Catabolism of proteins and amino acids
Dec 16, 2015
Transaminase Roles
Transaminases equilibrate amino groups among available -keto acids.
This permits synthesis of non-essential amino acids, using amino groups from other amino acids & carbon skeletons synthesized in a cell. Thus a balance of different amino acids is maintained, as proteins of varied amino acid contents are synthesized.
Although the amino N of one amino acid can be used to synthesize another amino acid, N must be obtained in the diet as amino acids (proteins).
Example of a Transaminase reaction: Aspartate donates its amino group, becoming the -keto
acid oxaloacetate. -Ketoglutarate accepts the amino group, becoming the
amino acid glutamate.
a s p a r t a t e - k e t o g l u t a r a t e o x a l o a c e t a t e g l u t a m a t e
A m i n o t r a n s f e r a s e ( T r a n s a m i n a s e )
C O O
C H 2
C H 2
C
C O O
O
C O O
C H 2
HC
C O O
N H 3+
C O O
C H 2
C H 2
HC
C O O
N H 3+
C O O
C H 2
C
C O O
O + +
The prosthetic group of Transaminase is pyridoxal phosphate (PLP), a derivative of vitamin B6.
p y rid o x a l p h o sp h a te (P L P )
NH
CO
P
O O
O
O H
C H 3
CH O
H 2
In the resting state, the aldehyde group of pyridoxal phosphate is in a Schiff base linkage to the -amino group of an enzyme lysine residue.
NH
CO
P
OO
O
O
CH3
HC
H2
N
(CH2)4
Enz
H
+
RHC COO
NH2
Enzyme (Lys)-PLP Schiff base
Amino acid
The -amino group of a substrate amino acid displaces the enzyme lysine, to form a Schiff base linkage to PLP. The (+) charged N of PLP acts as an electron sink, to facilitate catalysis. Lysine extracts H+, promoting tautomerization, followed by reprotonation & hydrolysis.
NH
CO
P
OO
O
O
CH3
HC
H2
N
HC
H
+
R COO Enz Lys NH2
Amino acid-PLP Shiff base (aldimine)
What was an amino acid leaves as an -keto acid.
The amino group remains on what is now pyridoxamine phosphate (PMP). A different -keto acid reacts with PMP and the process reverses, to complete the reaction.
NH
CO
P
O O
O
OH
CH3
CH2
NH2
H2
R C COO
O
Enz Lys NH2
Pyridoxamine phosphate (PM P)
-keto acid
Deamination of Amino Acids
Transaminases also function to funnel amino groups from excess dietary amino acids to those amino acids (e.g., glutamate) that can be deaminated.
Carbon skeletons of deaminated amino acids can be catabolized for energy, or used to synthesize glucose or fatty acids for energy storage.
Only a few amino acids are deaminated directly.
Glutamate Dehydrogenase catalyzes the major reaction that accomplishes net removal of N from the amino acid pool. It is one of the few enzymes that can use NAD+ or NADP+ as e acceptor. Oxidation at the -carbon is followed by hydrolysis, releasing NH4
+.
O O CH 2C
H 2C C C O O
O
+ N H 4+
N A D (P )+
N AD(P)H
O O CH 2C
H 2C C C O O
N H 3+
Hglu tam ate
-ke toglu tara te
G lu tam ate D ehydrogenase
H 2O
Summarized above: the role of transaminases in funneling amino N to glutamate, for deamination via Glutamate Dehydrogenase, producing NH4
+.
Amino acid -ketoglutarate NADH + NH4+
-keto acid glutamate NAD+ + H2O
Transaminase Glutamate Dehydrogenase
Most terrestrial land animals convert excess nitrogen to urea, a compound less toxic than ammonia, prior to excreting it.
The Urea Cycle occurs mainly in liver.
The 2 nitrogen atoms of urea enter the Urea Cycle as NH3 (most via Glutamate Dehydrogenase) and as amino N of aspartate.
H 2 N C
O
N H 2
u r e a
Carbamoyl Phosphate Synthase is the committed step of the Urea Cycle, and is subject to regulation.
Carbamoyl Phosphate Synthase is allosterically activated by N-acetylglutamate. This derivative of glutamate is synthesized when cellular [glutamate] is high, signaling excess of free amino acids due to protein breakdown or dietary intake.
H 2N C O PO 32
O
HC O 3 + N H 3 + 2 A TP
+ 2 A D P + P i
C arbam oyl Phosphate Synthase
carbam oyl phosphate
Hyperammonemia Disease
Hereditary deficiency of any of the Urea Cycle enzymes leads to hyperammonemia - elevated [ammonia] in blood.
Total lack of any Urea Cycle enzyme is lethal. Elevated ammonia is toxic, esp. to the brain. If not treated immediately after birth, severe mental retardation results.
Postulated mechanisms for toxicity of high [ammonia]
High NH3 would drive Glutamine Synthase:
glutamate + ATP + NH3 glutamine + ADP + Pi
This would deplete glutamate – a neurotransmitter & precursor for synthesis of the neurotransmitter GABA.
Depletion of glutamate & high ammonia level would drive Glutamate
Dehydrogenase reaction to reverse:
glutamate + NAD(P)+ -ketoglutarate + NAD(P)H + NH4
+
The resulting depletion of -ketoglutarate, an essential Krebs Cycle intermediate
would impair energy metabolism in the brain.
Hyperammonemia Disease
Treatment of deficiency of Urea Cycle enzymes (depends on which enzyme is deficient):
limiting protein intake to the amount barely adequate to supply amino acids for growth, while adding to the diet the -keto acid analogs of essential amino acids.
Liver transplantation has also been used, since liver is the organ that carries out Urea Cycle.
Other disorders associated with the urea cycle
•Citrulinemia - lack of argininosuccinate synthase activity1-2 g citruline is excreted per day
•Argininosuccinicaciduria - absence of argininosuccinase activityhigh levels of argininosuccinate in blood, urine, cerebrospinal fluid
•Hyperargininemia -low levels of arginase activityelevated levels of arginine in blood and cerebrospinal fluid