamate.
i
?o,"
CH,
l-r_,
'!
CH,
H2N-C-CO2H
t-
HrN-f-COrH
HH
Aspartic acid
Glutamic acid
ia
Amide side chains Asparagine and glutamine are the amides of aspartic
and glutamic acids, respectively.Their side chains are €lectricallv neutral
at pH 7.0.
1
I
l8.l The Amino Acids
O
O.
\a,
I
,NH,
CH,
I-
H2N-C-COzH
I
H
Asparagine
\a,
I
555
,NH,
CH,
I
f",
HZN-C-CO2H
I
H
Glutamine
Basic side chains This group consists of ttrree amino acids containing
weakly basic nitrogens. The nitrogens on the side chains of lysine and arginine are such sufficiently strong bases that they remove a proton from
water at neutral pH. At pH 7.0, therefore, the side chains of arginine and
Iysine are positively charged. The nitrogens of the side chains of histidine
are more weakly basic than those of lysine and arginine. In solution at pH
7.0, the side chains of only about half of the histidine molecules are positively charged at any moment.
NH,
I
NH,
I
C:NH
I
I
CH,
NH
HC-NH
CH,
CH,
NCH
CH,
CH,
t-
I
I
I
CH,
I
H,N-C-CO,H
I
H
Lysine
I
I
CH,
I
H,N-C-CO,H
I
Il
\/
C
I
CH,
I
H,N-C-CO,H
I
H
H
Arginine
Histidine
Sulfur-containing side chains Methionine and cysteine are two common
sulfur-containing amino acids. Cysteine is often found linked to another
cysteine 4rough formation of a disulfide bond (-S-S-)
to form the
amino acid cystine.Cystine is not considered one of the 20 common amino
acids, since it is formed from two molecules of cysteine.
H
I
CH.
I
b
I
H2N-C-CO2H
I
CHr
I
s
I
I
SH
CH,
S
CH,
CH,
CH,
I
I
H2N-C-CO2H
I
H
Cysteine
I
I
HzN-C-CO2H
I
H
Methionine
I
H2N-C-CO2H
I
H
Cystine
556
CHAPTER
18 Amino Acids,Peptides,and Proteins
PRACTICEEXERCISE
I8.I
Categorizethe following amino acids according to their.side-chain
groups:
(a) leucine
(b) lysine
(c) serine
(d) tyrosine
18.2Stereoisomers
of amino ocids
AIMS: To stote the handedness
of omino ocidsfound in noture.
To drow Fischerproiection formulosfor olpho omino
ocids.
Focus
All the amino acids but glycine
have handedness.
A glance at the structures of common amino acids will show that, except for
glycine, there are four different groups attached to the alpha carbons. This
is a criterion for handednessin organic molecules (seeSec.16.1).Amino
acids fit this rule and may exist as D or r forms that are nonsuperimposable
mirror images of each other. The handedness of amino acids is related to
that of o- and r-glyceraldehyde as follows:
CHO
CHO
I
H-toH
c02H
HOtH
lr
RR
o-Glyceraldehyde
r-Glyceraldehyde
CO,H
CO,H
l-tHtNH,
HrNtH
tl
RR
Although the amino acids found in
nature are mostly the r forms, the
n forms of a few amino acids are
found in the cell walls of certain
bacteria.
'lxRmplE'lg.l'''j
r ' r r rI r l
R
o-Amino acid
r-Amino acid
Configuration of r-amino acids
Nature shows a marked preference for the r forms of amino acids.With
few exceptions, the amino acids found in lMng systems belong to the r
family.
o r a w i n g ' r i s c h a i ' t r o i afco r m ' u t a s,, :
'' i-:.':::"',,
',',i'
'
.'
Draw the Fischer projection formula of r-alanine
sotuTtoN
Place the carboxylic acid group at the top and the methyl group (the R
group) at the bottom. For the l isomer, the amino group is on the left.
COOH
g,N-_]-H
-t
CH.