Nitrogen & Urea

Question 1

aminotransferases

Answer 1

reversibly shuttle amino groups b/t intermediates

Question 2

pyridoxal phosphate

Answer 2

derived from vitamin B6; coenzyme bound to aminotransferases and req for transamination mech

Question 3

What happens to excess AAs?

Answer 3

catabolized to gluconeogenic and/or lipogenic intermediates (via transfer to alpha-ketoglutarate by aminotransferases –> generate glutamate)

Question 4

glutamate dehydrogenase

Answer 4

catalyzes oxidative deamination of glutamate

- produces ammonia and alpha-ketoglutarate

Question 5

alkaptonuria

Answer 5

pathophys: deficiency in homogentisate oxidase - enzyme in pathway for tyrosine degradation
Sx: homogentisate accumulates, mostly excreted in urine, but also deposits on the joints over time–> severe arthritis

Question 6

maple syrup urine disease (MSUD)

Answer 6

Pathophsy: deficiency in branched-chain alpha-keto acid dehyrogenase
- accumulation of leucine, isoleucine, valine, and their keto-acids
Sx: neurological deficients b/c buildup of branched-chain alpha-keto acids, maple syrup odor in urine, death

Question 7

homocystinuria

Answer 7

Pathophys: deficiency in enzyme cystathionine beta-synthase (converts homocysteine –> cystathionine)
Sx: osteoporosis, mental retardation, increased risk of CV disease

Question 8

location of glutamate dehydrogenase

Answer 8

abundant in liver, localizes to the mitochondrial matrix

Question 9

regulation of glutamate dehydrogenase rxn

Answer 9

+ ATP and GTP

+ glutamate levels high

Question 10

catalytic enzymes of the urea cycle + their locations

Answer 10

1. carbamoyl phosphate synthetase I (mito. matrix)
2. ornithine transcarbaoylase (mito. matrix)
3. argininocuccinate synthase (cytosol)
4. argininosuccinate lysase (cytosol)
5. Arginase (cytosol)

Question 11

rate limiting step in the urea cycle (and how it is regulated)

Answer 11

enzyme carbamoyl phosphate synthetase I (CPS I)

- regulated by N-acetyl glutamate

Question 12

how is the urea cycle regulated long-term?

Answer 12

transcriptional control

Question 13

what happens to urea levels in starvation or a protein-rich diet?

Answer 13

urea cycle enzyme levels are elevated

Question 14

how much ATP is consumed in synthesizing one molecule of urea?

Answer 14

3 ATPs

Question 15

alanine aminotransferase

Answer 15

alanine + alpha-KG –> pyruvate + glutamate

reversible

Question 16

aspartate aminotransferase

Answer 16

aspartate + alpha-KG –> OAA + glutamate

reversible

Question 17

phenylketonuria

Answer 17

deficiency in phenylalanine metabolism

Sx: neuro deficits