INTRAMOLECULAR ELECTRON-TRANSFER IN CYTOCHROME-O OF ESCHERICHIA-COLI - EVENTS FOLLOWING THE PHOTOLYSIS OF FULLY AND PARTIALLY REDUCED CO-BOUND FORMS OF THE BO(3) AND OO(3) ENZYMES

The events which follow photolysis of CO-inhibited fully reduced and C O-bound mixed-valence cytochrome o have been studied in two variants o f the enzyme, one of which contains heme B at the low-spin site (bo3) and the other of which contains heme O (oo3). For this, isolated enzym e was prepared from three different strains of Escherichia coli which produce these two variants in different relative amounts [Puustinen, A ., Morgan, J. E., Verkhovsky, M., Thomas, J. W., Gennis, R. B., & Wiks trom, M. (1992) Biochemistry 31, 10363-10369]. In both types of enzyme microsecond electron redistribution was observed from the oxygen-bind ing heme to the low-spin heme. In the bo3 enzyme, the rate was similar to that in the bovine enzyme (3 mus), but in the oo3 enzyme, it was s everal times slower. However, in both types of cytochrome o, the same electron redistribution process was also apparently observed on other time scales, some faster and some slower. The rate of CO rebinding in the mixed-valence enzyme was found to be slower than in the fully redu ced enzyme, apparently because of the subpopulation of oxidized oxygen -binding heme produced by the electron redistribution. The extent of t his electron redistribution, and thus the inter-heme DELTAE(m), can be calculated from this change in rate. The heme B and heme O containing low-spin sites have E(m) values about 20 and 50 mV lower, respectivel y, than the oxygen-binding heme.