Uncoupling reaction

H20 served as the electron donor and methyl viologen as the electron acceptor. 0 consumption was measured with a Clark-type electrode and phosphorylation was measured colorimetrically. Data are presented as averaged I values SD obtained with three Isolations of thylakoids. Average specific activities were o9 3 ymoles 0, consumed and 171 16 ymoles Pi esterified/mg Chi h for the coupled reactions, and 223 i 5 ymoles 0 consumed/mg Chi h for the uncoupled reaction. 

FIGURE 4 The reactions catalyzed by prolyl 4-hydroxylase, (a) The normal reaction, coupled to proline hydroxylation, which does not require ascorbate. The fate of the two oxygen atoms from 02 is shown in red. (b) The uncoupled reaction, in which a-ketoglutarate is oxidatively decarboxylated without hydroxylation of proline. Ascorbate is consumed stoichiometrically in this process as it is converted to dehydroascorbate. 

The chemical significance of Eqs. 4.18, 4.26, or 4.27 depends entirely on whether the several mechanistic assumptions (uncoupled reactions, gamma distribution of rate coefficients with the same coefficient of variation, etc.) that were used to derive them can be verified independently by molecular-scale experiments. Unless this kind of substantiation is possible, the adherence of data on adsorption-desorption reactions to these equations has no unique mechanistic implication.2... 

P-450 is also able to catalyse reductive reactions. Thus if the enzyme becomes activated in the absence of substrate it can react as an oxidase, reducing oxygen to superoxide, peroxide and water. Whether such an uncoupled reaction can occur in vivo, however, is not clear [219], At low concentrations of oxygen, P-450 can act as a reductase. Thus it is able to initiate lipid peroxidation either directly via reduction of lipid hydroperoxides or via reducing CCI4 to CHCI3 [217],... 

Myllyla R, Majamaa K, Gunzler V, Hanauskeabel HM, Kivirikko 109. KI. Ascorbate is consumed stoichiometrically in the uncoupled reactions catalyzed by prolyl 4-hydroxylase and lysyl hydroxylase. J. Biol. Chem. 1984 259 5403-5405. 

Loida, P.J. and S.G. Sligar (1993). Molecular recognition in cytochrome P450—mechanism for the control of uncoupling reactions. Biochemistry 32, 11530-11538. 

It seems wise to differentiate the uncoupling reaction from the earlier described autoxidation of reduced rtochrome P4S0, which forms hydrogen peroxide via O2 radicals as the reduction product of dioxy n. Both reactions, however, can occur in vivo and are physiological side reactions due to the un edficity of the microsomal monooxygenase system. 

Armed as we are now with the KAM theorem, the Center Manifold theorem, and the Stable Manifold theorem, we can begin to visualize the phase space of reaction dynamics. Returning to our original system (see Uncoupled Reaction Dynamics in Two Degrees of Freedom ), we now realize that the periodic orbit that sews together the half-tori to make up the separatrix is a hyperbolic periodic orbit, and it is not a fixed point of reflection. From our previous visualization of uncoupled phase-space dynamics, we know that the separatrix is completely nontwisted. In the terminology of Poliak and Pechukas, the hyperbolic periodic orbit is a repulsive PODS. ... 

Loida PJ, Sligar SG (1994) Molecular recognition in cytochrome P450 control of uncoupling reactions via site-directed mutagenesis. 8th international conference on cytochrome P450, pp 463-466... 

Reactions shown in central region are referred to as uncoupling reactions or shunt reactions. (Reproduced with permission in modified form from [11] 2012, Wiley-VCH, Weinheim.)... 

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