OBSOLETE. A cytochrome b-like protein that has a sulfur atom ligated to the iron of the prosthetic group (heme-thiolate); enzymes: typically monooxygenases acting on, typically, lipophilic substrates. The characteristic mode of action of these enzymes is not electron transfer (some P450 enzymes probably do not even involve the reversible Fe(II)/Fe(III) equilibrium), but rather oxygen atom transfer.