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

在叶绿素生物合成的最后阶段，暗操作类固氮酶原叶绿素氧化还原酶(DPOR)还原Pchlide的C17=C18双键，形成Chla的直接前体--叶绿素。DPOR的晶体结构表明，质子供体的空间排列决定了Pchlide还原的立体专一性。在本研究中，通过阻止质子转移来捕获反应中间产物，并通过吸收光谱和电子自旋共振光谱对它们进行检测。我们认为，DPOR反应包括：1)通过从[4Fe-4S]-簇到Pchlide的单电子转移反应生成Pchlide阴离子自由基，2)通过单质子转移生成第二个Pchlide中性自由基，3)第二次电子转移消除Pchlide自由基，4)第二质子转移完成反应。DPOR是一种独特的自由基酶，具有固氮酶的共同结构。

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.