Ferredoxin-sulfite Reductase from a Cyanobacterium, Spirulina platensis

Abstract

Ferredoxin-sulfite reductase (Fd-SiR) [hydrogen-sulfide: ferredoxin oxidoreductase, EC 1.8.7.1] from a cyanobacterium, Spirulina platensis, has been purified to homogeneity by a novel procedure. Subunit analysis by sodium dodecyl sulfate gel electrophoresis yielded a single protein band with a molecular weight of 63, 000. In the presence of 0.1 M potassium sulfate, gel filtration produced a value of 120, 000, indicating the presence of a dimer in this ionic environment. A plot of ferredoxin concentration versus enzymatic (Fd-SiR) activity yielded a sigmoidal curve, giving a Hill coefficient (ñ) of 2.2. Purified Fd-SiR, in the oxidized form, has absorption maxima at 279, 388, and 590 nm. Thus the enzyme has the properties of a siroheme-containing protein.