Proteins Biochemistry Essay Question And Answers

Proteins

Name the sulfur-containing amino acids. Describe the metabolism of any one of them.
Answer:

Sulfur-containing amino acids:

• Cysteine
• Cystine
• Methionine

Metabolism of methionine:

• The metabolism of methionine is divided into three parts.

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1. Utilization of methionine for transmethylation reaction.
2. Conversion of methionine to cysteine and cystine.
3. Degradation of cysteine and its conversion to specialized products.

1. Transmethylation reactions.

• The transfer of methyl group from active methionine to an acceptor is known as transmethylation.
• For this methionine need to be first activated to S-adenosylmethionine.
• It occurs by the transfer of an adenosyl group from ATP to the sulfur atom of methionine by the methionine S- adenosyl – transferase enzyme.
• As a result, sulfur becomes a sulfonium atom.
• 3 ATP molecules are consumed in it.
• Functions of S-adenosylmethionine.
  • Transfer methyl group to an acceptor from methionine.
  • Gets converted to S-adenosyl homocysteine.
• This is hydrolyse to

1. Homocysteine.

• • Remethylated to methionine by N-methyl tetrahydrofolate.
  • This methionine can be reused.
• Adenosine.
• -S-adenosyl methionine is also involved in the synthesis of polyamines like spermine.

2. Conversion of methionine to cysteine.

• Homocysteine, a precursor of cysteine is initially synthesized from methionine.
• Homocysteine then condenses with serine to form cystathionine by PLP-dependent cystathionine synthase.
• Cleavage and deamination of cystathionine to cysteine and a-ketobutyrate occur by cystathionine synthase and cystathionine.

3. Degradation of cysteine

1. Cysteine and cystine are interconvertible by an NAD+ – dependent cystine reductase.
2. Meraceptothanolamine.
   • Cysteine on decarboxylation produces it.
   • Mercaptoethanolamine is involved in the biosynthesis of coenzyme A.
3. Cysteine sulfite.
   • Cysteine oxidizes to cysteine sulfinate by cysteine dioxygenase enzyme.
   • It cleaves alanine to sulfite. S
   • Sulfite is converted to sulfate and excreted in the urine.
4. Cysteic acid.
   • Cysteine sulfinate oxidizes to cysteic acid.
   • This produces
     1. Taurine by decarboxylation and
     2. Pyruvate by degradation.
5. Cysteine can be degraded by desulfhydrase to liberate sulfur, ammonia, and pyruvate.