18
chapter 2
Amino Acids
COOH ^^-a-C arbon
H,N
I
R
FIGURE 2-1
Basic structure of an a-amino acid.
CHO
CHO
1. Nonpolar (hydrophobic),
2. Polar, negatively charged (acidic),
3. Polar, positively charged (basic), and
4. Polar, neutral (un-ionized).
CHO
H— Ç— OH
CH2OH
D-Glyceraldehyde
CHO
I
OH— Ç— H
CH2OH
L-Glyceraldehyde
Within each class, R-groups differ in size, shape, and
other properties. Figure 2-3 shows the structure of each
amino acid according to this classification with the R-
group outlined. Ionizable structures are drawn as they
would exist at pH 7.0. The three-letter and one-letter ab-
breviations for each amino acid are given in Table 2-1. A
-yl
ending on amino acid residue indicates that the carboxyl
group of an amino acid is linked to another functional
group (e.g., in a peptide bond).
The eight
essential amino acids
(Table 2-1) are those
which humans cannot synthesize and which must be
supplied in the diet. The remaining amino acids are
synthesized in the body by various biochemical pathways
(Chapter 17).
ÇHO
H"—C—*OH
CH2OH
ÇHO
HO«^Ç— H
CH2OH
Fischer perspective formulas
CHO
H-
CHO
-OH
CH2OH
HO-
-H
CH2OH
Fischer projection formulas
Nonpolar Amino Acids
Glycine
Glycine is the smallest amino acid and has an H atom
as its R-group. It is the only a-amino acid that is not
optically active. The small R-group provides a mini-
mum of steric hindrance to rotation about bonds; there-
fore, glycine fits into crowded regions of many peptide
chains. Collagen, a rotationally restricted fibrous protein,
has glycyl residues in about every third position in its
polypeptide chains. Glycine is used for the biosynthesis
of many nonprotein compounds, such as porphyrins and
purines.
Glycine and taurine are conjugated with bile acids, prod-
ucts derived from cholesterol, before they are excreted into
the biliary system. Conjugated bile acids are amphipathic
and are important in lipid absorption (Chapter 12). Glycine
also is a neurotransmitter; it is inhibitory in the spinal cord
and excitatory in the cerebral cortex and other regions of
the forebrain.
Nonketotic hyperglycinemia
(NKH) is an in-
born error of glycine degradation in which a large amount
of glycine accumulates throughout the body. NKH causes
ÇOOH
ÇOOH
H— Ç—
n h
2
h
2
n
*~
ç
~ *
h
CH
3
ch
3
D-Alanine
L-Alanine
FIGURE 2-2
Different representations of the configurational stereoisomers of
glyceraldehyde and of alanine.
severe consequences in the central nervous system (CNS)
and leads to death (Chapter 17).
COCT
I
+ H3N— C— H
I
H
Glycine
