342
chapter 17
Protein and Amino Acid Metabolism
possesses two binding sites for ATP. One ATP is utilized
in activation of bicarbonate by forming an enzyme-bound
carboxyphosphate that reacts with ammonium ion to form
an enzyme-bound carbamate, with elimination of inor-
ganic phosphate. Carbamoyl phosphate is generated when
the second ATP reacts with the enzyme-bound carbamate,
with release of ADP and free enzyme.
In humans, there are two immunologically distinct car-
bamoyl phosphate synthases, one mitochondrial (CPSI)
and the other cytosolic (CPSII). CPSI is involved in ure-
agenesis, uses NH
3
exclusively as the nitrogen donor,
and requires binding of NAG for activity. CPSII uses
glutamine as substrate, is not dependent on NAG for
activity, and is required for synthesis of pyrimidine
is transported out of the mitochondria by the citrulline-
omithine antiporter.
Formation o f Argininosuccinate
The condensation of citrulline and aspartate to argini-
nosuccinate is catalyzed by argininosuccinate synthase in
the cytosol and occurs in two steps. In the initial step,
the ureido group is activated by ATP to form the enzyme-
bound intermediate adenylylcitrulline. In the second step,
nucleophilic attack of the amino group of aspartate
displaces AMP and yields argininosuccinate. The overall
reaction is shown below:
n h
2
NH
C O O -
1
c = o
I
j
0 = N H + —-C H
1
NH
|
C O O "
1
NH
1
C H
2
j
C H
2
C H N H 3+
j
C H
2
c o o -
i , .
.
4T
1
+1
A T P 4"
♦ = *
{
+
C H
2
1
1
C H
2
C H
2
J
C H
2
1
1
C O O "
C H
2
1
C H N H 3+
1
1
C H N H 3+
C O O "
1
C O O "
L-Citrulline
L-Aspartate
L-Argininosuccinate
(Chapter 27). Normally, the mitochondrial membrane is
not permeable to carbamoyl phosphate, but when the
concentration increases, carbamoyl phosphate spills into
the cytosol and promotes synthesis of orotic acid and
uridine 5'-phosphate.
The reaction is driven forward by hydrolysis of pyrophos-
phate to inorganic phosphate. Argininosuccinate forma-
tion is considered as the rate-limiting step for urea synthe-
sis. This reaction incorporates the second nitrogen atom
of the urea molecule donated by aspartate.
Formation o f Citrulline
Formation o f Arginine and Fumarate
Ornithine carbamoyltransferase
(ornithine
transcar-
Argininosuccinate lyase in cytosol catalyzes cleavage
bamoylase) catalyzes the condensation between car-
of argininosuccinate to arginine and fumarate:
bamoyl phosphate and ornithine to yield citrulline in
mitochondria:
0
I
n h
2
C O "
n h
2
N H
2
1
T ransferred
1
1
1
1
carb am o yl
C = N H + — CH
c
— n h 2+
N H 3+
O
II
O
g ro u p
1
1
1
1
1
NH
C H
2
NH
C O O "
C H
2
1
N H
1
!
C H
2
1
C O O "
J
= t
1
C H
2
1
+
C H
0
C H
2
C H
2
1
1
I
1
1
1
C H
2
C H
2
H C
H
2
N — C — 0 — P 0 32"
+
C H
2
<
1
1
C H
2
1
+ P2~
+ H +
i
C H
2
1
C H
2
1
C O O "
C arb am o yl
C H N H 3+
C H
2
1
1
C H N H 3+
p h o sp h ate
1
1
v n i i n i
j
1
C O O '
C H N H
3
+
j
3
C O O "
C O O "
C O O "
(.-Argininosuccinic acid
L-Arginine
F um arate
L-O rnithine
L-Citrulline
This is the pathway for synthesis of arginine, a nonessen-
Although the equilibrium constant strongly favors cit-
tial amino acid; however, in the event of physiological
rulline formation, the reaction is reversible. Citrulline
deficiency, as in premature infants, or a defect in any of
