L29- Nitrogen Metabolism Flashcards
More than 95% of the amino acids in plasma are replaced every ___ minutes.
10
Increased levels of which of the following compound(s) reflects an increase in protein intake: urea, ammonia, uric acid, and/or creatinine?
Urea.
Name two physiological examples in which it would be advantageous for an individual to be in positive nitrogen balance.
Childhood and pregnancy.
In which form is excess nitrogen excreted?
Urea.
What are the three general steps in which amino nitrogens from amino acids are excreted?
- Transfer of the amino group to a common carrier such as glutamine 2. Release of the transferred amino group as ammonia 3. Incorporation of ammonia into urea.
Which amino acids cannot be modified via transamination reactions?
Lysine and threonine.
List the three most commonly used alpha-ketoacids for transaminase reactions.
Pyruvate, alpha-ketoglutarate and oxaloacetate.
Which coenzyme is required for all aminotransferase enzymes?
Pyridoxal phosphate (PLP).
From which vitamin is pyridoxal phosphate derived?
Vitamin B6.
In the absence of an amino acid, the coenzyme pyridoxal phosphate is covalently bound to a _______ (amino acid) residue at the active site of the aminotransferase.
Lysine.
What is the role of glutamate dehydrogenase with regards to the production of ammonia?
It catalyzes the release of the alpha-amino group from glutamate by oxidative deamination.
In which tissue or organ, and in which cellular compartment, is the enzyme glutamate dehydrogenase found?
Liver mitochondria.
True or False. Glutamate dehydrogenase can use either NAD+ or NADP+ as a coenzyme.
True.
What are the allosteric activators of the deamination reaction catalyzed by glutamate dehydrogenase?
GDP and ADP.
What are the allosteric inhibitors of the deamination reaction catalyzed by glutamate dehydrogenase?
GTP and ATP.
Glutamate dehydrogenase can use either NAD+ or NADPH as a coenzyme. When is each coenzyme preferentially used?
NAD+ is generally used for deamination of glutamate, producing alpha-ketoglutarate and ammonia. NADPH is used for the reverse reaction, trapping ammonia as glutamate.
Which enzyme catalyzes the irreversible addition of ammonia to glutamate, trapping ammonia as glutamine?
Glutamine synthetase.
What is the reaction catalyzed by asparagine synthetase?
This enzyme catalyzes the transfer of the amide nitrogen of glutamine to aspartate using ATP, forming asparagine, glutamate, AMP and PPi.
Which enzyme catalyzes the irreversible release of ammonia from glutamine?
Glutaminase.
What is the physiologic function of the glutaminase isoform found in the liver?
It couples ammonia production to urea synthesis for excretion of nitrogen.
What is the physiologic function of the glutaminase isoform found in the kidneys?
It produces NH3, which gets protonated to NH4+ in the kidney tubules. This permits the conservation of other cations such as Na+ and K+, and therefore helps to regulate blood pH and combat acidosis.
What is the source of the two nitrogens found in urea?
One enters the urea cycle as free ammonia while the other enters the urea cycle as the amino group on aspartate.
How many ATP molecules are required for the production of one urea molecule through the urea cycle?
3 ATP’s are used (but 4 high-energy bonds are cleaved).
In which tissue or organ does the urea cycle occur?
Liver.
