Some Properties and Amino Acid Sequence of Plastocyanin from a Green Alga, Ulva arasakii

Abstract

Plastocyanin was purified from a multicellular, marine green alga, Ulva arasakii, by conventional methods to homogeneity. The oxidized plastocyanin showed absorption maxima at 252, 276.8, 460, 595.3, and 775 nm, and shoulders at 259, 265, 269, and 282.5 nm; the ratio A276.8/A595.3 was 1.5. The midpoint redox potential was determined to be 0.356 V at pH 7.0 with a ferri- and ferrocyanide system. The molecular weight was estimated to be 10,200 and 11,000 by SDS-PAGE and by gel filtration, respectively. U. arasakii also has a small amount of cytochrome c6, like Enteromorpha prolifera. The amino acid sequence of U. arasakii plastocyanin was determined by Edman degradation and by carboxypeptidase digestion of the plastocyanin, six tryptic peptides, and five staphylococcal protease peptides. The plastocyanin contained 98 amino acid residues, giving a molecular weight of 10,236 including one copper atom. The complete sequence is as follows: AQIVKLGGDDGALAFVPSKISVAAGEAIEFVNNAGFPHNIVFDEDAVPAGVDADAISYDDYLNSKGETV VRKLSTPGVY G VYCEPHAGAGMKMTITVQ. The sequence of U. arasakii plastocyanin is closet to that of the E. prolifera protein (85% homology). A phylogenetic tree of five algal and two higher plant plastocyanins was constructed by comparing the amino acid differences. The branching order is considered to be as follows: a blue-green alga, unicellular green algae, multicellular green algae, and higher plants.