PHASE-DIAGRAMS OF A CRYSTALLINE MEMBRANE-PROTEIN, BOVINE HEART CYTOCHROME-C-OXIDASE, IN THE SALTING-IN REGION

Phase diagrams were determined for cytochrome c oxidase crystals, based on the protein concentration changes with time in the supernatant due to crystal growth. The solubility of the protein decreases when the concentration of sodium phosphate buffer, pH 7.4, decreases in the range of 0.5mM-10mM, as well as when the concentration of Brij-35, a detergent stabilizing this enzyme, increases in the examined range (0%-14%). The precipitated protein was all crystalline without any amorphous materials. These results indicate a "salting-in" of the enzyme due to the phosphate ion at any fixed Brij-35 concentration. Thus, this protein is most readily crystallized either by addition of Brij-35 or by concentration of the protein solution without changing the buffer concentration (with an ultrafiltration apparatus). Nucleation and crystal growth rates were also determined. Instantaneous nucleation always occurred even when the initial protein concentration was only 24% higher than the solubility of the crystal. The present results suggest that the mechanism of crystal growth of large membrane proteins is significantly different from that of small water-soluble proteins.