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In enzymology, protochlorophyllide reductases are enzymes that catalyze the conversion from protochlorophyllide to chlorophyllide a. They are oxidoreductases participating in the biosynthetic pathway to chlorophylls.
There are two structurally unrelated proteins with this sort of activity, referred to as light-dependent and dark-operative. The light- and NADPH-dependent reductase is part of the short-chain dehydrogenase/reductase superfamily and is found in plants and oxygenic photosynthetic bacteria, 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. This enzyme may be evolutionary older; due to its bound iron-sulfur clusters is highly sensitive to free oxygen and does not function if the atmospheric oxygen concentration exceeds about 3%. It is possible that evolutionary pressure associated with the great oxidation event resulted in the development of the light-dependent system.
The light-dependent version uses NADPH:
While the light-independent or dark-operative version uses ATP and ferredoxin: