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Biochemistry - 3 Questions

1
Biochemistry

A vitamin B12 deficiency may affect heme synthesis by reducing the concentration of which of the following? Choose the one best answer.

1) Acetyl-CoA
2) Succinyl-CoA
3) Glycine
4) Alanine

📝 Explanation:

Vitamin B12 participates in two reactions in the body-conversion of homocysteine to methionine and conversion ofmethylmalonyl-CoA to succinyl-CoA.

Methylmalonyl-CoA is produced via various amino acid degradation pathways, and from oddcarbon chain fatty acid oxidation. In the absence of B12, succinyl-CoA would only be produced as an intermediate of the TCA cycle, and if it were removed from the cycle for heme synthesis, energy production may suffer.

Glycine is usually obtained from the diet, although in a B12 deficiency a functional folate deficiency may also develop, leading to an inhibition of serine hydroxymethyltransferase, the enzyme that converts serine to glycine, and requires free tetrahydrofolate.

Succinyl-CoA and glycine are the precursors for heme synthesis. A B12 deficiency would not inhibit the production of acetyl-CoA, succinate, or alanine. 

2
Biochemistry

Hyperuricemia in Lesch-Nyhan syndrome is due to a defect in which of the following pathways?

1) Purine biosynthesis
2) Pyrimidine biosynthesis
3) Purine salvage
4) Pyrimidine salvage

📝 Explanation:

Uric acid is a purine derivative, increased by purine salvage reactions that convert purines, purine ribonucleosides, and purine deoxyribonucleoside to mononucleotides (incorrect answer 4).
 
Such salvage reactions require much less energy than de novo synthesis (incorrect answers 1, 2). The liver is the major site of purine nucleotide biosynthesis and provides excess purines for other tissues that cannot synthesize purines. 

A defect in hypoxanthine-guanine phosphoribosyl transferase, one of the enzymes of purine salvage, is responsible for purine overproduction and subsequent hyperuricemia observed in Lesch-Nyhan syndrome.

3
Biochemistry
Urea formation take place in liver by:
1. Salvage pathway
2. Krebs cycle
3. Krebs Henseleit cycle
4. none

📝 Explanation:

The synthesis of urea takes place in the liver by the process called ''Ornithine cycle'' or "Urea cycle"
This cycle was discovered by Hans Krebs and Kurt Henseleit in the year 1932.
The Ornithine cycle is completed in 5 steps:
1. Conversion of ammonia into carbamoyl phosphate
2.The transfer of carbamoyl group from carbamoyl phosphate to ornithine
3. Releasing of the formed citrulline into the cytosol
4. Cleavage of argininosuccinate
5. Hydrolysis of Arginine: The final step involves the hydrolysis of Arginine into urea and ornithine

Arginase is the sixth and final enzyme of this cycle.

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