From a dimer to a monomer: Construction of a chimeric monomeric isocitrate dehydrogenase

Many isocitrate dehydrogenases (IDHs) are dimeric enzymes whose catalytic sites are located at the intersubunit interface, whereas monomeric IDHs form catalytic sites with single polypeptide chains. It was proposed that monomeric IDHs were evolved from dimeric ones by partial gene duplication and fusion, but the evolutionary process had not been reproduced in laboratory. To construct a chimeric monomeric IDH from homo-dimeric one, it is necessary to reconstitute an active center by a duplicated region; to properly link the duplicated region to the rest part; and to optimize the newly formed protein surface. In this study, a chimeric monomeric IDH was successfully constructed by using homo-dimeric Escherichia coli IDH as a start point by rational design and site-saturation mutagenesis. The ~67 kDa chimeric enzyme behaved as a monomer in solution, with a K_m of 61 μM and a k_cat of 15 s^-1 for isocitrate in the presence of NADP⁺ and Mn²⁺ . Our result demonstrated that dimeric IDHs have a potential to evolve monomeric ones. The evolution of the IDH family was also discussed.