Week 1b - Urea Cycle Link to Gluconeogenesis

Question 1

Pyrophosphate is rapidly hydrolyzed so

Answer 1

4 molecules of ATP are consumed to synthesize 1 molecule of urea

Question 2

The synthesis of fumarate by the urea cycle is important because

Answer 2

it is a precursor for glucose syntheses

Question 3

Fumarate as a precursor for glucose synthesis

Answer 3

• fumarate is hydrolyzed to malate
• malate is oxidized to oxaloacetate
• oxaloacetate can be converted into glucose by gluconeogenesis or transaminated to aspartate

Question 4

Carbamoyl phosphate synthetase generates carbamoyl phosphate for the urea cycle and

Answer 4

the first step in pyrimidine biosynthesis

Question 5

Carbamoyl phsophate synthetase in mammals

Answer 5

2 distinct isoenzymes of the enzyme

Question 6

The carbamoyl phosphate synthetase used in pyrimidine synthesis differs in from its urea cycle counterpart in 2 important ways

Answer 6

1. this enzyme utilizes glutamine as its nitrogen source rather than NH3
   - the side chain of glutamine is hydrolyzed within one domain of the enzyme, and the ammonia generated moves through a tunnel in the enzyme to a second active site, where it reacts with carboxy phosphate
2. this enzyme is part of a large enzyme called CAD that comprises 3 distinct enzymes for pyrimidine synthesis

Question 7

A catalytic site in one isoenzyme has been adapted to act as

Answer 7

an allosteric site in another isoenzyme having a different physiological role