M4: Amination Reactions L25 Flashcards
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
B. Chymotrypsin
D. Carboxypeptidase A
E. Pepsin
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
C. Glutamine
D. Alanine
What kind of reaction does glutamine synthetase take part in? Glutaminase?
Glutamine synthetase: Amination
Glutaminase: Deamination
What are the 2 enzymes for deamination reactions in the liver (and kidney)?
- Glutaminase
2. Glutamate dehydrogenase
What drives the glutaminase reaction forward?
Consumption of NH4 by the urea cycle drives the reaction forward.
Describe the Glutamate dehydrogenase reaction.
- Glutamate produced from the glutaminase reaction contains 1 NH3
- 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.
How is the glutamate dehydrogenase reaction regulated?
Regulated by energy state:
- low: ADP (and NAD(P)+) = allosteric activator
- HIGH: GTP (and NAD(P)H) = allosteric inhibitor
Is the glutamate dehydrogenase reaction reversible?
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.
What are the transamination pairs? Whats the difference between them?
Amino acid (has amine group) : Alpha-Keto Acid (has ketone instead of amine group)
- Alanine : Pyruvate
- Glutamate : alpha-KG
- Aspartate : Oxaloacetate
What’s the basis of a transamination reaction?
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).
Which of the following is a required co-factor in transamination reactions? A. NH3 B. Vitamin B12 C. Vitamin C D. Pyridoxal Phosphate
D. Pyridoxal Phosphate (PLP)
What is the enzyme used for transamination reactions? What is the necessary co-enzyme/prosthetic group?
Enzyme: Transaminases AKA amino transferases.
Co-enzyme/prosthetic group: Pyridoxal Phosphate (PLP)
What is funneling?
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).
What is PLP derived from?
Vitamin B6, which has to be ingested, it can’t be made in the body.
Explain the interconversion of PLP and PMP.
- In transamination reactions, PLP is going to gain an amino group from an amino acid (in the aldehyde position) = PMP
- PMP Gives the amine group to a keto acid to generate one keto acid and another amino acid thus regenerating PLP
What are the key components of PLP?
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).
Explain the ping pong mechanism of transamination.
L25 S27-29
What can lead to Type 2 Tyrosinemia? What are the symptoms?
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
What is the Glucose-Alanine cycle? What’s a different name for it?
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.
Describe the Glucose-Alanine (Cahill) reaction.
L25 S37
What is the difference between ALT-2 and ALT-1?
Both are Alanine Transaminases
ALT-2
• skeletal muscle
• uses pyruvate as keto-acid
ALT-1
• liver
• uses a-KG as keto-acid
What amino acids does transamination not work for? What’s the alternative reaction?
Lysine, Proline, Threonine
Alternative reaction: oxidative deamination.
