Abstract
To understand the specificity and efficiency of protein-protein interactions promoting electron transfer, we evaluated the role of electrostatic forces in precollision orientation by the development of two new methods, computer graphics alignment of protein electrostatic fields and a systematic orientational search of intermolecular electrostatic energies for two proteins at present separation distances. We applied these methods to the plastocyanin/cytochrome c interaction, which is faster than random collision, but too slow for study by molecular dynamics techniques. Significant electrostatic potentials were concentrated on one-fourth (969 A2) of the plastocyanin surface, with the greatest negative potential centered on the Tyr-83 hydroxyl within the acidic patch, and on one-eighth (632 A2) of the cytochrome c surface, with the greatest positive potential centered near the exposed heme edge. Coherent electrostatic fields occurred only over these regions, suggesting that local, rather than global, charge complementarity controls productive recognition. The three energetically favored families of pre-collision orientations all directed the positive region surrounding the heme edge of cytochrome c toward the acidic patch of plastocyanin but differed in heme plane orientation. Analysis of electrostatic fields, electrostatic energies of precollision orientations with 12 and 6 A separation distances, and surface topographies suggested that the favored orientations should converge to productive complexes promoting a single electron-transfer pathway from the cytochrome c heme edge to Tyr-83 of plastocyanin. Direct interactions of the exposed Cu ligand in plastocyanin with the cytochrome c heme edge are not unfavorable sterically or electrostatically but should occur no faster than randomly, indicating that this is not the primary pathway for electron transfer.
Highlights
To understand the specificity and efficiency of pro- redox partners
The three energetically favored families of pre- charged residues areconcentrated in an elongated acidic collision orientations all directed the positive region patch, composed of two distinctive clusters that are highly conserved in higher plant plastoacidic patch of plastocyanin but differed inheme plane cyanins (Guss andFreeman, 1983;Colman et al, 1978).These orientation
The physiological importance of PC,coupled with a fundamental interestin the mechanisms of electron transfer between metalloproteins, has Biological electron transport depends upon efficient mechanisms for electron transfer (Mayo et al, 1986), and upon specificity in the recognition of appropriate led to many studies of interactions of PCwith both inorganic and biological redox reagents
Summary
Electrostatic Asymmetry of PCand Cyt c-In poplar PC, negatively charged residues are clustered in two groups (residues 42-44 and 59-61) that surround Tyr-83 (Fig. lA),while in Cyt c positively charged residues surround the exposed heme edge(Fig. 1B) These asymmetric distributions of charged residues were reflected in theelectrostatic potentials mapped onto themolecular surfaces, showing that both molecules have highly unbalanced, but complementary, charge distributions (Fig. 2). The maximum charge docking brought into imidazole ring edge of Cu ligand His-87 and the surrounding proximity the regions with the most extreme electrostatic surface were electrostatically close to neutral.Witha dis- potentials, i.e. the portion of the acidic patch nearGlu-59 and tance-dependent dielectric model, the acidic patch remained Glu-61 on PC, and the positive region near Lys-13 on Cyt c. The electrostatic field vectors calculated from the PCstruc- surface of the exposed heme edge of Cyt c
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