Enzymatic systems with homology to nitrogenase

Abstract

Nitrogenase-like dark operative protochlorophyllide oxidoreductase (DPOR) is involved in the two-electron reduction of protochlorophyllide to form chlorophyllide during chlorophyll biosynthesis. Formation of bacteriochlorophyll additionally requires a structurally related enzyme system which is termed chlorophyllide oxidoreductase (COR). During DPOR catalysis, the homodimeric subunit ChlL(2) transfers electrons to the corresponding heterotetrameric catalytic subunit (ChlN/ChlB)(2). Analogously, subunit BchX(2) of the COR enzymes delivers electrons to subunit (BchY/BchZ)(2). The ChlL(2) protein is a dynamic switch protein triggering the ATP-dependent transfer of electrons via a [4Fe-4S] cluster onto a second [4Fe-4S] cluster located on subunit (ChlN/ChlB)(2). This initial electron transfer step of DPOR catalysis clearly resembles nitrogenase catalysis. However, the subsequent substrate reduction process was proposed to be unrelated since no molybdenum-containing cofactor or a P-cluster equivalent is employed. To investigate the transient interaction of both subcomplexes ChlL(2) and (ChlN/ChlB)(2) and the resulting electron transfer processes, the ternary DPOR enzyme holocomplex was trapped as an octameric (ChlN/ChlB)(2)(ChlL(2))(2) complex after incubation with non-hydrolyzable ATP analogs. Electron paramagnetic resonance spectroscopic experiments of various DPOR complexes in combination with circular dichroism spectroscopic experiments of the ChlL(2) protein revealed a detailed redox catalytic cycle for nucleotide-dependent DPOR catalysis.