Crystal structure of the oxidised and reduced acidic cytochrome c(3) from Desulfovibrio africanus

Unique among sulphate-reducing bacteria, Desulfovibrio africanus has two pe riplasmic tetraheme cytochromes c(3), one with an acidic isoelectric point which exhibits an unusually low reactivity towards hydrogenase, and another with a basic isoelectric point which shows the usual cytochrome c(3) react ivity. The crystal structure of the oxidised acidic cytochrome c(3) of Desu lfovibrio africanus (Dva.a) was solved by the multiple anomalous diffractio n (MAD) method and refined to 1.6 Angstrom resolution. Its structure clearl y belongs to the same family as the other known cytochromes c(3), but with weak parentage with those of the Desulfovibrio genus and slightly closer to the cytochromes c(3) of Desulfomicrobium norvegicum. In Dva.a, one edge of heme I is completely exposed to the solvent and surrounded by a negatively charged protein surface. Heme I thus seems to play an important role in el ectron exchange, in addition to heme III or heme IV which are the electron exchange ports in the other cytochromes c(3). The function of Dva.a and the nature of its redox partners in the cell are thus very likely different. By alignment of the seven known 3D structures including Dva.a, it is shown that the structure which is most conserved in all cytochromes c(3) is the f our-heme cluster itself. There is no conserved continuous protein structure which could explain the remarkable invariance of the four-heme cluster. On the contrary, the proximity of the heme edges is such that they interact d irectly by hydrophobic and van der Waals contacts. This direct interaction, which always involves a pyrrole CA-CB side-chain and its bound protein cys teine S-gamma atom, is probably the main origin of the four-heme cluster st ability. The same kind of interaction is found in the chaining of the hemes in other multihemic redox proteins. The crystal structure of reduced Dva.a was solved at 1.9 Angstrom resolutio n. The comparison of the oxidised and reduced structures reveals changes in the positions of water molecules and polar residues which probably result from changes in the protonation state of amino acids and heme propionates. Water molecules are found closer to the hemes and to the iron atoms in the reduced than in the oxidised state. A global movement of a chain fragment i n the vicinity of hemes III and IV is observed which result very likely fro m the electrostatic reorganization of the polypeptide chain induced by redu ction. (C) 1999 Academic Press.