In enzymology, precorrin-6A reductase () is an enzyme that catalyzes the chemical reaction
The three substrates of this enzyme are precorrin-6A, reduced nicotinamide adenine dinucleotide phosphate (NADPH) and a proton. Its products are precorrin-6B and oxidised NADP<sup>+</sup>.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH=CH group of an acceptor with NAD or NADPH as donor. The systematic name of this enzyme class is precorrin-6B:NADP+ oxidoreductase. Other names in common use include precorrin-6X reductase, precorrin-6Y:NADP+ oxidoreductase and CobK. This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in aerobic bacteria.