Karnataka PGET 2007 Biochemistry MCQ Answer 05

The correct answer is B

The requirement of rats and humans for the essential amino acids phenylalanine and methionine is substantially reduced if tyrosine, which is formed from phenylalanine, or cysteine, which is formed from methionine, is added to the diet.

Cysteine is a nonessential amino acid, which means that it is manufactured from other amino acids in the liver; it does not have to be obtained directly through the diet. It is synthesized in the liver from methionine.

Cysteine is not only an essential constituent of proteins, but it also lies in the major route of incorporation into all organic sulfur compounds in the body. Cysteine is formed from methionine and serine. Methionine is converted to S-adenosyl methionine (SAM) and on to homocysteine. Homocysteine then reacts with serine under the influence of the enzyme cystathionine beta-synthetase. This series of steps requires adequate levels of vitamin B-12, vitamin B-6, and betaine. A variety of genetic defects have been found relating to these enzymes, which may then dictate the increased need in certain individuals for enhanced levels of vitamin B-12 and vitamin B-6, or betaine to stimulate proper synthesis of cysteine. Cysteine can also be converted to taurine, which is a component of bile salts as well as a neurotransmitter.

Category: Karnataka PGET 2007 MCQs