Changing the substrate specificity of P450cam towards diphenylmethane by semi-rational enzyme engineering

A focused library comprising nine residues of the active site of P450cam monooxygenase resulting in similar to 300 000 protein variants was screened for activity on diphenylmethane (DPM). The assay was based on the depletion of NADH by an in vitro reconstituted P450cam system in a 96-well scale. The throughput was increased by the parallel cultivation, purification and analysis of 20 variants per well (cluster screening). Thus similar to 20 000 protein variants could be screened in summary of which five were found to transform DPM with a specific activity of up to 75% of the wild-type activity on D-camphor and a coupling rate of 7-18%. One variant converting DPM to 4-hydroxydiphenylmethane (4HDPM) was subjected to site-directed mutagenesis and saturation mutagenesis, which revealed the particular importance of positions F87, Y96 and L244 for substrate selectivity and the possibility for further improvements of this variant. Moreover, a reduction in size of the amino acid at position 396 decreased specific activity dramatically but increased coupling and switched the main product formation from 4HDPM towards diphenylmethanol.