M4: Amination Reactions L25

Question 1

A loss of function mutation in which of the following
enzyme may cause delayed absorption of dietary
amino acids in the GI-tract?
Select ALL that apply.

A. Endopeptidase H
B. Chymotrypsin
C. Pancreatic lipase
D. Carboxypeptidase A 
E. Pepsin

Answer 1

B. Chymotrypsin
D. Carboxypeptidase A
E. Pepsin

Question 2

Which is a non-toxic and abundant carrier of nitrogen in circulation?
Select ALL that apply

A. NH3
B. NH4+
C. Glutamine
D. Alanine
E. Glutamate synthetase

Answer 2

C. Glutamine

D. Alanine

Question 3

What kind of reaction does glutamine synthetase take part in? Glutaminase?

Answer 3

Glutamine synthetase: Amination

Glutaminase: Deamination

Question 4

What are the 2 enzymes for deamination reactions in the liver (and kidney)?

Answer 4

1. Glutaminase

2. Glutamate dehydrogenase

Question 5

What drives the glutaminase reaction forward?

Answer 5

Consumption of NH4 by the urea cycle drives the reaction forward.

Question 6

Describe the Glutamate dehydrogenase reaction.

Answer 6

1. Glutamate produced from the glutaminase reaction contains 1 NH3
2. NAD(P)+ is used as a coenzyme to glutamate dehydrogenase and water to cause a release of NH4+ and NAD(P)H to generate alpha-KG.
   - Consumption of NH4 by the urea cycle AND consumption of aKG by the citric acid cycle drive the reaction forward.

Question 7

How is the glutamate dehydrogenase reaction regulated?

Answer 7

Regulated by energy state:

• low: ADP (and NAD(P)+) = allosteric activator
• HIGH: GTP (and NAD(P)H) = allosteric inhibitor

Question 8

Is the glutamate dehydrogenase reaction reversible?

Answer 8

The forward reaction is an oxidative deamination and has a positive ∆G˚’. We compensate for this by rapidly pushing alpha-KG into the CAC, so there is very little alpha-KG in the liver.
The reverse reaction is glutamate synthesis and it is an amination reaction.

Question 9

What are the transamination pairs? Whats the difference between them?

Answer 9

Amino acid (has amine group) : Alpha-Keto Acid (has ketone instead of amine group)

1. Alanine : Pyruvate
2. Glutamate : alpha-KG
3. Aspartate : Oxaloacetate

Question 10

What’s the basis of a transamination reaction?

Answer 10

By coupling reactions together you can transfer an amino group from one amino acid to form another a.a.
Ex: Alanine + a-KG -> Pyruvate + glutamate
By transfer of an amino group from one amino acid (alanine) to a keto group (a-KG) to generate a second amino acid (glutamate) and a keto group (pyruvate).

Question 11

Which of the following is a required co-factor in transamination reactions?
A. NH3
B. Vitamin B12
C. Vitamin C
D. Pyridoxal Phosphate

Answer 11

D. Pyridoxal Phosphate (PLP)

Question 12

What is the enzyme used for transamination reactions? What is the necessary co-enzyme/prosthetic group?

Answer 12

Enzyme: Transaminases AKA amino transferases.

Co-enzyme/prosthetic group: Pyridoxal Phosphate (PLP)

Question 13

What is funneling?

Answer 13

Funneling the alpha-amino group of amino acids to a-keto glutarate to get glutamate and an a-keto acid (glutamate plays a central role in Nitrogen metabolism).

Question 14

What is PLP derived from?

Answer 14

Vitamin B6, which has to be ingested, it can’t be made in the body.

Question 15

Explain the interconversion of PLP and PMP.

Answer 15

1. In transamination reactions, PLP is going to gain an amino group from an amino acid (in the aldehyde position) = PMP
2. PMP Gives the amine group to a keto acid to generate one keto acid and another amino acid thus regenerating PLP

Question 16

What are the key components of PLP?

Answer 16

Key components:
• C 4’aldehyde group
- will form a covalent link (aldimine) with the alpha-amino group of the enzyme residue (or AA substrate) called a Schiff base.

• Phosphate group

• called “the handle”
• for binding to the PLP coenzyme during transamination.

• 3’OH group
- improves catalytic efficiency.

• Pyridine ring
- serves as an electron sink involved in resonance for the stabilization of carbanion intermediates (stabilization).

Question 17

Explain the ping pong mechanism of transamination.

Answer 17

L25 S27-29

Question 18

What can lead to Type 2 Tyrosinemia? What are the symptoms?

Answer 18

Inherited autosomal recessive defect in Tyrosine Catabolism. This is caused by a deficiency in Tyrosine Aminotransferase (Transaminase) in the liver: Tyrosine wont be made into 4-hydroxyphenylpyruvate (uses the alpha-kg/glutamate pair). This is all due to “non-sense” Mutations; the protein is not expressed.
Symptoms:
- Photophobia/ Cornea Keratitis
- Sensitive skin/ blistering lesions on palms and soles
- Mental retardation/ microecephaly
- Behavioral problems

Question 19

What is the Glucose-Alanine cycle? What’s a different name for it?

Answer 19

Also called the “Cahill Cycle”.
Conversion of glucose to pyruvate, pyruvate to alanine by transamination. Alanine can travel through the blood all the way back to the liver, and in the liver you do the opposite reaction to make glucose which can be further returned to the muscle to generate energy.

Question 20

Describe the Glucose-Alanine (Cahill) reaction.

Answer 20

L25 S37

Question 21

What is the difference between ALT-2 and ALT-1?

Answer 21

Both are Alanine Transaminases
ALT-2
• skeletal muscle
• uses pyruvate as keto-acid

ALT-1
• liver
• uses a-KG as keto-acid

Question 22

What amino acids does transamination not work for? What’s the alternative reaction?

Answer 22

Lysine, Proline, Threonine

Alternative reaction: oxidative deamination.