Lecture 22 - Amino Acid Catabolism and the Urea Cycle

Question 1

Overview of Amino Acid Catabolism

Answer 1

Normal synthesis and degradation of cell proteins
- rate of synthesis = rate of degradation
Excess amino acids if diet rich in protein
Starvation or Diabetes
- carbs not always available
- amino acids then used for fuel

Question 2

How do we break down protein from diet?

Answer 2

Enzymes in stomach 
- pepsinogen 
- HCl
Enzymes in small intestine
- trypsinogen 
- chymotrypsinogen

Question 3

Transport of Amino Acids across Membranes

Answer 3

Amino acids have specific protein carriers in membranes
Amino acid transport system is active transport
Requires energy
Maintains higher concentration of amino acids inside relative to outside of cell

Question 4

What happens to N in amino acid oxidation?

Answer 4

Released from amino acid
Synthesis of:
- creatine
- transmitters
- purines
- pyrimidine

Question 5

What happens to C (from C skeleton) in amino acid oxidation?

Answer 5

Can be synthesised as:

• glucose
• glycogen
• fatty acids
• steroids

Question 6

What happens to the ammonium produced from the breakdown of amino acids?

Answer 6

Incorporated into ammonium containing molecules

Can be utilised to form Carbamoyl phosphate which then enters urea cycle

Question 7

Removal of Amino Group

Answer 7

Transamination
- transfer and removal of a-amino group from amino acid to a-keto acid
Glutamate Dehydrogenase
- removal of a-amino group from glutamate to free ammonium and a-ketoglutarate
Glutaminase
- removal of amino group from side chain of glutamine to glutamate and free ammonium

Question 8

Transaminase Reactions

Answer 8

Facilitate transfer of amino groups from one compound to another
Reversible
Takes place in liver

Question 9

How do tissues and muscles decrease levels of ammonium?

Answer 9

Extrahepatic tissues convert ammonium into glutamine for transport by blood to liver
Muscles convert ammonium to alanine, via glucose-alanine cycle, for transport by blood to liver

Question 10

Ammonium Transporters

Answer 10

Glutamine
- between tissues
Glutamate
- within tissues and between cytosol and mitochondria
Alanine 
- principally between muscle and liver

Question 11

Which amino acids are ketogenic?

Answer 11

Tyrosine
Isoleucine
Phenylalanine
Tryptophan 
Leucine
Lysine

Question 12

What does it mean for an amino acid to be ketogenic or glucogenic?

Answer 12

Ketogenic - amino acid that can be degraded directly into acetyl-CoA
Glucogenic - amino acid that can be converted to glucose through gluconeogenesis