Energetics of the quinone electron acceptor complex in Rubrivivax gelatinosus

The pH and temperature dependences of the free energy stabilization of the Q-A and Q-B semiquinone anions (QA and QB are respectively the primary and secondary quinone electron acceptors) were studied in antenna-reaction centre complex from Rubrivivax (R.) gelatinosus. This was achieved by measuring the rate constants of the P+Q-A (kAP) and P+Q-B (kBP) (P is the primary electron donor) charge recombination processes by flash-induced absorption spectroscopy. Despite the high primary sequence analogies of the QA and QB protein pockets between R. gelatinosus and the much more studied species as Rps. viridis, Rb. sphaeroides and Rb. capsulatus, the energetic behaviour of the quinone complex of R. gelatinosus appears to be somewhat different: (i) above pH 10, kAP decreases, whereas it increases in Rps. viridis; this suggests the presence of a protonatable group that stabilizes I- (I is a bacteriopheophytin electron acceptor) rather than Q-A; (ii) the pH dependence of kBP is unusually flat in the range 4-7.5, possibly reflecting that a substantial part of the P+Q-B charge recombination proceeds via the direct route through the protein by an electron tunnelling mechanism, at variance to what is observed in the three species mentioned above; (iii) the very substantial increase of kBP observed above pH 7.5 is reasonably well described by the presence of two apparent protonatable groups: pK1QB = 9.4, pK1Q-B = 11 and pK2QB = 8.5, pK2Q-B = 9.4. The latter group was not reported in Rps. viridis, Rb. sphaeroides or Rb. capsulatus. We conclude that the apparent pK values measured here in R. gelatinosus may reflect the contribution as a whole of several and/or distant groups rather than of well-defined residues.