Highly purified hydroxylamine oxidoreductase derived from Nitrosomonas europaea: Some physicochemical and enzymatic properties

Hydroxylamine oxidoreductase [EC 1.7.3.4] of Nitrosomonas europaea was purified to an electrophoretically homogeneous state and some of its properties were studied.The molecular weight of the enzyme as determined by gel filtration on Sephadex G-150 and by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate is 175,000-180,000, while the minimum molecular weight per heme determined from the dry weight and heme content is 17,500. The enzyme is a C-type cytochrome; its reduced form shows absorption peaks at 418 (γ peak), 521 (β peak), 553 (α peak), and 460 nm (due to an unidentified chromophore). Although the α peak at 553 nm has a shoulder at 559 nm, the enzyme does not possess protoheme or a cytochrome b subunit. It seems likely that the enzyme molecule possesses heme c molecules in different states.The enzyme reacts rapidly with various eukaryotic cytochromes c, but does not react with "bacterial-type" cytochromes c. Although the enzyme does not react with cytochrome c-552 (N. europaea), another C-type cytochrome of the organism, cytochrome c-554 (N. europaea) acts as an electron acceptor for the enzyme.