Chapter 26.1, Problem 2P

Interpretation Introduction

Interpretation:

The reason why cysteine has R configuration while all the others have S configuration.

Concept introduction:

Except glycine, all the other 19 amino acids have chiral centers and hence can occur as two enantiomers. But the naturally occurring enantiomers resemble the L sugars, i.e. placing the CO₂- group at the top and pointing the side chain downwards and the NH₃ group on the left. These usually have S configuration, which is dependent on the priority of the attached molecules. But this is not the case for cysteine as it has R configuration.

To determine:

Why cysteine is the L amino amino acid that occurs in R configuration naturally?