On the role of high-potential iron-sulfur proteins and cytochromes in the respiratory chain of two facultative phototrophs

The capability of high potential iron–sulfur proteins (HiPIPs) and soluble cytochromes to shuttle electrons between the bc 1 complex and the terminal oxidase in aerobically grown cells of Rhodoferax fermentans and Rhodospirillum salinarum , two facultative phototrophs, was evaluated. In Rs. salinarum , HiPIP and a c -type cytochrome (α-band at 550 nm, E m,7 =+290 mV) are both involved in the electron transfer step from the bc 1 complex to the terminal oxidase. Kinetic studies indicate that cytochrome c 550 is more efficient than HiPIP in oxidizing the bc 1 complex, and that HiPIP is a more efficient reductant of the terminal oxidase as compared to cytochrome c 550 . Rs. salinarum cells contain an additional c -type cytochrome (asymmetric α-band at 556 nm, E m,7 =+180 mV) which is able to reduce the terminal oxidase, but unable to oxidize the bc 1 complex. c -type cytochromes could not be isolated from Rf. fermentans , in which HiPIP, the most abundant soluble electron carrier, is reduced by the bc 1 complex (zero-order kinetics) and oxidized by the terminal oxidase (first-order kinetics), respectively. These data, taken together, indicate for the first time that HiPIPs play a significant role in bacterial respiratory electron transfer.