Redox reactions kinetic constraints

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

From such figures, it can be concluded that in normal in vivo conditions, proteins which are under thermodynamic control of the glutathione redox buffer should be predominantly in the PSH form when Reaction (32) is feasible, whereas they will be predominantly in the P(S)2 form when Reaction (33) is feasible. Formation of protein mixed-disulfides PSSG is therefore likely to be of greater importance than that of P(S)2 in situations of oxidative stress. In such conditions however, non-steady-state concentrations of GSH and GSSG will often prevail and protein activities will be primarily under control of kinetic constraints. 

This, of course, is subject to both thermodynamic and kinetic constraints. Specifically, the initial reaction must occur predominantly (although not necessarily exclusively) at the center having the appropriate redox potential (the higher potential for reaction 6a and the lower potential for reaction 6b), and rapidly enough so that the secondary transfers shown in these equations are observable. 

If a second redox protein is present under the conditions represented by Eq. 3 or 5, secondary protein-protein ET reactions can occur, again subject to thermodynamic and kinetic constraints. In this situation, however, the constraints can be modulated by having the higher potential (under oxidizing conditions) or lower potential (under reducing conditions) protein present in stoichiometric excess, thereby favoring removal of electrons from (or entry of electrons into) the appropriate protein in the initial reaction with exogenous flavin. 

The mentioned thermodynamic prerequisite that the formal potential of the substrate redox system must be more positive than the formal potential of the catalyst redox system means that, in principle, reduction of S is easier compared to Cat , but that kinetic constraints essentially hinder this process at potentials where the catalyst is oxidized. Then, the direct reduction of S does not proceed electrochemically at potentials where Cat is reduced (or maybe even at no accessible potential at all) but only via homogeneons redox reaction (Equation (3.2)) with CaC". In this context, the regeneration of the catalyst leads to much steeper concentration profiles of the catalyst in the diffusion reaction layer that is, to a steeper concentration gradient that (see Chapter 1) means larger current. 

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