X-ray structure of a truncated form of cytochrome f from Chlamydomonas reinhardtii

A truncated form of cytochrome f from Chlamydomonas reinhardtii (an importa nt eukaryotic model organism for photosynthetic electron transfer studies) has been crystallized (space group P2(1)2(1)2(1); three molecules/asymmetri c unit) and its structure determined to 2.0 Angstrom resolution by molecula r replacement using the coordinates of a truncated turnip cytochrome f as a model. The structure displays the same folding and detailed features as tu rnip cytochrome f, including (a) an unusual heme Fe ligation by the alpha-a mino group of tyrosine 1, (b) a cluster of lysine residues (proposed dockin g site of plastocyanin), and (c) the presence of a chain of seven water mol ecules bound to conserved residues and extending between the heme pocket an d K58 and K66 at the lysine cluster. For this array of waters, we propose a structural role. Two cytochrome f molecules are related by a noncrystallog raphic symmetry operator which is a distorted proper 2-fold rotation. This may represent the dimeric relation of the monomers in situ; however, the he me orientation suggested by this model is not consistent with previous EPR measurements on oriented membranes.