Molecular mechanism of acetoacetyl-CoA enhanced kinetics for increased bioplastic production from Cupriavidus necator 428

Abstract

Polyhydroxyalkanoates are gaining importance due to their biodegradable nature and close analogy to plastics. Polyhydroxybutyrate (PHB) is the most widely used bioplastic from polyalkanoate family, which is produced by a legion of bacterial species via phbCAB operon encoding β-ketothiolase (PhaA), NADPH-dependent acetoacetyl-coenzyme A (acetoacetyl-CoA) reductase (PhaB) and polyhydroxyalkanoate synthase (PhaC). Augmentation in the activity of these enzymes is promising for increased PHB production which is achieved by enzyme engineering strategies including non-structural and structural approaches. Our study is deployed on directed evolution-based experimentally reported mutants of PhaB enzyme with increased efficiency due to impact on critical structural factors. We have analyzed and compared the native PhaB with two of its variants Q47L and T173S in complex with their cofactor i.e. NADPH as well as the substrate i.e. acetoacetyl-CoA, via long range molecular dynamics simulations. Interaction profile, MMPBSA, essential dynamics, and free energy landscape analysis revealed that the enzyme efficiency is critically affected by cofactor interactions. It was also observed that mutants have higher equilibrium constant with lesser but optimal affinity for substrate and cofactor than the wild type, which might be the reason for increased efficiency of the mutants via enhanced substrate and cofactor exchange rate. Our study provides insights into the cofactor and substrate binding affinities to PhaB enzyme at atomistic level, which will facilitate designing of highly efficient PhaB enzymes for increased PHB production. Communicated by Ramaswamy H. Sarma