Form and Function of Cytochrome b-559

Abstract

Cytochrome b-559 is an integral part of all Photosystem II reaction centers. The cytochrome is composed of two polypeptides that are linked by a single heme. The linkage is created by two histidine residues that serve as axial ligands for the protoheme. Each polypeptide provides one histidine. The structure of cytochrome b-559 is not known, nor is its organization in the Photosystem II reaction center. Although genetic and biochemical data show that cytochrome b-559 is composed of two different subunits, $α$ and $β$, it is not known if the native protein is a heterodimer($α$$β$) or two homodimers ($α$2 and $β$2). The function of cytochrome b-559 is also a mystery, although it is clear that the cytochrome is not involved in the primary electron transfer reactions of Photosystem II. Among its more perplexing features is the occurrence of two different thermodynamic forms, one high potential and one low potential, that differ by 300 mV Furthermore, although cytochromes typically function as electron carriers, cytochrome b-559 is a reluctant electron donor or acceptor. Under physiological conditions its detectable light-induced turnover is negligible. When conditions are selected to promote a significant light-induced turnover, the rate of electron transfer to or from the cytochrome is slow compared to the rate of photosynthetic electron transport. The sluggish behavior of cytochrome b-559 has prompted suggestions that it is involved in secondary electron transport that serves to protect Photosystem II against excess light. This chapter focuses on the structural organization of cytochrome b-559 in Photosystem II and current models to explain its role in photosynthesis.