2bq4

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Revision as of 15:40, 5 November 2007

CRYSTAL STRUCTURE OF TYPE I CYTOCHROME C3 FROM DESULFOVIBRIO AFRICANUS

Overview

In Desulfovibrio metabolism, periplasmic hydrogen oxidation is coupled to, cytoplasmic sulfate reduction via transmembrane electron transfer, complexes. Type II tetraheme cytochrome c3 (TpII-c3), nine-heme cytochrome, c (9HcA) and 16-heme cytochrome c (HmcA) are periplasmic proteins, associated to these membrane-bound redox complexes and exhibit analogous, physiological function. Type I tetraheme cytochrome c3 (TpI-c3) is thought, to act as a mediator for electron transfer from hydrogenase to these, multihemic cytochromes. In the present work we have investigated, Desulfovibrio africanus (Da) and Desulfovibrio vulgaris Hildenborough, (DvH) TpI-c3/TpII-c3 complexes. Comparative kinetic experiments of Da, TpI-c3 and TpII-c3 using electrochemistry confirm that TpI-c3 is much more, efficient than TpII-c3 as an electron acceptor from hydrogenase (second, order rate constant k = 9 x 10(8) M(-1) s(-1), K(m) = 0.5 microM as, compared to k = 1.7 x 10(7) M(-1) s(-1), K(m) = 40 microM, for TpI-c3 and, TpII-c3, respectively). The Da TpI-c3/TpII-c3 complex was characterized at, low ionic strength by gel filtration, analytical ultracentrifugation and, cross-linking experiments. The thermodynamic parameters were determined by, isothermal calorimetry titrations. The formation of the complex is mainly, driven by a positive entropy change (deltaS = 137(+/-7) J mol(-1) K(-1), and deltaH = 5.1(+/-1.3) kJ mol(-1)) and the value for the association, constant is found to be (2.2(+/-0.5)) x 10(6) M(-1) at pH 5.5. Our, thermodynamic results reveal that the net increase in enthalpy and entropy, is dominantly produced by proton release in combination with water, molecule exclusion. Electrostatic forces play an important role in, stabilizing the complex between the two proteins, since no complex, formation is detected at high ionic strength. The crystal structure of Da, TpI-c3 has been solved at 1.5 angstroms resolution and structural models, of the complex have been obtained by NMR and docking experiments. Similar, experiments have been carried out on the DvH TpI-c3/TpII-c3 complex. In, both complexes, heme IV of TpI-c3 faces heme I of TpII-c3 involving basic, residues of TpI-c3 and acidic residues of TpII-c3. A secondary interacting, site has been observed in the two complexes, involving heme II of Da, TpII-c3 and heme III of DvH TpI-c3 giving rise to a TpI-c3/TpII-c3 molar, ratio of 2:1 and 1:2 for Da and DvH complexes, respectively. The, physiological significance of these alternative sites in multiheme, cytochromes c is discussed.

About this Structure

2BQ4 is a Single protein structure of sequence from Desulfovibrio africanus with CA and HEC as ligands. Structure known Active Site: AC1. Full crystallographic information is available from OCA.

Reference

The type I/type II cytochrome c3 complex: an electron transfer link in the hydrogen-sulfate reduction pathway., Pieulle L, Morelli X, Gallice P, Lojou E, Barbier P, Czjzek M, Bianco P, Guerlesquin F, Hatchikian EC, J Mol Biol. 2005 Nov 18;354(1):73-90. Epub 2005 Sep 29. PMID:16226767

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