A Tale of Two Subunits: How the Neomorphic R132H IDH1 Mutation Enhances Production of αHG

Heterozygously expressed single-point mutations in isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2, respectively) render these dimeric enzymes capable of producing the novel metabolite alpha-hydroxyglutarate (alpha HG). Accumulation of alpha HG is used as a biomarker for a number of cancer types, helping to identify tumors with similar IDH mutations. With IDH I, it has been shown that one role of the mutation is to increase the rate of conversion from alpha KG to alpha HG. To improve our understanding of the function of this mutation, we have detailed the kinetics of the normal (isocitrate to alpha KG) and neomorphic (alpha KG to alpha HG) reactions, as well as the coupled conversion of isocitrate to alpha HG. We find that the mutant IDH1 is very efficient in this coupled reaction, with the ability to form alpha HG from isocitrate and NADP(+). The wild type/wild type IDH1 is also able to catalyze this conversion, though it is much more sensitive to concentrations of isocitrate. This difference in behavior can be attributed to the competitive binding between isocitrate and alpha KG, which is made more favorable for alpha KG by the neomorphic mutation at arginine 132. Thus, each partial reaction in the heterodimer is functionally isolated from the other. To test whether there is a cooperative effect resulting from the two subunits being in a dimer, we selectively inactivated each subunit with a secondary mutation in the NADP/H binding site. We observed that the remaining, active subunit was unaffected in its associated activity, reinforcing the notion of each subunit being functionally independent. This was further demonstrated using a monomeric form of IDH from Azotobacter vinelandii, which can be shown to gain the same neomorphic reaction when a homologous mutation is introduced into that protein.