Structure-directed construction of a high-performance version of the enzyme FabG from the photosynthetic microorganism Synechocystis sp. PCC 6803.

PhaB (acetoacetyl-CoA reductase) catalyzes the reduction of acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA in polyhydroxybutyrate (PHB) synthesis and FabG (3-ketoacyl-acyl-carrier-protein reductase) catalyzes the β-ketoacyl-ACP to yield (R)-3-hydroxyacyl-ACP in fatty acid biosynthesis. Both of them have been classified into the same group EC 1.1.1. PhaB is limited with substrate specificities, while FabG was considered as a potential PhaB due to broad substrate selectivity despite of low activity. Here, X-ray crystal structures of FabG and PhaB from the photosynthetic microorganism Synechocystis sp. PCC 6803 were resolved. Based on them, a high-performance FabG on acyl-CoA directed by structural evolution was constructed that may serve as a critical enzyme to partition carbon flow from fatty acid synthesis to PHA.