The structure of plastocyanin from the ­cyanobacterium Phormidium laminosum

The crystal structure of the `blue' copper protein plastocyanin from the cyanobacterium Phormidium laminosum has been solved and refined using 2.8 Å X-­ray data. P. laminosum plastocyanin crystallizes in space group P4₃2₁2 with unit-cell dimensions a = 86.57, c = 91.47 Å and with three protein molecules per asymmetric unit. The final residual R is 19.9%. The structure was solved using molecular replacement with a search model based on the crystal structure of a close homologue, Anabaena variabilis plastocyanin (66% sequence identity). The molecule of P. laminosum plastocyanin has 105 amino-acid residues. The single Cu atom is coordinated by the same residues - two histidines, a cysteine and a methionine - as in other plastocyanins. In the crystal structure, the three molecules of the asymmetric unit are related by a non-crystallographic threefold axis. A Zn atom lies between each pair of neighbouring molecules in this ensemble, being coordinated by a surface histidine residue of one molecule and by two aspartates of the other.