Transcript Catabolism of proteins and amino acids Reactions in the attachment of ubiquitin to proteins.

Catabolism of proteins and amino acids
Reactions in the attachment of ubiquitin to
proteins
Relationships among major pathways in nitrogen
catabolism
The alpha amino acid nitrogen is channeled into
glutamate.
Transaminase Roles
Transaminases equilibrate amino groups among available aketo 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).
Transaminases function in amino acid catabolism
and biosynthesis
COO
COO
COO
CH2
COO
CH2
CH2
CH2
CH2
CH2
HC
NH3+
COO
+
C
O
COO
C
O
COO
+
HC
NH3+
COO
aspartate a-ketoglutarate oxaloacetate glutamate
Aminotransferase (Transaminase)
Example of a Transaminase reaction:
 Aspartate donates its amino group, becoming the a-keto
acid oxaloacetate.
 a-Ketoglutarate accepts the amino group, becoming the
amino acid glutamate.
H
O
O
P
O
O
C
H2
C
OH
O

N
H
CH3
pyridoxal phosphate (PLP)
The prosthetic group of Transaminase is pyridoxal
phosphate (PLP), a derivative of vitamin B6.
R
H
C
COO
Enz
(CH2)4
NH2
Amino acid
N+
HC
O
O
H2
C
P
O
H
O
O

N
H
CH3
Enzyme (Lys)-PLP Schiff base
In the resting state, the aldehyde group of pyridoxal
phosphate is in a Schiff base linkage to the e-amino group of
an enzyme lysine residue.
EnzLysNH2
R
H
C
COO
N+
HC
O
O
H2
C
P
O
H
O
O

N
H
CH3
Amino acid-PLP Shiff base (aldimine)
The a-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.
O
EnzLysNH2
CH2
O
O
H2
C
P
O
NH2
R
C
COO
a-keto acid
OH
O

N
CH3
H
Pyridoxamine phosphate (PMP)
What was an amino acid leaves as an a-keto acid.
The amino group remains on what is now pyridoxamine
phosphate (PMP). A different a-keto acid reacts with PMP and
the process reverses, to complete the reaction.
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.

OOC
H2
C
H2
C
glutamate
NH3+
C
H
H 2O
COO
NAD(P)+
NAD(P)H
O
H2 H2

OOC C C
a-ketoglutarate
C
COO + NH4+
Glutamate Dehydrogenase
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 a-carbon is followed by hydrolysis,
releasing NH4+.
Glutamate Dehydrogenase
Amino acid
a-keto acid
a-ketoglutarate
glutamate
Transaminase
+
NADH + NH4
+
NAD + H2O
Glutamate
Dehydrogenase
Summarized above: the role of transaminases in
funneling amino N to glutamate, for deamination
via Glutamate Dehydrogenase, producing NH4+.
Amino Acid Oxidase
Glutamine Synthetase
Glutaminase
Muscle and liver play major roles in maintaining
steady-state levels of amino acids
Glucose-Alanine Cycle
Amino acid exchange between organs
O
H2N
C
NH2
urea
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.
Urea Cycle
HCO3 + NH3 + 2 ATP
O
H2N
C
Carbamoyl Phosphate
Synthase
OPO 32 + 2 ADP + Pi
carbamoyl phosphate
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.
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)+ 
a-ketoglutarate +
NAD(P)H + NH4+
The resulting depletion of a-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 a-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 activity
1-2 g citruline is excreted per day
•Argininosuccinicaciduria absence of argininosuccinase activity
high levels of argininosuccinate in blood, urine, cerebrospinal fluid
•Hyperargininemia low levels of arginase activity
elevated levels of arginine in blood and cerebrospinal fluid