Activation energy, biological electron

Jortner, J., 1976, Temperature dependent activation energy for electron transfer between biological molecules J. Chem. Phys. 64 486034867. 

The acceleration mechanism of redox mediators are presumed by van der Zee [15]. Redox mediators as reductase or coenzymes catalyze reactions by lowering the activation energy of the total reaction. Redox mediators, for example, artificial redox mediators such as AQDS, can accelerate both direct enzymatic reduction and mediated/indirect biological azo dye reduction (Fig. 3). In the case of direct enzymatic azo dye reduction, the accelerating effect of redox mediator will be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the azo dye. Possibly, both reactions will be catalyzed by the same nonspecific periplasmic enzymes. In the case of azo dye reduction by reduced enzyme cofactors, the accelerating effect of redox mediator will either be due to an electron shuttle between the reduced enzyme cofactor and redox mediator or be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the coenzymes. In the latter case, the addition of redox mediator simply increases the pool of electron carriers. 

It is evident that the preceding considerations do not apply to all biological electron transfer systems. Even in the bacterial reaction center, the transfer between the two quinones Qa Qbj which takes place over 18 A [18], is characterized in Rhodobacter sphaeroides by a large entropic contribution, which has been attributed to the high solvent exposure of Qg [126]. By using the activation energy value reported in Ref. [126], two very different X values may be deduced from Eq. (23) = 0.1 eV and Aj = 2.5 eV. The previous considerations... 

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