Enzyme-Substrate Complex Formation and Electron Transfer in Nitrogenase-Like Dark-Operative Protochlorophyllide Oxidoreductase (DPOR).

Nitrogenase-like dark-operative protochlorophyllide oxidoreductase (DPOR) is a two-component metalloenzyme involved in (bacterio)chlorophyll biosynthesis. DPOR enables photosynthesis in photosynthetic bacteria by catalyzing the MgATP hydrolysis-dependent, stereoselective two-electron reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This requires the repeated transient association of DPOR's two component proteins (BchL and BchNB), and involves a series of individual and unresolved sequence of events (including MgATP-hydrolysis, electron transfer, protein association/dissociation, substrate binding, etc.). DPOR shares structural and mechanistic similarities with nitrogenase, although the spectroscopic properties of Pchlide and Chlide permit the reaction to be followed in situ with visible spectroscopy. Here, we investigate DPOR's mechanism through vis-spectroscopy in the absence of an electron donor in the system, where we were able to observe the formation of the enzyme-substrate (ES) complex prior to substrate reduction (electron transfer and MgATP hydrolysis). The determination of rate constants for ES formation as well as overall electron transfer reveals the complex rate-limiting interplay between these two processes. Further, we observe evidence of cooperativity for ES complex formation in DPOR, which may be the origin of cooperativity during enzymatic turnover.