A thermodynamic model for the cooperative functional properties of the tetraheme cytochrome c₃ from Desulfovibrio gigas

Abstract

A thermodynamic model is presented to describe the redox behaviour of the tetraheme cytochrome c3 from Desu(fbvihrio gigns. This molecule displays different intrinsic redox potentials for the four hemes and during thc redox titration process, interactions among different hemes occur, thus altering the values of redox potentials according to which of the hemes are oxidized [Santos, H., Moura, J. J. G., Moura, I., LeGall Chem. 245, 6241 -62501 coupled to a proton-linked equilibrium between two tertiary structures. Such a formalism, which requires a reduced number of parameters, is able to fully account quantitatively for the pH dependence of the NMK redox-titration curves. The 'redox-Bohr' effcct is discussed in terms of the available structure and thermodynamic data and a functional mechanism is proposed.Cytochrome c3 from De.su&vihrio gigas is a monomeric tetraheme protein which displays an oxidation/reduction equilibrium characterized by having hemes with different (and pH dependent) midpoint redox potentials. Moreover, during the redox process they are affected by the oxidation state of the surrounding hemes (interacting potentials), as demonstrated by previous N M R studies [I]. The cooperativity network resulting from these interacting potentials, has been previously discussed in terms of the functional properties of this electrontransfer protein [2]. In addition, recent observations by resonance-Raman [3] and EPK spectroscopy [4, 51 have indeed supported a similar behaviour for the redox-titration curve in cytochrome c3 from different sources.Although the X-ray structures of cytochrome c3 from D . gig0.r and from closely related and similarly behaving cytochromes have bcen resolved to high resolution [6 -81, providing evidence of the close spatial relationship of the four hemes, the detailed physical and structural mechanism underlying such a cooperative beliaviour has not been clarified up to now.It has been pointed out that electron transfer can take place over large distances through protein interiors [9-1 I]. This concept implies that changes in the tertiary conformation of the protein moiety can be very important in modulating the mutual influence of two redox centers within the same protein. In this way, the effectiveness of a preferential pathway for the electron transfer between two redox centers through the protein can be either magnified or dissipated by conformational changes of the tertiary structural arrangement. Such a mechanism can become very important whenever a relatively However, the presence of four functionally interacting hemes makes the phenomenological analysis of the redoxequilibrium process very complicated even for a single pH value, requiring the introduction of 16 pH-dependent binding parameters whose physical and numerical significance cannot be easily characterized [l]. 'This prompted us to elaborate a model according to a formalism, which follows from the interaction network IzH4 [12] coupled to an equilibrium bctween two tertiary structures, to carry out a more straightfor...

Structural and functional characterization of cytochrome c3 from D. desulfuricans ATCC 27774 by 1H‐NMR

“…The haems are covalently bound to the polypeptide chain by two thioether bridges and bis-histidinyl axial coordination and have very low redox potentials. For the D. gigas and D. vulgaris cytochromes, the microscopic haem mid-point redox potentials were shown to be dependent on the redox state of the protein and pH [3][4][5][6], thus establishing a network of redox and redox-Bohr cooperativities. In the case of D. vulgaris, this was used to explain how cytochrome c3 can act as a charge separation device for electrons and protons originating from the oxidation of molecular hydrogen via hydrogenase, leading to energy transduction [7].…”

Solution structure of Desulfovibrio vulgaris (Hildenborough) ferrocytochrome c 3 : structural basis for functional cooperativity 1 1Edited by P. E. Wright

“…This is still the case for the tetrahaem cytochromes c 3 from Desulfovibrio spp., even though considerable progress has been made in rationalising their thermodynamic properties (Santos et al, 1984;Coletta et al, 1991;Turner et al, 1994Turner et al, , 1996Park et al, 1996;Louro et al, 1996;Salgueiro et al, 1997) and high-resolution X-ray structures are available for several cytochromes c 3 in the oxidised form (Higuchi et al, 1984;Czjzek et al, 1994;Matias et al, 1996;Simo Ä es et al, 1998). Tetrahaem cytochrome c 3 from Desulfovibrio vulgaris (Hildenborough: DvHc 3 ) is a small (14 kDa; 107 amino acid residues) soluble protein whose crystal structure in the oxidised state has been determined by X-ray diffraction (Matias et al, 1993;Simo Ä es et al, 1998).…”

“…Although the molecular basis for the ®ne regulation of several types of cooperativity mechanisms has been successfully established (Perutz, 1990), including redox-linked regulation in enzyme catalysis (Williams et al, 1997;Fu È lo È p et al, 1995;Fukuyama et al, 1995;Chen et al, 1994), no unequivocal structural basis for the cooperativity involved in electronic to protonic energy transduction has been established. This is still the case for the tetrahaem cytochromes c 3 from Desulfovibrio spp., even though considerable progress has been made in rationalising their thermodynamic properties (Santos et al, 1984;Coletta et al, 1991;Turner et al, 1994Turner et al, , 1996Park et al, 1996;Louro et al, 1996;Salgueiro et al, 1997) and high-resolution X-ray structures are available for several cytochromes c 3 in the oxidised form (Higuchi et al, 1984;Czjzek et al, 1994;Matias et al, 1996;Simo Ä es et al, 1998).…”

Electrochemical study of an electron shuttle diheme protein: The cytochrome c from T. thermophilus

“…Nonetheless, the EPR titrations suggest that the potentiometric properties of the individual hemes are divergent in the fragments and full-length protein and they demonstrate the existence of cooperative interactions between them. This behavior was also suggested in other multi redox center proteins, including diheme cyt c4 [33], cyt c oxidase [47,48] and tetraheme cyt c3 [49,50]. Interestingly, as a consequence of the interaction scheme, the N-terminal domain is easier to reduce than the C-terminal domain in the full-length protein.…”