VERTEBRATE AND PLANT-TYPE [2Fe-2S] FERREDOXINS: ELECTRON SHUTTLES OF DIFFERENT STRUCTURE

Jürgen J. Müller¹, Matthias Rottmann², Alexander Müller¹, Rita Bernhardt², Udo Heinemann^1,3

¹Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Str. 10, D-13122 Berlin, Germany
²FR 12.4 - Biochemie, Universität des Saarlandes, P.O. Box 15 11 50, D-66041 Saarbrücken, Germany
³Institut für Kristallographie, Freie Universität Berlin, Takustr. 6, D-14195 Berlin, Germany
E-mail: jjm@mdc-berlin.de; URL: http://www.mdc-berlin.de/~crystal

In the hydroxylase systems of vertebrates and some bacteria the [2Fe-2S] ferredoxins transfer electrons from an NAD(P)H-dependent reductase to different cytochromes P450. These ferredoxins are structurally different from plant-type ferredoxins for which crystal structures have been determined recently [1].

The crystal structure of a truncated oxidized vertebrate ferredoxin Adx(4-108) (adrenodoxin) has been determined by Müller et al. [2] at 1.85 Ä resolution by multiple-wavelength anomalous-dispersion phasing techniques making use of the iron atoms in the protein's [2Fe-2S] cluster and refined to a crystallographic R value of 0.195. The protein displays the compact (a+b) fold typical for [2Fe-2S] ferredoxins. The polypeptide chain is organized into a larger core domain (ubiquitin super-fold) and a small interaction domain which comprises 35 residues including all those previously determined to be involved in binding to adrenodoxin reductase and cytochrome P450scc. The [2Fe-2S] cluster is completely buried within a protruding bulge with the most exposed Fe atom being 3.7 Ä below the molecular surface.

Seven plant-type ferredoxins, structurally characterized so far [1], show nearly identical structures of the core domain but completely different small domains in comparison to adrenodoxin. The resolution of better than 2 Ä renders possible a detailed comparison of the vicinity of the redox center. Their redox potentials differ by -50 to -100 mV from that of adrenodoxin, mainly caused by structural and electrostatic differences near the [2Fe-2S] cluster. Electron pathway families have been compared for plant- and vertebrate type ferredoxins on the basis of the electron tunneling model [3] to get further insight into the electron transfer processes. Asymmetric charge distributions yield electric potentials and dipole moments which may explain predocking orientation events when ferredoxins interact with their redox binding partners.

[1] Holden, H.M. et al., J. Bioenergetics Biomembranes 26 (1994) 67-88.
[2] Müller, A. et al., Structure 6 (1998)269-280.
[3] Beratan, D.N. et al., Science 252 (1991), 1285-1288.