Novel insight into the copper-ligand geometry in the crystal structure of Ulva pertusa plastocyanin at 1.6-angstrom resolution - Structural basis forregulation of the copper site by residue 88

The crystal structure of plastocyanin from a green alga, Ulva pertusa, has been determined at 1.6-Angstrom resolution. At its copper site, U. pertusa plastocyanin has a distorted tetrahedral coordination geometry similar to o ther plastocyanins. In comparison with structures of plastocyanins reported formerly, a Cu(II)-S-delta(Met(92)) bond distance (2.69 Angstrom) is short er by about 0.2 Angstrom and a Cu(II)-S-gamma(Cys(84)) distance is longer b y less than 0.1 Angstrom in U. pertusa plastocyanin. These subtle but signi ficant differences are caused by the structural change at a His-Met loop (H is(87)- Met(92)) due to an absence of a O(Asp(85))-O-gamma(Ser(88)) hydroge n bond which is found in Enteromorpha prolifera plastocyanin. In addition, poplar and Chlamydomonas reinhardtii plastocyanins with a glutamine at resi due 88 have a weak cation-rr interaction with Tyr(83). This interaction len gthens the Cu(II)-S-delta(Met(92)) bond of poplar and C, reinhardtii plasto cyanins by 0.14 and 0.20 Angstrom respectively. As a result of structural d ifferences, U. pertusa plastocyanin has a less distorted geometry than the other plastocyanins. Thus, the cupric geometry is finely tuned by the inter actions between residues 85 and 88 and between residues 83 and 88, This res ult implies that the copper site is more flexible than reported formerly an d that the rack mechanism would be preferable to the entatic theory. The Hi s-Met loop may regulate the electron transfer rate within the complex betwe en plastocyanin and cytochrome f.