We investigated structural and functional properties of bovine cytochrome P450 steroid 21-hydroxylase (P450c21), which catalyzes hydroxylation at C-21 of progesterone and 17alpha-hydroxyprogesterone. The uncoupled H(2)O(2) formation was higher in the hydroxylation of progesterone (26% of NADPH consumed) than that of 17alpha-hydroxyprogesterone (15% of NADPH consumed), indicating that 17alpha-hydroxyprogesterone can better facilitate the O-O bond scission. In relation to this, it is noted that the O-O stretching mode (nu(O-O)) of the oxygen complex of P450c21 was sensitive to the substrate; the progesterone- or 17alpha-hydroxyprogesterone-bound enzyme gave single (at 1137 cm(-1)) or split nu(O-O) bands (at 1124 and 1138 cm(-1)), respectively, demonstrating the presence of two forms for the latter. In contrast to nu(O-O), no corresponding difference was observed for the Fe-O(2) stretching mode between two different substrate-bound forms. The Fe-S(Cys) stretching mode in the ferric state was also identical (349 cm(-1)) for each substrate-bound form, suggesting that modulation through the axial thiolate by the substrate is unlikely. Therefore, it is deduced that the hydroxyl group at C-17 of 17alpha-hydroxyprogesterone forms a hydrogen bond with the terminal oxygen atom of the FeOO complex in one form, yielding a lower nu(O-O) frequency with higher reactivity for O-O cleavage, whereas the other form in which the substrate does not provide a hydrogen bond to the oxygen ligand is essentially the same between the two kinds of substrates. In the hydrogen-bonded species, the substrate changes the geometry of the FeOO moiety, thereby performing the hydroxylation reaction more effectively in 17alpha-hydroxyprogesterone than in progesterone.