The catalytic efficiency of 21-hydroxylase for conversion of progesterone in humans is approximately 1.3 x 10^7 M-1s-1 at 37 °C. This makes it the most catalytically efficient P450 enzyme of those reported to date, and catalytically more efficient than the closely related bovine 21-hydroxylase enzyme.[1] C-H bond breaking to create a primary carbon radical is thought to be the rate-limiting step in the hydroxylation.[2]