报告题目：Functional evolution and constraint in an enzyme family
报 告 人：Antony Dean教授，University of Minnesota, USA
Understanding how and why some enzymes evolve new functions while others do not is an outstanding problem in molecular evolution.
Experiments show that an engineered NAD-dependent E. coli isocitrate dehydrogenase (IDH) is selectively inferior to the wildtype NADP-dependent IDH during competition for acetate, an oxidized carbon source where additional reducing power, in the form of reduced NADPH, is needed for biosynthesis. The selection intensifies as other sources of NADPH are deleted from the genetic background. Genome surveys show that all species capable of growing on acetate have an NADP-dependent IDH. Those that have only an NAD-dependent IDH cannot grow on acetate. The 3.5 billion-year-old switch from NAD to NADP is an adaptation to growth on oxidized carbon sources.
The related isopropylmalate dehydrogenases (IMDH) all use NAD. The reduced fitness of an engineered NADP-dependent E. coli IMDH is attributable to product inhibition by abundant cellular NADPH which forms a strong hydrophobic interaction with the gamma-isopropyl of the substrate/product. This inhibition explains why no IMDH has ever evolved the capacity to use NADP. In contrast, NADPH is a weak inhibitor of IDH because there is no attractive force between the uncharged ring and the adjacent negatively charged gamma-carboxylate of the substrate/product. Hence, IDH has always been free to evolve NADP use. The pattern of functional constraint and evolution across 3.5 billion years of life finds its explanation in terms of just two physiochemical interactions (the one hydrophobic and the other electrostatic) and a metabolic demand for additional reducing power during growth on acetate.