Beißinger, M., Sticht, H., Sutter, M., Ejchart, A, Haehnel, W. & Rösch, P. (1997) EMBO J., in press

Abstract:

Cytochrome c6 functions as a small, soluble electron carrier between the two membrane bound complexes cytochrome b₆f and photosystem I (PSI) in oxygenic photosynthesis. Although structurally very different, the copper containing protein plastocyanin acts as a functional homologue of the heme containing cytochrome c6. While both are encoded in many green algae and cyanobacteria, some of them like Synechococcus elongatus seem to use cytochrome c6 as the only electron carrier to PSI.
After resonance assignment based on homo- and heteronuclear NMR spectra, structure calculation was performed with restrained molecular dynamics calculations using 1586 interresidual NOEs and 28 dihedral angle restraints as experimental restrictions. The overall fold exhibiting four a-helices is similar to other class I c-type cytochromes. In addition to the secondary structure present in most homologues, a small antiparallel b-sheet is found in the vicinity of Met58, one of the axial heme ligands.
The current studies on cytochrome c6 from a primordial thermophilic cyanobacterium that has no plastocyanin shed some light on evolutionary aspects of electron transfer. The structural feature of a flat hydrophobic area as found in the new structure of cytochrome c6 is conserved in other cytochromes c6 and even in plastocyanin of higher plants. This is the docking region and includes the site of electron transfer to PSI and possibly to the cytochrome b₆f complex. The binding of cytochrome c6 to PSI in green algae as Chlamydomonas reinhardtii includes the additional interaction of a negative patch with a positive domain of PSI. This positive domain has not been inserted at the evolutionary level of cyanobacteria. However, the negatively charged surface region is also present in cytochrome c6 of Synechococcus elongatus although it is unfavorable due to electrostatic repulsion and a slower electron transfer to PSI as compared to plants. It is concluded that in the course of the evolution this negative patch was optimized most likely to improve the reaction with positively charged cytochrome f at a balance rate of electron transfer to PSI. As PSI is known in structural detail for this organism, the reported cytochrome c6 structure provides also the basis for mutagenesis studies to delineate the mechanism of electron transfer between both.

Solvent exposed hydrophobic surface area in the heme vicinity. The hydrophobic clusters are shown in yellow.

Electrostatic properties of the protein surface in the cytochromes from three different organisms. Positive and negative charged surface regions are represented by blue and red color, respectively.

Correspondence to:

Paul Rösch, Lehrstuhl für Struktur und Chemie der Biopolymere, Universität Bayreuth, D-95440 Bayreuth, Tel.: (+49) 921 / 55-3541, Fax: (+49) 921 / 55-3544, e-mail: Paul.Roesch@uni-bayreuth.de