Abstract
Electron bifurcation is here described as a special case of the continuum of electron transfer reactions accessible to two-electron redox compounds with redox cooperativity. We argue that electron bifurcation is foremost an electrochemical phenomenon based on (a) strongly inverted redox potentials of the individual redox transitions, (b) a high endergonicity of the first redox transition, and (c) an escapement-type mechanism rendering completion of the first electron transfer contingent on occurrence of the second one. This mechanism is proposed to govern both the traditional quinone-based and the newly discovered flavin-based versions of electron bifurcation. Conserved and variable aspects of the spatial arrangement of electron transfer partners in flavoenzymes are assayed by comparing the presently available 3D structures. A wide sample of flavoenzymes is analyzed with respect to conserved structural modules and three major structural groups are identified which serve as basic frames for the evolutionary construction of a plethora of flavin-containing redox enzymes. We argue that flavin-based and other types of electron bifurcation are of primordial importance to free energy conversion, the quintessential foundation of life, and discuss a plausible evolutionary ancestry of the mechanism.
Highlights
Life fundamentally depends on the free energy ( G) provided by the electrochemical disequilibrium between reduced, electron-donating and oxidized, electron accepting environmental substrates
Enzyme-bound flavin, a two-electron redox compound, was proposed as the crucial redox cofactor permitting the coupling between an endergonic one-electron transfer reaction to the low potential acceptor and a exergonic one reducing the high potential acceptor (Chowdhury et al, 2016)
It is worth noting that electron bifurcation obviously occurs in roughly the same region of inverted redox midpoint potentials as do “both together”-type 2-electron transfer reactions such as for example in NDH-2 (Figure 1)
Summary
Life fundamentally depends on the free energy ( G) provided by the electrochemical disequilibrium between reduced, electron-donating and oxidized, electron accepting environmental substrates (see Schoepp-Cothenet et al, 2013).
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