ИСТИНА

Cytochrome bd is a tri-heme (b558, b595, d) terminal oxidase that couples the electron transfer from quinol to O2 with generation of a proton-motive force. We studied photolysis and subsequent recombination of CO with cytochrome bd-I from Escherichia coli in one-electron reduced (MV) and fully reduced (R) states by timeresolved absorption spectroscopy at 532-nm excitation. Probing the Soret and visible band regions, it is found that CO photodissociation from MV enzyme causes fast (within 1.5 μs) electron transfer from heme d to heme b595, which is not reported earlier. Unlike cytochrome bd in the R state, in MV enzyme the apparent contribution of absorbance changes associated with CO dissociation from heme d is small, if any. Photodissociation of CO from heme d in MV enzyme is suggested to be accompanied by the immediate binding of an internal ligand at the opposite side of the heme. Through the following transition, CO recombines with heme d yielding a transient hexacoordinate state. Then the ligand slowly dissociates from heme d.