Download 26.4 The ureo cyde

790
CHAPTER
26 Metabolismof NitrogenCompounds
transaminase(GOT),which catalyzesthe formation of oxaloacetate,one of
the intermediates of the citric acid cycle, from aspartic acid.
o
o
HO-C-CH2CH-C-OH
cor
+ a-Ketoglutaricacid --
o
ll
oo
ll ll
HO-C-CH2C-C-OH
I
+ Glutamic acid
NHz
Asparticacid
Oxaloaceticacid
Damagedheart cells die and split open, but some of the GOT moleculesthat
spill into the blood are still active. The levels of serum GOT (SGOT)activity are
a measureof the extent of damageto heart muscle causedby a heart attack.
All knornmtransaminases require pyridoxal phosphate, a derivative of
pyridoxol (vitamin 86), as the cofactor.
HO
\//
c
o
o-P-o-cH,
I
o
Pyridoxal phosphate
OH
CH.
Pyridoxamine phosphate
The aldehyde carbon of pyridoxal phosphate is the acceptor of the amino
group of amino acids in the first stageof reactions catalyzedby transaminases.
An u-ketoacid and pyridoxamine phosphate are the products of the reaction.
Amino acid + Pyridoxalphosphate--* ct-Ketoacid
+ Pyridoxaminephosphate
The second stage of transamination consists of the transfer of the amino
group of the pyridoxamine phosphate to a-ketoglutarate with regeneration
of the pyridoxal phosphate.
a-Ketoglutaricacid + Pyridoxaminephosphate------Glutamicacid + Py'ridoxal
phosphate
PRACTICE
EXERCISE
26.2
Write an equation for the transamination
tarate as the acceptor.
of tyrosine with o-ketoglu-
PRACTICE
EXERCISE
26.5
How does an amino acid lose its amino group in transamination?
26.4 Theureo cyde
Focus
Ammonia produced from the
oxidation of glutamic acid is
excreted as urea.
AIMS: To exploin the net resultsof the ureo cycleon o
moleculorand o physiologic scole.To list two causesof o
negotivenitrogen bolonce.
Glutamic acid serves as the depot for receiving amino groups removed
from amino acids by transamination reactions. Since there is a limited
quantity of a-ketoglutarate in cells, it must be regenerated so that
transamination reactions and the citric acid cycle can continue. The o-
26.4 The UreaCycle
791
ketoglutarate is regenerated from glutamic acid by an oxidation reaction
catalyzed.by glutamate dehydrogenase. This en4rme uses NAD+ as the
coenzyrne. The remoual of the amino group by oxidation is calledoxidative
deamination.
Ho
o
o
+ H2o:#ry
Ho-A-cH,cH,J-A-oH
I
NHt
Glutamic
NAD*
+ Ho-A-cu,cH,A-A-ou
NHn*
NADH
Ammonium
ion
acid
oo
a-Ketoglutarate
With the regeneration of a-ketoglutarate, the goal of the cell has been
met-the cellular concentration of a-ketoglutarate is restored. Oxidative
deamination also has produced ammonia, however. And ammonia, even
in low concentrations, is especially toxic to brain cells and can result in
coma and death. A Closer Look Hyperammonemia examines some reasons for this toxicity. It is important that ammonia be removed from the
body-This is accomplished by the reactions of the urea cycle,shovvnin Figure 26.3.
Theureacycle ls a metabolic pathway by whirh land-dwelling animals
prepare wasteammonia and ammonium ionsfor excretion.Theexcretion of
nitrogen as urea helps to conserve bodywater and prevent dehydration. If
ammonia were excreted directly in the urine, the ammonia would have to
be kept very dilute. The resulting loss of large volumes of body water would
make dehydration an ever-presentdanger.IJrea, on the other hand, is very
soluble in water and is nontoxic except at high concentrations. Urea formation permits land-dwellers to eliminate large quantities of nitrogen while
losing relatively little water as urine. Other animals, primarily the fishes,
CO2+NH4++HrO@
T
t
ffi1n
t \a-l
-
\\
\
\
\
ffi
re{
Figure26.5
Theureacycle.
\
792
CHAPTER
26 Metabolismof NitrogenCompounds
excreteammonia as a waste product. Becausethey live in a water environment, fishesare not in dangerof dehydration.
PRACTICE
EXERCISE
26.4
\A/hattoxic substanceis produced when s-ketoglutarateis regenerated
from glutamic acid?How is this substanceremovedfrom the body?
Steps of the urea cycle
Let'sexaminethe chemicalreactionsof the urea cyclein more detail.Before
ammonia can enter the urea cycle,it is convertedto a more energeticform:
the compound carbamoyl phosphate.The formation of carbamoyl phos-
:
Hyperammonemia
Hyperammonemia is an increasein ammonia in
the blood to higher concentrations than the normal30 to 60 mM. This condition is usually caused
by the inability of patients to form urea fast
enough to keep ammonia in the blood at normal
levels. Severalconditions can lead to hlperammonemia. Some newborns exhibit a delay in the
development of urea cycle enzymes.The hyperammonemia in these infants clears up when the
enzymes begin to function. In adults, causesof
hlperammonemia include hereditary defects in
the enzymesof the urea cycle and blockageof the
blood flow through the liver, as might occur in
cirrhosis(seefigure).
Coma {an unconscious state) may occur in
casesof hyperammonemiathat are causedby
liver (hepatic) disease.The reason for hepatic
coma is uncertain, but it may be due to the depletion of AIP in the brain. To help us understand
how ammonia may contribute to ATP depletion,
let's review some aspectsof metabolism.You may
recall that the citric acid cycle is an important
sourceof reducingpower in the form of NADH
and FADH2 for oxidative phosphorylation and
that oxidative phosphorylation is responsiblefor
the production of most of the AIP needed to provide the energy to keep cells alive and healthy. In
hyperammonemia, ammonia may cross the
blood-brain barrier and enter brain cells. In the
brain cells, the excessammonia could drive the
reaction between glutamate and u-ketoglutarate
in the direction of the formation of glutamate.
The shift to glutamate would reduce the amount
of a-ketoglutarate available for the citric acid
cycle. As a result, the concentrations of NADH
and FADH2 also would decrease,less ATP would
be formed in oxidative phosphorylation, and
hepatic coma could resulifrom the depletion of
AIP. Some cases of hyperammonemia can be
treated by the addition of c-ketoacids to the diet
of the hyperammonemic patient. This diverts
some of the excessammonia into amino acids
through transamination of the s-ketoacids. The
amino acids are used by liody cells to make proteins instead of having the ammonia contribute
to the patient'shlperammonemia.
Cirrhosisof the liver impairsthe urea cycleand can
causehyperammonemia.
'
'
:
:
:
:
26.4 The Urea Cycle
79,
phate requires, in addition to ammonia, one bicarbonate ion and the
expenditure of two molecules ofAIP
oo
ilil
NH3 + HCO3 + 2ATP -------HzN-C-O-|-O-
+ 2ADP + Pi
o-
;tr"'.ffi:Jj
With the formation of carbamoyl phosphate, ammonia has been energetically primed for entry into the urea cycle. The cycle can be summarized in
four steps:
Step 7: Carbamoyl phosphate reacts with ornithine (seeSec.23.7). Ornithine, an amino acid not found in proteins, is already present in the cell.
The product of this reaction is citrulline.
H,N
C:O
NH,
HN
I
tt
?"'
cH,
I
I
CH,
NH,
CH,
*^
(D-O
tt-
/c:o-
CH,
l-
H2N-CH
H2N-CH
I
I
C
o/
+ P,
CH,
C
\ou
o/
Ornithine
Carbamoyl
phosphate
\oa
Citrulline
Step 2: Citrulline reacts with aspartic acid to give a complex molecule
called arginosuccinic acid. This reaction requires the expenditure of two
more high-energy phosphate bonds of AIP You need not memorize such
complex molecular structures, but you should be aware of the path that
nitrogen follows through the cycle.
OH
\./
q
H
H,N*
-\
|
|
c_N_c_H
,/l
HN
CH"
ll
H,N
C:O
I
HN
I
CHr
tCH,
tCHr
I
H2N-CH
I
C
ooH
\./
C
--z----
+ Hr\-CH
!
C H-,
|t - . ,
/\
o'
C
oH
9H'
|
c}j,
I
AMP + PPI
H,Ol
.)p.
c
//\
o'
oH
CHr
I
H2N-C-H
I
C
,/\
ooH
CitruIline
ATP
o/
Aspartic
acid
\ou
Arginosuccinic acid
794
CHAPTER
26 Metabolismof Nitrogen Compounds
Keep in mind that both the ngw nitrogens that have been added to
ornithine can originate as amino groups of amino acids: The first nitrogen
can come from ammonia produced by the deamination of an amino acid
and used to form carbamoyl phosphate; the second comes from succinic
acid.
Step 3: An enzyme now cleaves arginosuccinic acid into arginine and
fumaric acid. Fumaric acid is one of the intermediates of the citric acid
cvcle.
H,N*
-\
\.,
OH
tI
|
I
c_N_c_H
tl
"T
CH,
fn
t-
?''
C
o/ to,
H,N*
-\./
C
€9H'+
o
II
HN
I
C
C
t-
HO-C
CH,
t-,
tl
o
I
H2N-C-H
.
/'c-oH
ll
t'
CH,
CH,
H2N-C-H
H
I
C
C
\ott
o/
NH,
\ou
o/
Arginosuccinic acid
Arginine
Fumaric acid
We have followed the qmthesis of the nonessential amino acid arginine.
Step 4: The production of urea occurs when arginine is hydrolyzed to
ornithine and urea.
H,N-\./
NH,
C
H
I
HN
I
l*
I
CH,
I-
H-N-H
CH,
tCH,
tCHr
t-
H2N-C-H
C
o/
\o
Arginine
+ H,O _-
CHr
t-
o
ll
+ NH2-C-NH2
CHr
I
H2N-C-H
I
C
{
OH
Ornithine
(as its ammoniumion)
Urea
The ornithine reenters the urea cycle by reacting with another molecule of
carbamoyl phosphate, After entering the bloodstream, urea is flltered by
i
the kidneys and disposed of in the urine.
26.5 Catabolismof Amino Acids
Nitrogen
795
balance
Normal adults generally maintain a nitrogen balance-the quantity of
nitrogen excreteddaily equals the intake. About B0% of the nitrogen
excretedin the urine is in the form of urea, and almost all the body'swaste
nitrogen of metabolism is excretedin the urine; the nitrogen compounds
in the feces come mainly from indigestible materials. Children have a
positive nitrogen balance-fhe excretion of less nitrogen than is consumed.The nitrogen balanceis positive becausechildren are growing and
their cells are making new proteins and other nitrogen compounds. Several conditions result in a negative nitrogen balance-the excretion of
more nitrogen than is consumed.During starvation and certain diseases,
the carbon skeleton of amino acids derived from the breakdown of muscle proteins must be catabolized as an energy source. Since no new protein is available to eat, starving people excretemore nitrogen than they
consume.The lack of even one essentialamino acid in the diet resultsin a
negative nitrogen balance. With an essential amino acid missing, the
other amino acids cannot be used to make complete proteins.Theseother
amino acids are deaminated, and the nitrogen is excretedas urea.
26,5 Catobolismof smino ocids
AIMS: To distinguish betweenglucogenicond ketogenicomino
ocids.To discusshow omino ocids con be usedfor energy
production,gluconeogenesis,
ond the synthesisof fats.
Focus
The carbon skeletons of amino
acids are converted to intermediates of glucose metabolism
and fatty acid metabolism.
Once a,-ketoacidshave been formed from amino acid by transamination
reactions, their carbon skeletons are subjected to further chemical
changes.One set of amino acids is converted to pyruvate, oxaloacetate,or
a-ketoglutarate (Fig. 26.4).Amino acids that are conuertedto theseintermediates are called glucogenic, since thesecompounds are also important to
glucosemetabolism. Pyruvate,formed at the end of glycolysis,and oxaloacetate are intermediates of the citric acid cycle.The remainder of the amino
acids are converted to acetyl CoA, which is also a product of fatty acid
metabolism. The amino acids that are conuerted to acetyl CoA are called
ketogenic.
The conversion of all the amino acids to intermediates of glucose or
fatty acid metabolism demonstrates the highly organized character of
metabolism and the economy of nature. By using a single, central pathway
for the metabolism of sugars, fats, and amino acids, the cell greatly
decreasesthe number of en4,rnes and chemical steps that otherwise might
be required to accomplish the same task.
Cells have priorities for the utilization of amino acids present in the
amino acid pool. In a normal individual, well nourished with carbohydrates,fats, and proteins, the synthesisof nonessentialamino acids,proteins, and other nitrogen-containing compounds is at the top of the prior-