Primary structures of P450 proteins. (a) Typical features of an ER-bound P450 protein (class II enzyme). The function of the different domains and regions indicated by colors are described in the text. (b) Variants of this canonical structure most commonly found: 1, soluble class I; 2, mitochondrial class I; 3, membrane-bound or plastidial class III. The three-dimensional folding of these structures can be viewed at [17,18]. A good (model) picture of membrane-bound P450 can be seen at .