Structure of an intermolecular electron-transfer complex: p-cresol methylhydroxylase at 6.0-A resolution

The structure of p-cresol methylhydroxylase [4-cresol:(acceptor) oxidoreductase (methyl-hydroxylating), EC 1.17.99.1], a flavocytochrome c, has been determined at 6.0-A resolution. The structure analysis is based on two heavy-atom derivatives with anomalous scattering and 2-fold averaging about a noncrystallographic axis. The molecule is an alpha 2 beta 2 tetramer with a cytochrome subunit of Mr approximately 8500 and a flavoprotein subunit of Mr approximately 49,000. The flavoprotein subunits are tightly packed about the molecular 2-fold axis, whereas the cytochrome subunits are located on the outside of the molecule, each in a depression on the surface of a flavoprotein subunit. The results of this study have led to the following conclusions. The alpha 2 beta 2 quaternary structure of the enzyme is different from alpha beta as originally thought. The orientation of the cytochrome subunit and the surface complementarity of the cytochrome and flavoprotein subunits are clearly defined. The cytochrome subunit is similar in size to other small bacterial cytochromes but probably forms a distinct subclass. The titration (by substrate) behavior of the enzyme and other kinetic properties are rationalized by its quaternary structure.