The fidelity of gramicidin S synthetase

The amino acid analog L-cyclohexylalanine, which may be considered an analog of the hydrophobic amino acids leucine and valine, was found to thioester-bind to the heavy enzyme of gramicidin S synthetase. The results indicate that it preferably binds to the thiol site of leucine, although binding to the valine site also occurs. In the synthesis of the cyclic decapeptide by gramicidin S synthetase, the results suggest that L-cyclohexylalanine can replace L-leucine and L-valine. We have also found that in the synthesis of the cyclic decapeptidase L-leucine can replace L-valine and vice versa. The results further indicate that the previously reported thioester binding of D and L-phenylalanine to the heavy enzyme takes place at the thiol site of leucine.