Higher Plant and Cyanobacterial Photosystem I: Connected Cytochrome Pathways

Abstract

Oxygenic photosynthesis is the principal converter of sunlight into chemical energy on earth. The conversion of solar energy is catalyzed by four multi-subunit membrane protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b6-f complex (cytb6f) and ATP-synthase (FOF1). These protein complexes are connected by soluble electron carriers that are vital not only for the proper function of ATP and NADPH production but also to render the system highly efficient in different organisms and various environments, some of which are quite harsh. While the main fabric of the membrane complexes is highly conserved, their surfaces and interaction with the soluble factors provide the specificity and fine regulation of the operating system. One of the prime examples for this phenomenon is the cyanobacterial photosynthetic electron transport chain that is situated alongside with respiratory complexes, yet it stays unique by virtue of the interacting soluble components. Cyanobacteria contain many different cytochromes that potentially can donate electrons to both PSI and cytochrome oxidase, yet the two systems can operate separately in a synchronous mode. The crystal structure determination of photosynthetic and respiratory protein complexes shed light on the various partial reactions and explains how they can function alongside each other.