Review High potential iron–sulfur proteins and their role as soluble electron carriers in bacterial photosynthesis: tale of a discovery Stefano Ciurli* & Francesco Musiani Laboratory of Bioinorganic Chemistry, Department of Agro-Environmental Science and Technology, University of Bologna Viale Giuseppe Fanin 40, 40127 Bologna, Italy; *Author for correspondence (e-mail: stefano.ciurli@unibo.it; fax: +39-051-209-6203) Received 1 October 2004; accepted in revised form 22 November 2004 Key words: bacterial photosynthesis, electron transfer, high potential iron–sulfur protein, molecular modeling, photosynthetic reaction center, protein docking, tetraheme cytochrome subunit Abstract This review is an attempt to retrace the chronicle of the discovery of the role of high-potential iron–sulfur proteins (HiPIPs) as electron carriers in the photosynthetic chain of bacteria. Data and facts are presented through the magnifying lenses of the authors, using their best judgment to filter and elaborate on the many facets of the research carried out on this class of proteins over the years. The tale is divided into four main periods: the seeds, the blooming, the ripening, and the harvest, representing the times from the discovery of these proteins to the most recent advancements in the understanding of the relationship between their structure and their function. Prologue HiPIPs are small proteins (9–10 kDa) discovered in the early 1960s, when they were shown to pos- sess a high reduction potential (ca. +350 mV) (Bartsch 1963) and an inorganic cofactor of [Fe 4 S 4 (SCys) 4 ] stoichiometry (Dus et al. 1967) (Figure 1a). The presence of the iron–sulfur cluster was confirmed by early X-ray diffraction studies that identified this core moiety as a distorted cubane (Strahs and Kraut 1968; Carter et al. 1974). The three-state hypothesis (Figure 1b), according to which the iron–sulfur cluster assumes the [Fe 4 S 4  3þ /[Fe 4 S 4  2þ /[Fe 4 S 4  1þ oxidation state, was proposed by Carter et al. to account for the different reduction potentials observed for HiPIPs (using the 3+/2+ redox couple, Figure 1c) and for the structurally analogous low-potential ferre- doxins (that use the 2+/1+ redox couple) (Carter et al. 1972). This review is a critical excursus of the major advancements that have led to the current view of the role of HiPIP in bacterial photosyn- thesis, schematically shown in Figure 2. We sug- gest that the reader refer to this Figure throughout the article in order to better follow the plot of the story. In this review, no attempt is made to com- prehensively cover the structural features of HiP- IPs, nor to extensively report on their spectroscopic properties. For these aspects, the reader is referred to a recent review covering these topics (Carter 2001). Let the tale begin... The seeds In 1961, Hori reported the isolation of a ‘brown protein’ from a halotolerant denitrifying Micrococcus, later designated as Paracoccus denitrificans. This ‘brown protein’ was expressed both under aerobic and anaerobic conditions, and in the latter case nitrate was used as terminal electron acceptor. The ‘brown protein’ was shown to have a reduction potential of +0.36 V and to behave as an electron donor in the cytochrome Photosynthesis Research (2005) 85: 115–131 Ó Springer 2005