Protein Catabolism

Question 1

What are the dietary sources of energy?

Answer 1

-Carbohydrates -> glucose
-fats -> fatty acids
-proteins -> amino acids

Question 2

Explain protein catabolism general .

Answer 2

-quantitatively less important as a fuel than carbohydrate and fat metabolism
-sources: diet and protein turnover
-variables: diet composition and food availability

Question 3

Explain protein catabolism dietary.

Answer 3

-typical ingestion -100g/day
-proteins denatured by low pH in the stomach
-digested to amino acids in the intestine
-pepsin, trypsin, chymotrypsin, elastase
-amino acids taken up by specific transporters

Question 4

Explain turnover of endogenous proteins.

Answer 4

-normal protein turnover- 300g/day
-damaged or exogenous proteins (junk) are degraded in lysosomes
-targeted protein degradation: tagged with ubiqitin and degraded by proteasomes

Question 5

General amino acid catabolism.

Answer 5

-twenty amino acids, so diverse mechanisms.
-general, 2 issues:
removal of the alpha-amino group and excretion of the NH4+
metabolism of the remaining carbon skeleton

Question 6

What are the 3 excretory forms of nitrogen?

Answer 6

-Ammonia as ammonium ions
-Urea
-Uric acid

Question 7

What is transamination?

Answer 7

-first the alpha-amino group is transferred from amino acids to aplha-ketogluatarate
-transamination with alpha-ketoglutarate to form alpha-ketoacid and glutamate
-amino groups are transferred to alpha-ketoglutarate, forming L-glutamate

Question 8

What is oxidative deamination?

Answer 8

-next, glutamate “gives up” amino group as NH4+ (glutamate dehydrogenase reaction) uses NADPH

Question 9

What is transdeamination?

Answer 9

-together transamination and oxidative deamination are known as transdeamination

Question 10

How does glutamine transport ammonia in the blood stream?

Answer 10

-outside of the liver:
glutamate + NH4+ + ATP -> glutamine + ADP (glutamine synthetase)
glutamine transported to the liver, then
glutamine -> glutamate + NH4+ (gluta,omase)
Glucose-alanine cycle

Question 11

How is ammonia excreted?

Answer 11

-when breakdown exceeds need, must excrete
-highly toxic
-plants: recycle almost all nitrogen
-fish- excrete ammonia
-birds, reptiles - excrete uric acid
-mammals- excrete urea

Question 12

Explain the Urea cycle general.

Answer 12

-liver only
-function only to excrete ammonia as urea

Question 13

What are the 2 fates of the carbon skeletons?

Answer 13

1. Glucogenic amino acids can be converted to glucose and degraded to:
   pyruvate, a-ketoglutarate, succinly-CoA, fumarate, oxaloacetate
   -All but lysine and leucine are glucogenic own humans
2. Ketogenic amino acids degraded to:
   acteyl-CoA and acetoacetyl-CoA
   -can be converted to fatty acids or ketone bodies but not glucose
   Some are both ketogenic and glycogenic. Lysine and Leucine are exclusively ketogenic

Question 14

What is the energy yield from protein catabolism?

Answer 14

-varies slightly with amino acid composition
-most energy yielded in the Krebs cycles,
-similar to carbohydrate, 17kJ/g, 4kcal/g