Chp 18 Urea Flashcards
(27 cards)
What is nitrogen fixation? Describe this process.
The process of reducing atmospheric N₂ (non-reactive) to ammonia (nucleophilic). Some symbiotic bacteria directly convert N₂ to NH₃ using the enzyme nitrogenase, which is an α₂β₂ complex that requires electrons from ferredoxin/flavodoxin and consumes 16 ATP per N₂ molecule.
Transamination reactions are catalyzed by what class of enzymes? Which coenzyme is required?
Transamination reactions are catalyzed by transaminase and require the coenzyme pyridoxal phosphate (PLP).
What is the overall equation for the Urea Cycle?
NH₃ + HCO₃⁻ + aspartate + 3 ATP → urea + fumarate + 2 ADP + 2 Pi + AMP + PPi
Which enzyme catalyzes the amino group transfer between glutamine and asparagine?
Asparagine synthetase catalyzes this reaction.
Lysine, methionine, and threonine are derived from what?
Aspartate
Lysine, methionine, and threonine are derived from aspartate. What is the common intermediate?
Aspartate β-semialdehyde.
How is alanine produced?
Alanine is directly produced via transamination of pyruvate.
Which cycle converts ornithine to arginine?
Ornithine is converted to arginine via the urea cycle.
What are the common nitrogen donors for transamination?
Glutamate and aspartate.
What does arginine serve as a precursor for?
Arginine serves as the precursor for nitric oxide, a biologically unusual free radical.
What structural polymer is derived from phenylalanine?
Phenylalanine serves as the monomer for lignin, a structural polymer found in wood.
Which five amino acids are degraded via transamination to their corresponding α-keto acids?
Alanine (Ala), Asparagine (Asn), Aspartate (Asp), Glutamate (Glu), and Glutamine (Gln).
The catabolic pathways of which three amino acids converge on glutamate?
Arginine, histidine, and proline.
What are the two possible catabolic pathways for serine?
Serine can be catabolized to glycine (further degraded) and pyruvate through a dehydration reaction.
Describe glycine catabolism.
Glycine catabolism is catalyzed by the glycine-cleavage system, which uses prosthetic groups and co-substrates like PLP (pyridoxal phosphate), lipoamide, NAD⁺, tetrahydrofolate (THF), and FAD.
What are the three routes of threonine catabolism?
Threonine can be catabolized to: Propionyl CoA (ketogenic), Glycine and acetaldehyde (glucogenic), and Acetyl CoA (ketogenic).
What are the first three common steps in the catabolism of leucine (Leu), valine (Val), and isoleucine (Ile)?
Transamination, Oxidative decarboxylation, Dehydrogenation.
Describe the initial and final products of methionine catabolism.
Methionine is initially converted to S-adenosyl methionine (SAM), a biological methylating agent, and ultimately degraded to propionyl CoA.
How is cysteine broken down?
Cysteine is broken down to pyruvate in three steps.
What is the first step of phenylalanine (Phe) catabolism?
Phenylalanine is initially converted to tyrosine (Tyr) by the enzyme phenylalanine hydroxylase.
What is lysine broken down to, and is it ketogenic or glucogenic?
Lysine is a ketogenic amino acid that is broken down to two molecules of acetyl CoA.
What is the first stage of nitrogen waste excretion?
The first stage of nitrogen waste excretion is the conversion of ammonia to carbamoyl phosphate, catalyzed by carbamoyl phosphate synthase.
Where do the nitrogen atoms in excreted urea come from?
Every molecule of excreted urea contains one nitrogen atom from carbamoyl phosphate and one nitrogen atom from aspartate.
Where does the urea cycle occur?
It occurs in the liver, with steps taking place both in the mitochondrial matrix and the cytosol.
