Towards structural determination of the water-splitting enzyme - Purification, crystallization, and preliminary crystallographic studies of photosystem ii from a thermophilic cyanobacterium

A photosystem II preparation from the thermophilic cyanobacterium Synechoco ccus elongatus, which is especially suitable for three-dimensional crystall ization in a fully active form was developed. The efficient purification me thod applied here yielded 10 mg of protein of a homogenous dimeric complex of about 500 kDa within 2 days. Detailed characterization of the preparatio n demonstrated a fully active electron transport chain from the manganese c luster to plastoquinone in the Q(B) binding site. The oxygen-evolving activ ity, 5000-6000 mu mol of O-2/(h.mg of chlorophyll), was the highest so far reported and is maintained even at temperatures as high as 50 degrees C, Th e crystals obtained by the vapor diffusion method diffracted to a resolutio n of 4.3 Angstrom The space group was determined to be P2(1)2(1)2(1) with f our photosystem II dimers per unit cell. Analysis of the redissolved crysta ls revealed that activity, supramolecular organization, and subunit composi tion were maintained during crystallization.