Amino acid oxidation and the production of urea

Question 1

How much of their oxidative energy do humans derive from the catabolism of AA’s?

Answer 1

A small fraction

Question 2

What are AA’s derived from? (3 answers)

Answer 2

1. The normal breakdown (recycling) of cellular proteins
2. Degradation of ingested proteins
3. Breakdown of body proteins

Question 3

When would body proteins be broken down to AA’s?

Answer 3

This would happen instead of other fuel sources during starvation or uncontrolled diabetes mellitus

Question 4

What do proteases do?

Answer 4

Degrade ingested proteins in the stomach and small intestine

Question 5

What are most proteases initially synthesized as?

Answer 5

Inactive zymogens

Question 6

An early step in catabolism of AA’s…

Answer 6

is the separation of the amino group from the carbon skeleton.

Question 7

What usually happens to the amino group after separation from carbon skeleton?

Answer 7

It is transferred to alpha-ketoglutarate to form glutamate

Question 8

What kind of reaction is the transfer of the amino group to alpha-ketoglutarate? (forming glutamate) What coenzyme is required?

Answer 8

Transamination. Pyridoxal phosphate.

Question 9

What happens to glutamate?

Answer 9

Transported to liver mitochondria, where glutamate dehydrogenase liberates the amino group as ammonium ion (NH4+).

Question 10

How is ammonia formed in other tissues transported to the liver?

Answer 10

As the amide nitrogen of glutamine or, when comes from skeletal muscle, as the amino group of alanine

Question 11

What happens to the pyruvate produced by deamination of alanine in the liver?

Answer 11

Converted to glucose, which is transported back to muscle as part of glucose-alanine cycle

Question 12

Is ammonia toxic?

Answer 12

Yes. Highly toxic to animal tissues

Question 13

Explain steps of urea cycle (3 steps)

Answer 13

1. Ornithine combines with ammonia, in the form of carbamoyl phosphate, to form citrulline.
2. 2nd amino group transferred to citrulline from aspartate to form arginine - the immediate precursor to urea.
3. Arginase catalyzes hydrolysis of arginine to urea and ornithine

Question 14

What is regenerated in each turn of the urea cycle

Answer 14

Ornithine

Question 15

How are the urea cycle and the citric acid cycle interconnected?

Answer 15

Urea cycle results in net conversion of oxaloacetate to fumarate, both of which are intermediates in the Citric acid cycle

Question 16

State 2 ways in which the activity of the urea cycle is regulated

Answer 16

1. At the level of enzyme synthesis

2. By allosteric regulation of the enzyme that catalyzes the formation of carbamoyl phosphate.

Question 17

What happens to the carbon skeletons of AA’s after the removal of amino groups?

Answer 17

Undergo oxidation to compounds that can enter the TCA cycle for oxidation to H20 and CO2

Question 18

What can AA’s be converted to? (depending on degradative end product)

Answer 18

Some to ketone bodies, some to glucose, some to both

Question 19

In what situation can AA degradation be critical to survival?

Answer 19

In conditions when AA’s are a significant source of energy

Question 20

Name the 5 intermediates through which the carbon skeletons of AA’s enter the TCA cycle

Answer 20

1. Acetyl-CoA
2. Alpha-ketoglutarate
3. Succinyl-CoA
4. Fumarate
5. Oxaloacetate
   (Some also degraded to pyruvate, which can then be converted to acetyl-CoA or oxaloacetate)

Question 21

Where are branched-chain AA’s (isoleucine, leucine, and valine) degraded?

Answer 21

Unlike other AA’s, they are degraded only in extrahepatic tissues