REVERSIBLE PHOTOCHEMISTRY IN THE ALPHA-SUBUNIT OF PHYCOERYTHROCYANIN - CHARACTERIZATION OF CHROMOPHORE AND PROTEIN BY ELECTROSTATIC CALCULATIONS

We used a macroscopic dielectric model to study the effects of solvati on and interaction between titratable and permanent partial charges on the protein conformational energy and the acid-base equilibria in the cyanobacterial photoreceptor phycoerythrocyanin, whose photoreversibl e photochromic response is attributed to a Z/E isomerization of the co valently bound tetrapyrrole chromophore. The calculations revealed the stabilization of the charged protonation state of the chromophore by a small set of strong local interactions. Although the protein is glob ular and water-soluble, the complex counterion structure has a strikin g similarity to the arrangement found for the photochemical active tra nsmembrane protein bacteriorhodopsin. This could be attributed to the fact that the protonation site in the cu-subunit of phycoerythrocyanin is buried in the interior of the protein. Due to the strong shielding from solvent, the interaction pattern is conserved upon a ground-stat e isomerization of the chromophore. The partial solvent exposure of th e isomerization site resulted in a drastic influence of the chromophor e configuration on the aqueous solvation energy of the protein. Implic ations for the sensitivity of the photochemistry to environmental fact ors and molecular binding are discussed. (C) 1993 John Wiley & Sons, I nc.