Highly enantioselective reduction of benzophenones by engineered Geotrichum candidum alcohol dehydrogenase

Biocatalytic approaches have gained increasing attention as sustainable alternatives to metal-catalyzed asymmetric reductions of ketones to obtain enantiopure alcohols, important intermediates for pharmaceutical synthesis. For example, enzyme-catalyzed reduction of substituted benzophenone analogs to produce chiral diaryl methanols has attracted interest, as they are the key intermediates in the synthesis of antihistamines. However, benzophenone analogs are difficult to be reduced by enzymes due to steric hindrance. Moreover, the similarities between the two groups adjacent to the carbonyl group make achieving high enantioselectivity in reduction challenging. In this study, we examined the reduction of benzophenone and its analogs by Geotrichum candidum acetophenone reductase (GcAPRD). However, the wild type did not exhibit activity toward benzophenone due to the substrate's bulkiness. Then, two mutants of GcAPRD (Trp288Ala and Phe56Ile/Trp288Ala) were applied to catalyze the reduction of benzophenone, resulting in high reduction yield (≥ 80%). In addition, both mutants exhibited catalytic activity toward methyl- and halogen-substituted benzophenones, especially toward 3- and 4-substituted substrates. Regarding enantioselectivity, Trp288Ala generally reduced both 3- and 4-substituted substrates to (R)-alcohols with up to 97% ee. In contrast, Phe56Ile/Trp288Ala reduced 3-substituted substrates to (R)-alcohols with up to 89% ee but reduced 4-substituted substrates to (S)-alcohols with up to 92% ee. At last, the reduction mechanism was investigated using molecular docking simulations. KEY POINTS: • GcAPRD mutants exhibited catalytic performance toward benzophenone analogs. • GcAPRD Phe56Ile/Trp288Ala exhibited substituent-dependent enantioselectivity. • Introducing Phe56Ile into GcAPRD Trp288Ala resulted in a clear enantiopreference.