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

Abstract

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.