How do oxidative deamination and transamination differ? What are the main enzymes for each (1,2)
Oxidative deamination releases amino group as free NH3, whereas transamination transfers it.
Oxidative deamination: glutamate dehydrogenase
Transamination: ALT and AST
Where does oxidative deamination mainly take place? How about transamination?
Oxidative deamination mainly takes place in liver & kidney
Transamination: liver, muscle.
How do transamination and oxidative deamination work together?
Amino groups of most AA are ultimately funneled to glutamate via transamination with a-ketoglutarate.
Glutamate is then deaminated by glutamate dehydrogenase to release free ammonia (regenerating a-ketoglutarate) for C skeleton.
Does glutamate dehydrogenase use NAD+ or NADP+ as a coenzyme?
Can actually use both. NAD+ is usually used in oxidative deamination (simultaneous loss of NH3 + oxidation of carbon skeleton), while NADP+ is used in reductive amination (sim gain of NH3 + reduction of C skeleton).
What allosterically activates and inhibits glutamate dehydrogenase? (2 each)
What does glutamate DH do during starvation? After protein-rich meal?
ADP & GDP activate; ATP & GTP inhibit.
Starvation: low cellular energy, need high AA degradation to get energy production --> increased synthesis of a-ketoglutarate for TCA
Protein rich: Also need GDH for degradation and ammonia production for excretion.
Why are NH3 levels highly regulated?
Bc hyperammonemia is highly toxic to brain.
How is free ammonia transferred from tissues to liver?
What enzyme? What does it require? What is the transporter?
1. Glutamine synthetase (many tissues) adds free ammonium to glutamate --> glutamine; requires ATP (serves as non-toxic ammonia transporter).
How many ammonia groups does glutamine carry?
What happens to glutamine in (1) liver and (2) kidney?
1. Glutaminase hydrolyzes glutamine --> glutamate & free ammonia.
In liver: NH4+ is incorporated into urea which is excreted via urine.
In kidney: NH4+ is directly excreted into urine.
What is urea cycle? Where does it take place? What chemically takes place to form urea?
Urea cycle is major route for ammonia disposal in LIVER
Amino group (aspartate) and ammonia from oxidative deamination combine with CO2 to form urea.
After formation of urea in liver, what happens? What does BUN stand for? When will BUN be high?
Urea leaves liver, travels through blood to kidneys and is secreted in urine.
BUN = Blood Urea Nitrogen
BUN will be high if there's kidney disease
What are the critical points of urea synthesis?
1. Initial rxn catalyzed by carbamoyl phosphate synthetase (controlled step).
2. 3 ATPs used
3. C and O from CO2, 1 N from NH3 and 1 N from aspartate
Insert picture of urea structure
What are rxns of urea cycle? 6
- 1. CO2 (provides C atom of urea) & free NH3 proovides one of N atoms for urea.
- 2. Carbamoyl phosphate synthetase I requires N-acetyl glutamate (activator) eventually forming citrulline.
- 3. Citrulline is transported out of mitochondrion
- 4. Amino group of aspartate provides 2nd N to urea.
- 5. Citrulline --> ornithine is regenerated and goes back to mitochondrion.
- 6. Arginase (liver only) cleaves arginine --> ornithine and urea.
- 7. Urea diffuses from liver --> blood --> kidney where it is filtered/excreted into urine.
What are ATP consuming steps of urea cycle? (2)
1. First step: CO2+NH3+ 2 ATP to form base for urea.
2. Combination of citrulline with aspartate to form arginonosuccinate.
(basically one for each part of urea).
What is N-acetylglutamate synthesized from? When?
N-acetylglutamate is synthesized from acetyl CoA + glutamate after a protein rich meal --> increases urea ccle.
What are the three major regulators of urea cycle? Determine whether long term or short term.
- 1. N-acetylglutamate (short-term) allosterically activates CPSI (activated by arginine)
- 2. High protein diet (long-term) use of products of AA metabolism for energy and increased urea.
- 3. Starvation: resuls in use of body protein as energy and thus also increases urea production. Upregulates autophagy and expression of urea cycle enzymes.
What causes hyperammonemia?
defect in ornithine trans carbamoylase.
Is urea cycle reversible? How many ATPs does it need? How many high energy bonds?
No, urea cycle = irreversible.
Requires 3 ATP; 4 high energy bonds.
What amino acid activates urea synthesis?
Arginine (activates N-acetylglutamate).
What are symptoms of hyperammonemia? (4) What are the biochemical consequences? (2)
Symptoms: vomiting, cerebral edema, irritability, coma.
Abnormal GABA and glutamine synthesis --> increases intracranial pressure.
Describe defective process, enzyme, and symptoms of albinism
Albinism: defective melanin synthesis from tyrosine.
Symptoms: lack of pigment
Describe defective process, enzyme, and symptoms (3) of carbamoyl phosphate synthetase I def
- Defective process: urea synthesis
- enzyme: carbamoyl phosphate synthetase I
- Symptoms: lethargy, convulsions, early death.
Describe defective process, enzyme, and symptoms (3) of Maple Syrup Urine?
- Defective process: branched chain AA degradation
- Enzyme: branched-chain a-keto acid dehydrogenase
- Symptoms: mental disease, retardation, early death.
Describe defective process, enzyme, and symptom (1) of PKU?
Cannot convert phe --> tyr, due to defective phenylalanine hydroxylase. Symptom: mental retardation.
What happens in between citrulline coing out and ornithine going back in?
Addition of N from aspartate
What happens during the 2nd addition of N to urea?
- 1. Fumarate --> Malate --> OAA
- 2. Glutamate transfers its amino group to OAA. This converts glutamate --> a-ketoglutarate and OAA --> asparatate.
Which has the amino group: glutamate or a-ketoglutarate?
glutamate. Everytime it passes it off to someone else, glutamate turns into a-ketoglutarate.