Degradation of amino acids
• Amino acid breakdown can yield:
– Acetyl-CoA
a-KG
– Succinyl-CoA
– OAA
– fumarate
a-KG is generated from five
amino acids
• Proline
• Glutamate
• Glutamine
• Arginine
• Histidine
Not a surprise
• Proline, glutamate, arginine, and glutamate
are synthesized from a-KG, here use
distinct enzymes for breakdown
• But, histidine is not; A completely different
pathway for histidine catabolism than for
anabolism,
• In this case, incoming amino acid
(glutamate) binds, donates its amino group
to pyridoxal phosphate, and leaves as an a-
keto acid (a-KG). Then, an incoming a-
deto acid binds and accepts the amino group
and leaves as an amino acid
Four amino acids are converted
to Succinyl-CoA
• Methionine
– Converted to homocysteine through methyl group
transfer, generates cysteine as converted to a-
ketobutyrate
• Isoleucine
– Transamination, oxidative decarboxylation to acetyl-
CoA and propionyl CoA
• Valine
– Transamination, decarboxylation to propionyl CoA
• Threonine
a-ketobutyrate generated and converted to propionyl
CoA
Propionyl-CoA is a common intermediate for
amino acids succinyl-CoA
Branched-chain a-keto acid
dehydrogenase complex
• In certain body tissues, this enzyme
catalyzes the oxidative decarboxylation of
valine, isoleucine, and leucine yielding
CO2, and acyl-CoA derivatives.
• Shares ancestry with pyruvate
dehydrogenase complex, a-KG
dehydrogenase complex – another example
of gene duplication
Branched-chain …complex
Asparagine and aspartate are
degraded to OAA
Fate of metabolites derived from
amino acids
• In addition to feeding the citric acid cycle,
amino acids can result in ketone bodies,
while others are gluconeogenic
Ketone bodies
• The six amino acids that are degraded to
acetoacetyl-CoA and/or acetyl-CoA (in blue
on previous slide) can be converted to
acetoacetate and b-hydroxybutyrate
Gluconeogenic amino acids
• Amino acids that are degraded to pyruvate,
a-KG, succinyl-CoA fumarate, and/or OAA
can be converted to glucose
Tempting to take a dietary perspective on
carbohydrate and protein metabolism, but…
We’ll just re-emphasize ammonia
metabolism
You seen this many, many times
• All aminotransferases have PLP
PLP enzymes
• Generally found in enzyme active site
covalently bound to amino group of lysine
PLP-mediated transformation
Aminotransferases exhibit Ping-
Pong kinetics
• Ping-pong – no ternary complex is formed
between substrates and enzyme; first
substrate binds, reacts, then that products
leaves before second substrate binds
Ammonia from amino acid
catabolism
• During amino acid breakdown, amino is
generally transferred to
glutamate (serves as nitrogen
source and sink)
• From there, amino group can be
released as ammonia
Transdeamination
• The combined action of aminotransferase
and glutamate dehydrogenase is called
transdeamination
• Glutamate dehydrogenase operates at an
important intersection of carbon and
nitrogen metabolism – as a result, highly
regulated
Linkage of TCA with aa
catabolism by allostery
• ADP is a positive effector of glutamate
dehydrogenase, while GTP is a inhibitor
Ammonia is toxic, so cells need
to get rid of it…
• Fix ammonia onto glutamate to form
glutamine and use as a transport mechanism
• Transport ammonia by glucose-alanine
cycle
• Excrete nitrogenous waste through urea
cycle
• Glucose-alanine cycle
Ammonia transport using alanine
• Alanine aminotransferase transfers the a-
amino group from glutamate to pyruvate,
forming alanine
• This shuttle funnels ammonia out of tissues
that have high glycolytic flux, to the liver,
which can remove ammonia via urea cycle
Dumping ammonia as urea
• The glutamine, glutamate, and alanine feed
the urea cycle
• The urea cycle generates urea, which can be
deposited as waste
• The urea cycle spans both the cytosol and
mitochondria and four intermediates – you
are responsible for the urea cycle
Entering the cycle
• Ammonia derived from glutamate
or glutamine is immediately linked
to bicarbonate – catalyzed by
carbamoyl phosphate synthetase I
(mitochondrial)
Starting the cycle
• Carbamoyl phosphate reacts with ornithine
to form citrulline with relase of inorganic
phosphate (similar to OAA and acetyl-CoA)
– catalyzed by ornithine transcarbamoylase
• Citrulline is passed to cytosol, where a
second amino group is introduced via
aspartate to form argininosuccinate –
catalyzed by argininosuccinate synthetase
with an ATP requirement
Producing urea
• Argininosuccinate lyase generates fumarate
and arginine from argininosuccinate
• Cytosolic enzyme arginase cleaves arginine
to yield urea and ornithine
• Ornithine is transported back to the
mitochondria to start another round of the
urea cycle
• Substrate channeling!! – except urea
Urea cycle and citric acid cycle
can be linked
• Fumarate generated by the urea cycle can be
converted to OAA in cytosol, and
transported to mitochondria for use in citric
acid cycle
• Conversely, transamination of OAA in
mitochondria yields aspartate, which is used
in the cytosolic urea cycle
“Kreb’s bicycle”
Urea cycle regulation
• More protein metabolism,
more urea production
• Carbamoyl phosphate
synthetase I is allosterically
activated by N-acetyl-
glutamate
The urea cycle is cost effective
• Looking at the urea cycle you observe three
ATP spent for every turn, BUT generation
of OAA from fumarate yields an NADH
which can be used to generate 2.5 to 3 ATP
via oxidative phosphorylation