Lecture 69

Question 1

What percentage of protein is turned over in our body every day?

Answer 1

5-10%

Question 2

What is the DRI for protein for adults?

Answer 2

0.8 g/kg/day (56 g/day for a man, 44 g/day for a woman)

Question 3

What is Cachexia?

Answer 3

• Loss of weight, muscle atrophy, fatigue, weakness and loss of appetite
• accompanies cancer, AIDS, CHF, or major trauma
• cannot be reversed nutritionally
• appears to be a response to inflammatory cytokines –> ubiquitin-proteosome pathway and inhibition of protein synthesis

Question 4

How do we measure protein insufficiency? Why?

Answer 4

Serum protein transthyretin (prealbumin), because it has a high turnover (half life 1-2 days) for a serum protein

Question 5

Which two AAs are used to transport nitrogen in the blood?

Answer 5

Alanine and Glutamine

Question 6

Which enzyme shuttles amino groups between AAs?

Answer 6

aminotransferases/transaminases

Question 7

What is the cofactor for aminotransferase?

Answer 7

Pyridoxal phosphate (derived from B6).

Question 8

What enzyme is used to synthesize non-essential AAs from glutamate?

Answer 8

transaminse/aminotransferase

Question 9

Describe the movement of nitrogen from skeletal muscle to the liver via transaminase.

Answer 9

1. Transaminase: a-ketoglutarate + AA –> glutamate + a-keto acid
2. ALT (alanine transaminase):glutamate + pyruvate –> a-ketoglutarate + alanine
3. alanine –> liver

Question 10

Describe what happens to alanine once it reaches the liver.

Answer 10

1. ALT (alanine transaminase): alanine + a-ketoglutarate –> glutamate + pyruvate
2. pyruvate –> glucose, glycogen, FAs (or oxidized for energy)
3. glutamate –> urea cycle

Question 11

Describe the movement of nitrogen from peripheral tissue to the liver/kidney via glutamine synthetase.

Answer 11

1. glutamine synthetase: glutamate + NH4+ –> glutamine
2. glutamine –> liver/kidney
3. glutaminase: glutamine –> glutamate + NH4+

Question 12

Describe the entry of glutamate into the urea cycle.

Answer 12

glutamate dehydrogenase: glutamate –> a-ketoglutarate + NH4+

(bidirectional enzyme. Uses NAD+ in catabolic reaction and NADP+ in anabolic reaction)

Question 13

What is the overall reaction for urea synthesis?

Answer 13

NH3 + HCO3- + aspartate + 3ATP –>

urea + fumarate + 2ADP + 4Pi

Question 14

Which AA donates a nitrogen to urea?

Answer 14

Aspartate

Question 15

How many high energy bonds are used in the formation of one molecule of urea?

Answer 15

4

Question 16

What is the significance of N-acetylglutamate (NAG)?

Answer 16

It is an absolute requirement for carbomoyl phosphate synthetase.

Question 17

Draw the urea cycle.

Answer 17

Well..?

Question 18

What happens to fumarate after it is formed in the urea cycle?

Answer 18

It is converted to oxaloacetate by the TCA cycle enzymes –> NADH.

Question 19

List 3 ways the urea cycle is regulated.

Answer 19

1. substrate availability
2. the synthesis of urea cycle is induced under conditions of a high protein diet or in prolonged fasting.
3. N-acetylglutamate is only synthesized when glutamate and acetyl-CoA levels are high. Arginine is a positive allosteric regular of NAGS.