Fig. 1.

Overview of substrate and reaction properties of C₂. (a) Structures of fully reduced flavin, C4a-hydroperoxyflavin and HPA; the face of flavin shown here is the re side. The pK_a value of the two-electron reduced flavin is 6.5, and many flavoenzymes are known to preferentially bind the anionic form of the reduced cofactor (4). On these bases, we have assumed that the C₂-bound flavin is in the anionic state. (b) Scheme of the catalytic cycle of HPA hydroxylase C₂ (7).