The only amino acid shown to make direct contact with the substrate during the course of the reaction was M112: the methyl makes a weak hydrophobic interaction with 3 during substrate binding in the proS (Figure ), whilst the amide backbone of M112 makes a weak hydrogen bond with 3 in the proR transition state (Figure ). Mutating M112 to alanine gave small reduction in enantioselectivity (Table , entry 5). Whilst alanine can also make hydrophobic interactions, its reduced chain length would preclude interactions with 3 in this instance and thus M112 could be helping to stabilise the substrates to a small extent. No difference would be expected for the proR pathway as alanine can still participate in amide backbone bonding. The lack of side chain interactions with the substrates and no clear results from the alanine scanning suggests that a rational approach to designing the active site via mutagenesis may not lead to improved enzyme. Indeed, the exposed nature of the active site on the side of the protein suggests that extensive backbone engineering to build up the protein bulk around the substrates may lead to more promising candidates for enantioselective catalysis.