Ferredoxin from a red alga, Porphyra umbilicalis.

A plant-algal type ferredoxin was isolated from the red alga, Porphyra umbilicalis. In its oxidised form the ferredoxin had absorption maxima at 277, (281), 323, 420 and 462 nm. Two atoms each of non-haem iron and labile sulphur were present per molecule protein. The midpoint potential of the protein was -400 mV and it effectively mediated electron transport in the NADP-photoreduction system of barley. The amino acid composition of Porphyra umbilicalis ferredoxin was determined as (Lys4, His2, Arg1, Asx10, Thr8, Ser7, Glx16-17, Pro3, Gly7, Ala8, Cys5, Val6, Met1, Ile5, Leu8, Tyr5, Phe2). The minimum molecular weight of approximately 11000 was confirmed by sedimentation-equilibrium studies in the analytical ultracentrifuge. Approaching half of the total amino acid sequence was determined by means of an automatic sequencer.