Multiple choice

In the peptidoglycan cross-linking chain, the third amino acid is L-lysine because

  1. this amino acid is positively charged

  2. this amino acid is hydrophillic

  3. this amino acid has a free amino group for the formation of peptide bond

  4. this amino acid has a large R-side chain
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C Correct answer
Explanation

This amino acid has a free amino group for the formation of peptide bond. Peptidoglycan of bacterial cell wall is made up of N-acetyl glucosamine (NAG)-N-acetyl muramic acid (NAM). To NAM, a tetra-peptide chain is attached consisting of L-alanine, D-glutamic acid, L-lysine and D-alanine. The L-lysine is attached to a penta-glycine chain sideways, which is further attached to D-alanine of the neighbouring NAG-NAM complex. Since L-lysine needs to form the penta-glycine chain, it should have a free amino group for peptide bond formation. This extensive cross linking gives rigid structure to the peptidoglycan moiety.

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