Cytochrome f from spinach and cyanobacteria: purification and characterization

Cytochrome f has been purified from spinach chloroplasts and from the photosynthetic membranes of the cyanobacterium Spirulina maxima. The spinach protein has an isoelectric point of 5.2 and gives a single band on isoelectric focusing gels. The S. maxima cytochrome shows a major band with a pI of 4.01 and a minor band with a pI of 3.97. S. maxima cytochrome f has a molecular weight approximately 38,000 and is monomeric, while the spinach protein is slightly smaller, approximately 36,000 daltons, and aggregates to form an octamer. S. maxima cytochrome f has an E'0 of +339 mV which is close to that of cytochromes f from higher plants. The NH2-terminal amino acid sequences of the cytochromes show striking similarities. Spinach cytochrome f shows a clear preference for oxidation by spinach plastocyanin and S. maxima cytochrome f is more readily oxidized by its in vivo reaction partner, cytochrome c553.