Reduction Potentials of Rieske Clusters:  Importance of the Coupling between Oxidation State and Histidine Protonation State

Abstract

Rieske [2Fe-2S] clusters can be classified into two groups, depending on their reduction potentials. Typical high-potential Rieske proteins have pH-dependent reduction potentials between +350 and +150 mV at pH 7, and low-potential Rieske proteins have pH-independent potentials of around -150 mV at pH 7. The pH dependence of the former group is attributed to coupled deprotonation of the two histidine ligands. Protein-film voltammetry has been used to compare three Rieske proteins: the high-potential Rieske proteins from Rhodobacter sphaeroides (RsRp) and Thermus thermophilus (TtRp) and the low-potential Rieske ferredoxin from Burkholderia sp. strain LB400 (BphF). RsRp and TtRp differ because there is a cluster to serine hydrogen bond in RsRp, which raises its potential by 140 mV. BphF lacks five hydrogen bonds to the cluster and an adjacent disulfide bond. Voltammetry measurements between pH 3 and 14 reveal that all the proteins, including BphF, have pH-dependent reduction potentials with remarkably similar overall profiles. Relative to RsRp and TtRp, the potential versus pH curve of BphF is shifted to lower potential and higher pH, and the pK(a) values of the histidine ligands of the oxidized and reduced cluster are closer together. Therefore, in addition to simple electrostatic effects on E and pK(a), the reduction potentials of Rieske clusters are determined by the degree of coupling between cluster oxidation state and histidine protonation state. Implications for the mechanism of quinol oxidation at the Q(O) site of the cytochrome bc(1) and b(6)f complexes are discussed.

Allostery in the ferredoxin protein motif does not involve a conformational switch

“…This value is in excellent agreement with previous reports achieved with solution titrations coupled with electron paramagnetic detection methods (21). It is important to note that while the addition of neomycin is extremely useful in the collection of protein-film voltammetry measurements, it has the potential to alter the observed redox potential of specific systems (43). However, in the case of mFdx the observed redox potential for mFdx is unchanged in the presence/absence of neomycin (21).…”

Formation and characterization of an all-ferrous Rieske cluster and stabilization of the [2Fe-2S] 0 core by protonation

“…This proton is distinct from the two protons that can be dissociated from the two histidine ligands. In the [2Fe-2S] 1ϩ state, they are fully bound at pH Ͻ 11 (14,18), and they remain bound at all observable pH values in the [2Fe-2S] 0 state. The data shown in Fig.…”

“…(6). However, the neutral and electronegative histidine ligands, the hydrogen bonds, and the control of solvent accessibility all combine to raise the [2Fe-2S] 2ϩ/1ϩ potentials of high-potential Rieske clusters (18), and therefore they influence the [2Fe-2S] 1ϩ/0 potential also. In an all-cysteine-ligated cluster, the [2Fe-2S] 1ϩ/0 potential is likely to be significantly lower.…”

Role of a novel disulfide bridge within the all-beta fold of soluble Rieske proteins

“…S6). The data could be fit with a single transition at pK a = 10.1 ± 0.2 (0 mM TCEP sample) or pK a = 8.9 ± 0.3 (5 mM TCEP sample), values which are intermediate between the values reported for bc 1 Rieske proteins [34] and Rieske ferredoxins [32] in the oxidized [2Fe-2S] 2? state.…”

“…5; closed circles). These redox potentials are intermediate between those of prototypic dioxygenase-type Rieske ferredoxins (E 0 & -180 mV, [31]) and respiratory and photosynthetic Rieske domains (E 0 & ?300 mV, [17]), and this wide range has been suggested to result from differences in solvent accessibility, hydrogen bonds, and the spatial distribution of ionizable side chains around the center among different types of Rieske proteins [29,32]. The status of the disulfide bond also influences the redox properties of the [2Fe-2S] cluster, as its reduction results in a 30-mV increase in the potential.…”