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When more amino acids are ingested than are needed to replace protein and other nitrogenous compounds, what is their fate?
They either become triacyglycerols or glucose.
When muscle protein is catabolized in muscle during a fast, what are the fates of the amino acids?
Some AA's are partially oxidized to produce energy in muscle and other tissues.
What is not used for energy in these tissues is released into the blood and travels to the liver as glutamine, alanine, and all the other amino acids. In the liver, the carbon skeletons are converted into glucose or used for energy. The nitrogen from these amino acids is incorporated into urea and excreted in the urine
What is another name for transamination reactions?
Be able to write a reaction for AST, ALT, or any other transamination reaction. What is the approximate Keq for these reactions?
- Aspartate transaminase = (AST)
- Aspartate + alpha-ketoglutarate <----AST----> Oxaloacetate (OAA) + Glutamate
- Alanine transaminase
- Alanine + alpha-ketoglutarate <----ALT-----> Pyruvate + Glutamate
Keq=1 so it is reversible.
What are the cofactors for ALT and AST. Are the coenzymes prosthetic groups? What vitamin are the cofactors derived from?
Pyridoxal Phosphate (PLP). The cofactor PLP is covalently bound (tightly bound) to the enzyme making it a prosthetic group. PLP is derived from Vitamin B6.
Around pH = 7.3, which is the most prevalent form, ammonia or ammonium ion? Use the pKa = 9.3 and the Henderson Hasselbalch Equation to prove it.
What are the major sources of free ammonia in the liver, muscle and other tissues?
1. Glutamate dehydrogenase
- 2. Deamidation.
3. Urease (located in the liver and gut)
4. Some amino acids do not use a transaminase. They just release their nitrogen and produce free ammonium ions during catabolism.
Be able to write the glutamate dehydrogenase reaction. Is this reaction readily reversible?
Glutamate + NAD(P)+ + H2O <_____GDH_____> NAD(P)H + alpha- ketoglutarate + NH4+ + H+
Yes, this reaction is readily reversible (Keq = 1). It can also use NAD+ as a cofactor as well as NAD(P)+.
Glutamate is important in the synthesis of nonessential amino acids. How does glutamate acquire the amino groups for this purpose?
Glutamate acquires amino acids for the synthesis of nonessential amino acids through transamination reactions with some other amino acid or from free ammonium ion through the GDH reaction. About 15 of the 20 amino acids will run a transamination reaction to regenerate glutamate by transferring their amino group to alpha-ketoglutarate.
Glutamate is important in the synthesis of urea. How does glutamate acquire the amino groups for this purpose? What does glutamate do with the amino groups?
Glutamate acquires amino groups from all the amino acids that will transaminate with it and from free ammonium ion. Glutamate will transfer (transaminate) amino groups to oxaloacetate to produce aspartate so that it can feed the amino group into the urea cycle. Glutamate can use GDH to produce free ammonium ion to feed into the urea cycle.
When excess NH4+ is produced in most cells of the body, it can be combined with glutamate and transported to the liver where it is released as free NH4+. Name the two enzymes and the intermediate in this process.
Enzymes: glutamine synthetase and glutaminase Intermediate: glutamine
Glutamate + NH4+ + ATP(peripheral tissues) <---glutamine synthetase---> Glutamine + ADP + Pi
Glutamine transported to liver
Glutamine <----glutaminase ----> Glutamate + NH4+ (liver)
Know the substrates, products, and five enzymes of the urea cycle. Which compartments are involved? What tissues are involved?
- HCO3 + NH4+ + 2ATP ______carbamoyl phosphate synthetase I (control enzyme)______> carbamoyl phosphate + 2ADP + Pi
(Mitochondria) Ornithine + carbamoyl phosphate _______ornithine transcarbamoylase_______> citrulline + Pi
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