THE 1.5-ANGSTROM CRYSTAL-STRUCTURE OF PLASTOCYANIN FROM THE GREEN-ALGA CHLAMYDOMONAS-REINHARDTII

The crystal structure of plastocyanin from the green alga Chlamydomona s reinhardtii has been determined at 1.5-angstrom resolution with a cr ystallographic R factor of 16.8%. Plastocyanin is a small (98 amino ac ids), blue copper-binding protein that catalyzes the transfer of elect rons in oxygenic photosynthesis from cytochrome f in the quinol oxidas e complex to P700+ in photosystem I. Chlamydomonas reinhardtii plastoc yanin is an eight-stranded, antiparallel beta-barrel with a single cop per atom coordinated in quasi-tetrahedral geometry by two imidazole ni trogens (from His-37 and His-87), a cysteine sulfur (from Cys-84), and a methionine sulfur (from Met-92). The molecule contains a region of negative charge surrounding Tyr-83 (the putative distant site of elect ron transfer) and an exclusively hydrophobic region surrounding His-87 ; these regions are thought to be involved in the recognition of react ion partners for the purpose of directing electron transfer. Chlamydom onas reinhardtii plastocyanin is similar to the other plastocyanins of known structure, particularly the green algal plastocyanins from Ente romorpha prolifera and Scenedesmus obliquus. A potential ''through-bon d'' path of electron transfer has been identified in the protein that involves the side chain of Tyr-83, the main-chain atoms between residu es 83 and 84, the side chain of Cys-84, the copper atom, and the side chain of His-87.