Dark-operative protochlorophyllide reductase and nitrogenase: common mechanism for reduction of stable multiple-bonds

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In final stages of biosynthesis of Chlorophylls (Chls), dark-operative protochlorophyllide (Pchlide) oxidoreductase (DPOR), a nitrogenase-like enzyme, reduces the C17=C18 double bond of Pchlide to form chlorophyllide, the direct precursor of Chl a. Crystallographic structure of DPOR suggested that the spatial arrangement of the proton donors determines the stereospecificity of the Pchlide reduction. In this study, reaction intermediates were trapped by blocking the proton transfer and they were detected by absorption and electron spin resonance spectroscopies. We proposed that the DPOR reaction consists of 1) generation of a Pchlide anion radical by a single electron transfer reaction from a [4Fe-4S]-cluster to Pchlide, 2) a second Pchlide neutral radical is formed by a single proton transfer, 3) a second electron transfer eliminates the Pchlide radical, and 4) a second proton transfer completes the reaction. DPOR is a unique radical enzyme with a structure common to nitrogenase.