SYLLABUS 15: Metabolism of the Alpha Amino Group of Amino Acids and the Urea Cycle Flashcards
what happens to excess aa?
metabolized to pyruvate, TCA cycle intermediates, and acetyl CoA
what happens to alpha amino groups of aa in excess?
all pooled into glutamate by transamination reactions
glutamate releases these amino groups as ammonia
ammonia is converted to urea in liver by urea cycle
where does most aa metabolism occur
muscle and liver
this is where glutamate - aa transamination reacitons occur
what happens in glutamate DH reaction?
where does it occur?
in the liver
takes the alpha amino groups from aa which were collected as glutamate and releases them to ammonia
that is, glutamate is oxidized to aKG + NH3 + NAD(P)H
uses NAD+ or NADP+ as cofactor
this is reversible
indicates that glutamate is a non-essential amino acid b/c can form it from aKG + NH3 + NADH
how does muscle ammonia get metabolized differently than liver ammonia? what reaction takes place?
whereas liver NH3 mixes w/ NH3 from GDH reaction and enters urea cycle, in muscle, must get NH3 to liver to undergo urea cycle, b/c muscle doesn’t have urea cycle
do not want to circulate this NH3 through blood b/c it is toxic to the brain
thus muscle NH3 reacts w/ glutamate to produce glutamine in glutamine synthase reaction
which amino acids undergo the glutamine synthase rxn?
Ser, His, Thr, Cys
what happens to product(s) of the glutamine synthase rxn?
glutamine leaves muscle, circulates to liver
aka glutamine is caryring NH3 produced in muscle to liver
in liver, the enzyme glutaminase hydrolyzes glutamine back to glutamate + NH3, and NH3 -> urea cycle. uses a water to do this.
after a high-protein meal, what is found in the blood? why?
large amounts of glutamate and glutamine
they carry a-amino groups from muscle to liver
what are the reactions of the urea cycle? where does each step occur?
- carbamoyl phosphate synthetase (CPS-I) in MITO
- ornithine transcarbamoylase in MITO
- argininosuccinate synthetase in CYTO
- argininosuccinate lyase in CYTO
- arginase in CYTO
describe the urea cycle
- in the mito, NH3 + CO2 react via CPS-I to form Carbamoyl Phosphate; takes 2ATP -> ADP + Pi
- RATE LIMITING STEP: Carbamoyl Phosphate reacts w/ Ornithine transcarbamoylase to form Citrulline; releases Pi
- Citrulline travels from Mito -> Cyto
In cyto, Citrulline reacts w/ Aspartate, uses an ATP -> ADP + Pi, via Argino Succinate Synthase to make Argino Succinate
- Argino Succinate Lyase acts on intermediate Argino succinate, makes Arginine, releasing Fumarate
- Arginase hydrolyzes Arginine w/ H2O, makes UREA
UREA -> URINE, ORNITHINE is regenerated and enter mito for next round
what is most highly deficient enzyme in urea cycle
ornithine transcarbomylase, rate limiting step of the cycle
is arginine an essential amino acid
only in children, when growing/developing, because cannot make enough Arg from urea cycle for growth/development
in adults, do not need high amounts of Arg, and make enough via urea cycle
what is the structure of urea
where do its parts come from
2 alpha amino groups: 1 from ammonia, 1 from alpha amino group of aspartate
C=O from CO2
how much of urea is made in CPT-1 step fo urea cycle
2/3
what activates CPS-1
NAGA, N-acetylglutamate
it’s formed from glutamate + acetyl CoA, activated by arginine
what steps of urea cycle require energy input?
how many high energy bonds are needed to produce 1 mol urea?
- CPT-1 requires 2ATP -> ADP + Pi
3, argino succinate synthase, requires an ATP -> ADP + Pi + Pi
thus need 4 high energy bonds broken to produce 1 mole of urea
where does aspartate for urea cycle come from?
what is its fxn in the urea cycle?
from aspartate itself or from glutamate via GOT reaction
it donates the 2nd NH2 of urea, but that NH2 could be from any other amino acid via the GOT reaction producing Asp
how can urea cycle and TCA cycle be linked?
fumarate produced in step 4, argino succinate lyase rxn, can enter TCA cycle and be converted to malate -> OAA
OAA can be converted to aspartate via GOT
thus aspartate is regenerated, can go back into urea cycle
what is most commin inborn deficiency in urea cycle?
deficiency of OTC, ornithine transcarbomylase
what is associated w/ deficiency of urea cycle enzyme
ammonia toxicity as NH3 - cannot be converted to urea for extretion
symptomps of deficiency of a urea cycle enzyme?
GI tract irritability
nausea
vomiting
lethargy
if very extensive deficiency, neurological disturbances, mental retardation, seizures, coma, death
why is NH3 toxic?
1) increases blood pH since NH3 is basic
2) depletes aKG out of the TCA cycle via glutamic DH reaction
3) forms excess of the excitatory neurotransmitter glutamine, via glutamine synthase rxn
how to treat a urea cycle deficiency?
- low protein diet to minimize excess aa
- clean out the gut - remove bacteria, yeast, which produce lots of NH3
- gene therapy, esp for OTC deficiency
- use drugs sucha s benzoate or phenylacetate, which can react w/ glycine or glutamine, remove aa
- maintain blood pH
how do we test for ammonia toxification
blood serum assay for NH3, esp in babies and children bc so dangerous
what are blood urea nitrogen levels diagnostic for
kidney function, b/c kidney filters and removes urea
what do blood GOT and GPT levels indicate
tissue damage as these v active transaminases leak out of injured tissue into blood
are GOT and GPT tissue specific
no
but highest levels are found in liver
how does benzoate work as a treament for NH3?
NH3 can make glycine
Benzoate + Glycine -> Hippurate
Hippurate gets excreted
how does phenylacetate work as a NH3 treatment?
NH3 -> Glutamate -> Glutamine
Phenylacetate -> Phenylacetate COA which reacts w/ Glutamine -> Phenylacetylglutamine
Phenylacetylglutamine gets excreted from body
why and when is urea synthesis necessary
Urea synthesis is necessary to excrete the alpha amino group from amino acids when
they are in excess in the body. These alpha amino groups are pooled as glutamate, and
glutamate is converted to urea in the liver so that these toxic amino groups can be
excreted.
why is ammon toxic?
what symptoms are assoc w/ NH3 intoxication?
Ammonia is toxic because it 1. Raises blood pH, 2. Depletes a-KG out of the TCA cycle via
the glutamic DH reaction, 3. Forms excess of the excitatory neurotransmitter glutamine,
via the glutamine synthase reaction
why is urea cycle a cycle?
because its ultimate product once urea has been made is
ornithine, which is recycled when it’s synthesized from the cyto -> shuttled back to the
mito to aid in undergoing another round of urea formation.
why is arginine a non essential aa, in adults?
Arginine is a non essential amino acid in adults because enough of it is produced via the
urea cycle – for children though, undergoing growth/development, they need to have
more arginine in-take than is produced in the urea cycle.
the mito membrane has transporter which exchanges citrulline for ornithine
wahts significance of this re: urea cycle?
The citrulline-ornithine transporter is crucial for the urea cycle to function as a cycle.
The cycle begins in the mito, where the first 2 steps take place, and produce citrulline,
which is translocated into the cyto to finish the rest of the urea production cycle, which
ultimately produces the product of ornithine, which is then relocated via this shuttle
back to the mito to aid in undergoing another round of urea formation. The cycle
wouldn’t be a functioning cycle without this transporter.
what intermediates pile up fo reach specific urea cycle enzyme inborn error of metabolism causing a deficiency?
Inborn errors of metabolism/what piles up
a. CPS1: NH4 piles up
b. Ornithine Transcarbamoylase: carbaoyl phosphate and ornithine pile up
c. Argino succinate synthase: citrulline piles up
d. Argino succinate lyase: argino succinate piles up
e. Arginase: arginine piles up
how do we prevent ammonia toxicity in individuals w/ a deficiency of a urea cycle enzyme?
prevent ammonia toxicity in individuals w/ a deficiency of a urea cycle enzyme by
advising them to have a low protein diet to minimize excess a.a. intake; clean out the
gut, removing bacteria and yeast, which can produce lots of NH3; some gene therapies
are in the works; use drugs like benzoate or phenylacetate, which can react with glycine
or glutamine, and remove amino acids; maintain blood pH.
what is BUN and what is its clinical significance?
BUN is a measure of the amount of blood urea nitrogen. It is diagnostic for kidney function because the kidney removes urea. If there is lots of urea in the blood, then it means kidneys aren’t functioning well.
