SYLLABUS 15: Metabolism of the Alpha Amino Group of Amino Acids and the Urea Cycle

Question 1

what happens to excess aa?

Answer 1

metabolized to pyruvate, TCA cycle intermediates, and acetyl CoA

Question 2

what happens to alpha amino groups of aa in excess?

Answer 2

all pooled into glutamate by transamination reactions

glutamate releases these amino groups as ammonia

ammonia is converted to urea in liver by urea cycle

Question 3

where does most aa metabolism occur

Answer 3

muscle and liver

this is where glutamate - aa transamination reacitons occur

Question 4

what happens in glutamate DH reaction?

where does it occur?

Answer 4

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

Question 5

how does muscle ammonia get metabolized differently than liver ammonia? what reaction takes place?

Answer 5

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

Question 6

which amino acids undergo the glutamine synthase rxn?

Answer 6

Ser, His, Thr, Cys

Question 7

what happens to product(s) of the glutamine synthase rxn?

Answer 7

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.

Question 8

after a high-protein meal, what is found in the blood? why?

Answer 8

large amounts of glutamate and glutamine

they carry a-amino groups from muscle to liver

Question 9

what are the reactions of the urea cycle? where does each step occur?

Answer 9

1. carbamoyl phosphate synthetase (CPS-I) in MITO
2. ornithine transcarbamoylase in MITO
3. argininosuccinate synthetase in CYTO
4. argininosuccinate lyase in CYTO
5. arginase in CYTO

Question 10

describe the urea cycle

Answer 10

1. in the mito, NH3 + CO2 react via CPS-I to form Carbamoyl Phosphate; takes 2ATP -> ADP + Pi
2. RATE LIMITING STEP: Carbamoyl Phosphate reacts w/ Ornithine transcarbamoylase to form Citrulline; releases Pi
3. Citrulline travels from Mito -> Cyto

In cyto, Citrulline reacts w/ Aspartate, uses an ATP -> ADP + Pi, via Argino Succinate Synthase to make Argino Succinate

4. Argino Succinate Lyase acts on intermediate Argino succinate, makes Arginine, releasing Fumarate
5. Arginase hydrolyzes Arginine w/ H2O, makes UREA

UREA -> URINE, ORNITHINE is regenerated and enter mito for next round

Question 11

what is most highly deficient enzyme in urea cycle

Answer 11

ornithine transcarbomylase, rate limiting step of the cycle

Question 12

is arginine an essential amino acid

Answer 12

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

Question 13

what is the structure of urea

where do its parts come from

Answer 13

2 alpha amino groups: 1 from ammonia, 1 from alpha amino group of aspartate

C=O from CO2

Question 14

how much of urea is made in CPT-1 step fo urea cycle

Answer 14

2/3

Question 15

what activates CPS-1

Answer 15

NAGA, N-acetylglutamate

it’s formed from glutamate + acetyl CoA, activated by arginine

Question 16

what steps of urea cycle require energy input?

how many high energy bonds are needed to produce 1 mol urea?

Answer 16

1. 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

Question 17

where does aspartate for urea cycle come from?

what is its fxn in the urea cycle?

Answer 17

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

Question 18

how can urea cycle and TCA cycle be linked?

Answer 18

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

Question 19

what is most commin inborn deficiency in urea cycle?

Answer 19

deficiency of OTC, ornithine transcarbomylase

Question 20

what is associated w/ deficiency of urea cycle enzyme

Answer 20

ammonia toxicity as NH3 - cannot be converted to urea for extretion

Question 21

symptomps of deficiency of a urea cycle enzyme?

Answer 21

GI tract irritability

nausea

vomiting

lethargy

if very extensive deficiency, neurological disturbances, mental retardation, seizures, coma, death

Question 22

why is NH3 toxic?

Answer 22

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

Question 23

how to treat a urea cycle deficiency?

Answer 23

1. low protein diet to minimize excess aa
2. clean out the gut - remove bacteria, yeast, which produce lots of NH3
3. gene therapy, esp for OTC deficiency
4. use drugs sucha s benzoate or phenylacetate, which can react w/ glycine or glutamine, remove aa
5. maintain blood pH

Question 24

how do we test for ammonia toxification

Answer 24

blood serum assay for NH3, esp in babies and children bc so dangerous

Question 25

what are blood urea nitrogen levels diagnostic for

Answer 25

kidney function, b/c kidney filters and removes urea

Question 26

what do blood GOT and GPT levels indicate

Answer 26

tissue damage as these v active transaminases leak out of injured tissue into blood

Question 27

are GOT and GPT tissue specific

Answer 27

no

but highest levels are found in liver

Question 28

how does benzoate work as a treament for NH3?

Answer 28

NH3 can make glycine

Benzoate + Glycine -> Hippurate

Hippurate gets excreted

Question 29

how does phenylacetate work as a NH3 treatment?

Answer 29

NH3 -> Glutamate -> Glutamine

Phenylacetate -> Phenylacetate COA which reacts w/ Glutamine -> Phenylacetylglutamine

Phenylacetylglutamine gets excreted from body

Question 30

why and when is urea synthesis necessary

Answer 30

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.

Question 31

why is ammon toxic?

what symptoms are assoc w/ NH3 intoxication?

Answer 31

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

Question 32

why is urea cycle a cycle?

Answer 32

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.

Question 33

why is arginine a non essential aa, in adults?

Answer 33

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.

Question 34

the mito membrane has transporter which exchanges citrulline for ornithine

wahts significance of this re: urea cycle?

Answer 34

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.

Question 35

what intermediates pile up fo reach specific urea cycle enzyme inborn error of metabolism causing a deficiency?

Answer 35

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

Question 36

how do we prevent ammonia toxicity in individuals w/ a deficiency of a urea cycle enzyme?

Answer 36

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.

Question 37

what is BUN and what is its clinical significance?

Answer 37

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.