24
chapter
2
Amino Acids
protein fibers. Examples of cross-linked amino acid struc-
tures are desmosine, isodesmosine (Figure 2-6), dehy-
drolysinonorleucine, lysinonorleucine, merodesmosine,
and dehydromerodesmosine (Chapter 10). Lysyl R-groups
participate in a different type of cross-linking in the forma-
tion of fibrin, a process essential for the clotting of blood.
In this reaction, the E-NH
2
group of one fibrin polypep-
tide forms a covalent linkage with the glutamyl residue of
another fibrin polypeptide (Chapter 36).
COO“
I
+ H3N— aC— H
I
0CH2
I
yCH2
I
<5CH2
I
eCH2
I
NH3+
Lysine
by diphenhydramine and pyrilamine. Hi-receptor antago-
nists are used in the treatment of allergic disorders. Secre-
tion of HC1 by the stomach (Chapter 12) and an increase
in heart rate are mediated by H
2
receptors. Examples of
H
2
-receptor antagonists of histamine action are cimeti-
dine and ranitidine, agents used in the treatment of gastric
ulcers.
Arginine
The positively charged guanidinium group attached to
the 5-carbon of arginine is stabilized by resonance between
the two NH
2
groups and has a pK' value of 12.48. Arginine
is utilized in the synthesis of creatine and it participates in
the urea cycle (Chapter 17).
The nitrogen of the guanidino group of arginine is con-
verted to nitric oxide (NO) by nitric oxide synthase. NO is
unstable, highly reactive, and has a life span of only a few
seconds. However, NO affects many biological activities,
including vasodilation, inflammation, and neurotransmis-
sion (Chapter 17).
Histidine
The imidazole group attached to the /3-carbon of his-
tidine has a pK' value of 6.0. The pK' value of his-
tidyl residues in protein varies depending on the nature
of the neighboring residues. The imidazolium-imidazole
buffering pair has a major role in acid-base regulation
(e.g., hemoglobin). The imidazole group functions as a
nucleophile, or a general base, in the active sites of many
enzymes and may bind metal ions. Histidine is nonessen-
tial in adults but is essential in the diet of infants and indi-
viduals with
uremia
(a kidney disorder). Decarboxylation
of histidine to yield
histamine
occurs in mast cells present
in loose connective tissue and around blood vessels, ba-
sophils of blood, and
enterochromaffin-like (ECL) cells
present in the acid-producing glandular portion (oxyntic
cells) of the stomach.
The many specific reactions of histamine are deter-
mined by the type of receptor (Hi or H2) present in the
target cells. The contraction of smooth muscle (e.g., gut
and bronchi) is mediated by Hi receptors and antagonized
COO“
I
+ H3N— C— H
I
CH2
I
CH2
I
<5CH2
I
NH
_____ l_____
I
^
! Guanidinium
L . „ .
. nh2.
. J
9ГОиР
Arginine
Neutral Amino Acids
Serine
The primary alcohol group of serine can form esters
W ith pliO SpliO iiC ctCiu ( F ig liic
2
-
6
) a i’iu glyC O SiucS w ith
COO"
COO“
1
H
1
+H3N— C— H
1
—с—H
1
H3N— C— H
1
1
H— C— OH
CH2
CH2
1
H
1
c------
CH
1
c------
CH
Serine
1
1
N ^
JMH
I
1
IsL
NH
H
sugars. The phosphorylation and dephosphorylation pro-
H
cesses regulate the biochemical activity of many proteins.
Histidine
Histamine
Active centers of some enzymes contain seryl hydroxyl
