Plastocyanin : Possible Significance of Quaternary Structure

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Abstract
Some properties of the blue Cu protein plastocyanin from the green alga Scenedesmus acutus were investigated and compared with that from spinach [Spinacia oleracea], including amino acid composition, isoelectric point and Cu content. The protein from Scenedesmus contained 2, that from spinach 4 Cu atoms per MW of 40,000. A combination of sodium dodecylsulfate/polyacrylamide gel electrophoresis and quantification of sulfhydryl [SH] groups indicated a strong preference for a species composed of 4 polypeptide chains of identical amino acid composition representing the enzymically active entity. Due to various treatments, the subunits of both plastocyanins were detected as either monomer species alone or as monomer and dimer in a ratio of 2:1 on sodium dodecylsulfate gels. The 4 -SH groups per molecule were of different reactivity: 2 -SH groups were detected after destruction of the chromophore; 2 more (forming an S-S bridge in the dimer) became evident after appropriate reduction. A KCN treatment for production of apoprotein was reported and the use of electrodialysis to improve incomplete apoprotein formation. These studies lend support to the proposal of a quanternary structure. Apoproteins were subjected to dodecylsulfate gel analysis, which proved to be an effective means of estimating both the extent of apoprotein formation and its reconstitution to the holoprotein.