Iron-sulfur Proteins: Part II Valence-specific Assignment in Oxidized Hipip through 1H NMR Spectroscopy

Abstract

High potential iron-sulfur proteins (HiPIP) are a small class of iron-sulfur proteins found in several photosyntetic bacteria [1]. HiPIP contain a [4Fe–4S] cluster [2] which is bound to the protein backbone as exemplified in Figure 1 for C. vinosum HiPIP [3–5]. It has been proposed that these metalloproteins may act as electron carriers that shuttle electrons from the bc1 complex to the bacteriochlorophyll contained in the photosynthetic reaction center [6]. This function is usually carried out by cytochrome c2, but some degree of complementarity has been found between cyt c2 and HiPIP in many species of bacteria, thus suggesting this hypothesis [6]. The redox process involves the [Fe4S4]3+/2+ couple [7]. The redox property peculiar to HiPIP is the high potential at which they transfer electrons (Eo‘ = +50 + +450 mV) [8] as compared to the much more widespread ferrodoxins (Eo’ = -250 + - 650 mV) [9, 10], containing an analogous [4Fe-4S] cluster, but in which the redox couple [Fe4S4]2+/1+ is involved. A large collection of spectroscopic data has been gathered on these proteins, in an effort to understand both the structural, electronic, and magnetic factors that characterize these clusters, and the relationships between these factors and the functional properties. In part I, [11] we have shown how 1H NMR represents a formidable tool for investigating these systems by monitoring the β-CH2 resonances of the cysteinyl residues bound to the paramagnetic cluster.