23 - Amino Acid Degradation Flashcards
True or false? Nitrogen is a component of proteins, carbs, lipids and nucleic acids
TRUE
Why are there no polymeric forms of nitrogenous compounds that are stored for release on demand (like glycogen or triacylglycerol)?
All proteins are functional
At steady state, 10-15% of energy requirement is from the OXIDATION of amino acids. Though, a carnivore can acquire up to 90% from amino acid oxidation after a meal.
In humans, what are the three metabolic states where amino acid oxidation occurs?
Basal: Amino acids generated by continual biosynthesis and degradation of cell proteins
High protein diet: Intake exceeds requirement for protein synthesis
Starvation of diabetes mellitus: carbohydrate is not available or is improperly utilized
What happens when there is a negative nitrogen balance (eg. not consuming enough)?
- Structural proteins (such as those in muscle tissue) will be catabolized to meet N requirement
- Structural proteins may be degraded even if only specific amino acids are absent
- When an individual amino acid accumulates above the level required for its utilization it will be catabolized (balance of N intake and excretion).
Where are most amino acids degraded?
What part of the amino acid is removed in catabolism?
Liver
Amino acid catabolism involves removal of the alpha amino group
During catabolism, most amino acids don’t release their alpha amino group as ammonia. What is it released as?
Most amino acids transfer the amino group by transamination (a non-oxidative reaction) and the carbon skeleton is incorporated into carbohydrate metabolism as alpha keto acid (can make glucose from gluconeogenesis!)
Though some AAs do produce an ammonia, which can go into the biosynthesis of nitrogenous molecules, or into carbomyl phosphate and the urea cycle.
An aspartate-argininosuccinate shunt connects the urea cycle and TCA cycle.
How is glutamic acid central to the degradation of amino acids (as it is in biosynthesis)?
It can be deaminated by an oxidative deamination, catalyzed by glutamate dehydrogenase, to give ammonia and an aKG.
This reversible reaction is also involved in the assimilation of ammonia.
List three common features of degradation pathways
- Removal of the amino group is followed by incorporation of the carbon skeleton into the citric acid cycle.
- Depending on the physiological status of the cell, the carbon skeleton will be used for further oxidation or for gluconeogenesis.
List the three ketone bodies
- acetoacetate
- beta-hydroxybutyrate
- acetone
How are ketogenic amino acids degraded?
- Degraded to ketone intermediates (acetyl-CoA or acetoacetyl-CoA), similar to fatty acid catabolism.
- Entry of these intermediates into the citric acid cycle as acetyl CoA does not result in any net carbohydrate synthesis in animals (just as fat cannot be converted to carbohydrate)
How are glucogenic amino acids degraded?
- Degraded to intermediates of TCA cycle or glycolysis that can be converted to glucose via gluconeogenesis (eg. pyruvate, oxaloacetate, fumarate, succinyl-CoA or aKG).
- Some amino acids enter the central pathways of carbohydrate metabolism at more than one point.
Most amino acids are glucogenic, not ketogenic.
List the five glucogenic AND ketogenic amino acids and the one exclusively ketogenic amino acid (the rest are glucogenic)
Both
- Tryptophan
- Phenylalanine
- Tyrosine
- Isoleucine
- Threonine
Ketogenic only
- Leucine
Since leucine is exclusively ketogenic and very abundant in proteins, this amino acid can contribute to ketone formation in diabetes mellitus.
Also, lysine releases glutamate during degradation, but consumes aKG, so it is considered by some to be only ketogenic.
List the five glucogenic AND ketogenic amino acids and the one exclusively ketogenic amino acid (the rest are glucogenic)
Both
- Tryptophan
- Phenylalanine
- Tyrosine
- Isoleucine
- Threonine
Ketogenic only
- Leucine
Since leucine is exclusively ketogenic and very abundant in proteins, this amino acid can contribute to ketone formation in diabetes mellitus.
Also, lysine releases glutamate during degradation, but consumes aKG, so it is considered by some to be only ketogenic.
How does amino acid degradation interact with carbohydrate metabolism?
- The glutamate dehydrogenase reaction releases ammonia in the liver for urea synthesis (how we get rid of excess amino groups)
- Glutaminase is also present in the liver (Takes glutamine and releases its ammonia to generate glutamic acid, which undergoes glutamate dehydrogenase to become aKG, which can add to gluconeogenesis (proteins can generate glucose if they need to)).
- Glutamic acid and glutamine are the major carriers of amino nitrogen, and these metabolites represent important branch points connecting nitrogen and carbohydrate metabolism.
What are aspartic acid and asparagine degraded to?
Asparagine to aspartate. Aspartate to oxaloacetate and fumarate
