Protochlorophyllide reductase

[4][5] There are two structurally unrelated proteins with this sort of activity, referred to as light-dependent (LPOR) and dark-operative (DPOR).

The light- and NADPH-dependent reductase is part of the short-chain dehydrogenase/reductase (SDR) superfamily and is found in plants and oxygenic photosynthetic bacteria,[6][7] while the ATP-dependent dark-operative version is a completely different protein, consisting of three subunits that exhibit significant sequence and quaternary structure similarity to the three subunits of nitrogenase.

It is part of the SDR superfamily, which includes alcohol dehydrogenase, and consists of a Rossman-fold NADPH-binding site and a substrate-specific C-terminal segment region.

The protochlorophyllide substrate is thought to bind to a cavity near the nicotinamide end of the bound NADPH.

[14] Similar to nitrogenase, the reduction mechanism relies on an electron transfer from the iron-sulfur cluster of the ATPase domain, through a secondary cluster on the catalytic heterotetramer and finally to the protochlorophyllide-bound active site (which, distinct from nitrogenase, does not contain FeMoco).