PLASTOCYANIN - STRUCTURAL AND FUNCTIONAL-ANALYSIS

Plastocyanin is one of the best characterized of the photosynthetic el ectron transfer proteins. Since the determination of the structure of poplar plastocyanin in 1978, the structure of algal (Scenedesmus, Ente romorpha, Chlamydomonas) and plant (French bean) plastocyanins has bee n determined either by crystallographic or NMR methods, and the poplar structure has been refined to 1.33 angstrom resolution. Despite the s equence divergence among plastocyanins of algae and vascular plants (e .g., 62% sequence identity between the Chlamydomonas and poplar protei ns), the three-dimensional structures are remarkably conserved (e.g., 0.76 angstrom rms deviation in the Ca positions between the Chlamydomo nas and poplar proteins). Structural features include a distorted tetr ahedral copper binding site at one end of an eight-stranded antiparall el beta-barrel, a pronounced negative patch, and a flat hydrophobic su rface. The copper site is optimized for its electron transfer function , and the negative and hydrophobic patches are proposed to be involved in recognition of physiological reaction partners. Chemical modificat ion, cross-linking, and site-directed mutagenesis experiments have con firmed the importance of the negative and hydrophobic patches in bindi ng interactions with cytochrome and Photosystem I, and validated the m odel of two functionally significant electron transfer paths in plasto cyanin. One putative electron transfer path is relatively short (appro ximately 4 angstrom) and involves the solvent-exposed copper ligand Hi s-87 in the hydrophobic patch, while the other is more lengthy (approx imately 12-15 angstrom) and involves the nearly conserved residue Tyr- 83 in the negative patch.