Question

Which amino acid R group can form hydrogen bonds with asparginine R group? 1) isoleucine 2) valine 3) tryptophan 4) alanine 5) More than one of the amino acids above can form hydrogen bonds with asparginine.

Answer 1

3) Tryptophan

R group of asparagine can form hydrogen bonds with R group of tryptophan.

All the given remaining amino acids isoleucine, alanine and valine are amino acids with non-polar side chains.

Tryptophan is the only given polar amino acid or the amino acid with a polar side.

Imino group of tryptophan (-NH-) which is present in its side chain is polar. There is a partial positive charge on hydrogen atom of imino group.

In asparagine the polarity of the side chain is due to the presence of an amide group (-CONH₂). In the carbonyl group oxygen is more electronegative atom.

A hydrogen bond is a bond which is formed between a hydrogen and another high electronegative atom.

Thus a hydrogen bond can be formed between the hydrogen in the imino group of tryptophan and oxygen of the amide group in asparagine.

It is the polarity of bond which contributes majorly to the formation of hydrogen bond. As there is no considerable bond polarity in the side chains of alanine, isoleucine and valine, their side chains can not form hydrogen bond with asparagine.