ABSTRACT
Type II isopentenyl diphosphate (IPP) isomerase catalyzes the interconversion of IPP and dimethylallyl diphosphate (DMAPP). Although the reactions catalyzed by the type II enzyme and the well-studied type I IPP isomerase are identical, the type II protein requires reduced flavin for activity. The chemical mechanism, including the role of flavin, has not been established for type II IPP isomerase. Recombinant type II IPP isomerase from Thermus thermophilus HB27 was purified by Ni2+ affinity chromatography. The aerobically purified enzyme was inactive until the flavin cofactor was reduced by NADPH or dithionite or photochemically. The inactive oxidized flavin-enzyme complex bound IPP in a Mg2+-dependent manner for which KD approximately KmIPP, suggesting that the substrate binds to the inactive oxidized and active reduced forms of the protein with similar affinities. N,N-Dimethyl-2-amino-1-ethyl diphosphate (NIPP), a transition state analogue for the type I isomerase, competitively inhibits the type II enzyme, but with a much lower affinity. pH-dependent spectral changes indicate that the binding of IPP, DMAPP, and a saturated analogue isopentyl diphosphate promotes protonation of anionic reduced flavin. Electron paramagnetic resonance (EPR) and UV-visible spectroscopy show a substrate-dependent accumulation of the neutral flavin semiquinone during both the flavoenzyme reduction and reoxidation processes in the presence of IPP and related analogues. Redox potentials of IPP-bound enzyme indicate that the neutral semiquinone state of the flavin is stabilized thermodynamically relative to free FMN in solution.
