Deck 13: Nitrogen Metabolism

Question 1

Many fish secrete ammonia as their nitrogen waste, but some secrete allantoin, which is a further degradation product of uric acid. The formation of allantoin, a more water-soluble molecule, is catalyzed by allantoinase. Describe how this enzyme might be used as a drug therapy in humans.

Answer 1

It has been observed that some fishes secrete allantoin instead of ammonia as a waste product. This product, allantoin, is formed with the help of allantoinase enzyme and it is water soluble.
This enzyme could be of therapeutic use in the human beings. This can be used to treat people with high uric acid in their blood. Uric acid, being poorly soluble in water, gets accumulated in the body. If this enzyme is given to patients with high uric acid, the enzyme forms allantoin (water soluble) from it and help its excretion via the kidney.

Question 2

Ammonia toxicity in humans may result from liver damage, but the most severe effects are found in the brain. Describe the nitrogen metabolism pathways involved.

Answer 2

Liver is the center for most of the metabolism of the humans. It detoxifies the body and converts the toxins into non-toxic forms. Ammonia is one of the toxic compounds that is generated from protein and amino acid metabolism. The ammonia that is generated in the cells is transported through the blood, in the form of glutamine. The liver converts the ammonia into less toxic form, the urea. Urea is then sent to excretion through the kidneys.
During chronic liver failure, ammonia is not properly metabolized. The concentration of this compound remains high in the blood. Ammonia crosses the blood brain barrier and starts the formation of glutamine in the brain. Also the high amount of ammonia in the brain increases the water content in the cerebrospinal fluid causing the cranial pressure to rise. This high cranial pressure is harmful for the brain, pressing the nerves of the cranium.

Question 3

Explain how two key portions of the pyridoxal phosphate molecule-the carbonyl group and the phosphate-are used in transaminase reactions and in phosphorylase reactions.

Answer 3

Pyridoxal phosphate or vitamin B6 is a coenzyme for several vital metabolic enzyme reactions. It acts as a coenzyme for most of the transamination, decarboxylation, deamination, and racemization reactions of amino acids.
The pyridoxal phosphate can be used in the transaminase and phosphorylation reaction in different ways. The pyridoxal phosphate forms Schiff's base along with the given amino acid 1. This Schiff's base then transfers the amino group from it to a keto acid 1 producing amino acid 2. The amino acid 1 then becomes a keto acid 2.
During phosphorylation reaction, pyridoxal phosphate does not act like above. Rather, it acts as a phosphate group donor only.

Question 4

How are the biopterin and flavin coenzymes similar?

Question 5

SAM is sold commercially as "SAM-e" as a natural product that can supposedly treat liver disease and depression. Yet, no controlled studies have shown such benefits. Why would SAM appear to have these actions? Would ingested SAM enter cells?

Question 6

How would urea synthesis from glycine differ from urea synthesis from alanine?

Question 7

Why are branched chain amino acids considered metabolically as a unit?

Question 8

After the formation of nitric oxide, what is the fate of the rest of the arginine molecule?