Electron Transfer within Protein Complexes

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DESCRIPTION (provided by applicant): The study of long-range electron transfer (ET) occupies a central place in modern biology and chemistry. However, the transfer of an electron/hole across a protein-protein interface almost invariably is controlled not by the ET process itself, but by the dynamics of conformational conversion among and within ensembles of interface configurations. Our overall aim is to understand how inter-protein ET is coupled to and controlled by these motions. During the current period we recognized that the 'forward' and 'back' steps of a flash- induced protein-protein photocycle of ET between a zinc-substituted hemoprotein and its redox partner involve different initial configurational ensembles, and hence should respond differently to the modulation of configuration interconversion. Measurements of these 'differential dynamics' will play an important role in our studies. They will be analyzed with a kinetic-dynamic (KD) model which incorporates for the first time the coupling of ET to configurational dynamics within a photocycle (Section D1). We pursue our aims through studies of three systems which exhibit complementary dynamic characteristics. Implementation of the KD model has begun with the measurements of inter-subunit ET within the structurally defined, mixed-metal, [Zn; Fe] hemoglobin (Hb) hybrids. It is our hypothesis that the ET photocycle monitored for the apparently rigid [Zn,Fe] hybrids is coupled to dynamics at the interface between [?1(Zn);?2(Fe)] ET partners (Section D2). The ET photocycle within the tightly-bound 1:1 complex between Zn-cytochrome c peroxidase (CcP) and cytochrome c (Cc) probes the coupling of ET to dynamics on a hierarchy of energy and time scales (Sections C1,2; D3). We hypothesize that ET near ambient is dominated by conversion between a few structures, but this 'freezes out' upon cooling to T ~0C and/or the addition of small solutes; transitions within the ensemble of substates that comprise the low-temperature structure then freeze out during a cooperative conformational transition in the range, 220-250K. Complexes of myoglobin(Mb) and cytochromeb5 (cytb5)(SectionsC1,3;D4)complement the other two systems in that they exhibit an ensemble of structures, only a few of which are reactive ('Dynamic Docking; DD). This leads to unexpected ET behaviors and gives a distinctly different cast to our aims. Mb charge- exchange mutations strengthen binding enough to shift the complex out of the DD regime and into a regime with strong 1:1 binding, and this results in remarkably fast intra-complex quenching of 3ZnMb. We shall (i) study the photophysics and dynamics of this process; (ii) explore how the ET/dynamics coupling changes as a DD complex with multiple bound structures is converted to 1:1 binding by tuning affinities through ionic strength variation and/or mutations; and (iii) exploit a linear free energy decomposition of the binding free energy of ET-active complexes, while experimentally testing its fundamental basis and interpretation.