Reaction rate ternary-complex mechanisms

The reaction of ground-state molecular oxygen with S(IV) proceeds at very slow rates in the absence of light or specific catalysts. Transition metals, such as Fe(III), which are capable of coordinating S(IV) and/or 02 accelerate the rate of S(IV) autoxidation. The mechanisms invoked to explain transition-metal-catalyzed S(IV) autoxidations can be classified into two categories (1) one-electron free-radical reactions and (2) two-electron polar reactions involving ternary complexes comprised of the transition metal, S(IV), and molecular oxygen. 

Another mechanism that may apply to a two substrate reaction is the ordered ternary-complex mechanism. For example, the complex EAB can be formed from EA by addition of B, but not from EB by addition of A the substrates must become attached in a particular order. This mechanism is represented in Figure 10.5, the second representation being that of Cleland. This mechanism also leads to a rate equation of the form of (10.26), but the significance of the constants is different. 

As stated previously, kinetic studies showed that the enzyme-catalyzed reaction follows an ordered ternary complex mechanism in which the binding of NAD occurs prior to that of lactate, and release of pyruvate precedes that of NADH. The use of rapid mixing methods allowed rates of many of the individual steps in the reaction sequence to be elucidated, including the hydride ion-transfer step in the catalytically active ternary complex. 

The overall direction of the reaction will be determined by the relative concentrations of ATP, ADP, Cr, and CrP and the equilibrium constant for the reaction. The enzyme can be considered to have two sites for substrate (or product) binding an adenine nucleotide site, where ATP or ADP binds, and a creatine site, where Cr or CrP is bound. In such a mechanism, ATP and ADP compete for binding at their unique site, while Cr and CrP compete at the specific Cr-, CrP-binding site. Note that no modified enzyme form (E ), such as an E-PO4 intermediate, appears here. The reaction is characterized by rapid and reversible binary ES complex formation, followed by addition of the remaining substrate, and the rate-determining reaction taking place within the ternary complex. 

The emphasis in kinetic studies of E-IIs has been on the analysis of the rates of phosphorylation of the sugar by the phosphoryl group donor. In the early studies the question was addressed whether phosphorylated E-II would be a catalytic intermediate in the reaction or whether the phosphoryl group would be transferred directly from the donor to the sugar on a ternary complex between the enzyme and its substrates [66,75,95-100]. This matter has been satisfactorily resolved by a number of other techniques in favor of the first option and possible reasons why some systems did not behave according to a ping-pong type of mechanism have been discussed [1]. 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

The Steady-State Mechanism If the interconversion step is not the sole rate-determining step and binding steps are not in rapid equilibrium, then a steady-state description of the reaction is applicable. If there is a single ternary complex, EXY, (thus, [EXY] = [EAB] + [EPQ]), the scheme can be depicted as... 

In their mechanism, presented as a series of proton equilibria in Scheme 10, the reaction is controlled by three steps (a) ionization of the zinc-bound water, which destabilizes the binary complex to an extent that substrate binding cannot occur (p/ 3, Scheme 10) (b) a stabilizing effect of alcoholate ion formation in the ternary complex. The pH dependence of this step is the result of ionization of the alcohol.1449 (c) The dissociation of the alcohol from the ternary complex. This is similar in rate to the dissociation of aldehydes, which might be expected for a substitution mechanism, both neutral species forming structurally similar ternary complexes. 

From the pseudo-first-order rate constants ku the second-order rate constants k2 are obtained by dividing kx by the alcohol concentration. It was found that the reaction rate constant kx is proportional to the alcohol concentration (at the same catalyst concentration). Table I gives the k2 values for the reaction between methanol and MDI catalyzed by dibutyltin dilaurate at 25.1°C. A plot of the k2 values vs. the dibutyltin dilaurate concentration (Figure 2) apparently deviates from a straight line, indicating that the mechanism of the catalyzed urethane formation in DMF differs from the mechanisms observed in apolar solvents (2-6). Most workers have assumed that in apolar solvents the mechanism involves formation of a complex between alcohol and dibutyltin dilaurate or the formation of a ternary complex between alcohol, isocyanate, and catalyst. In these cases, the relation between k2 and catalyst concentration differs from the relation observed in DMF. 

The steady-state rate equation for the random mechanism will also simplify to the form of Eq. (1) if the relative values of the velocity constants are such that net reaction is largely confined to one of the alternative pathways from reactants to products, of course. It is important, however, that dissociation of the coenzymes from the reactant ternary complexes need not be excluded. Thus, considering the reaction from left to right in Eq. (13), if k-2 k-i, then product dissociation will be effectively confined to the upper pathway this condition can be demonstrated by isotope exchange experiments (Section II,C). Further, if kakiB kik-3 -f- kikiA, then the rate of net reaction through EB will be small compared with that through EA 39). The rate equation is then the same as that for the simple ordered mechanism, except that a is now a function of the dissociation constant for A from the ternary complex, k-i/ki, as well as fci (Table I). Thus, Eqs. (5), (6), and (7) do not hold instead, l/4> < fci and ab/ a b < fc-i, and this mechanism can account for anomalous maximum rate relations. In contrast to the ordered mechanism with isomeric complexes, however, the same values for these two functions of kinetic coefficients would not be expected if an alterna-... 

Q. This finding eliminates a truly rapid equilibrium random mechanism, for which k and k must be much smaller than fc 4, k-i, k, and k-2, since the two exchange rates must then be equal. In fact, the differences between the two exchange rates show that the dissociation of A and/or P from the ternary complexes must be slow compared with that of B and/or Q, and also slow relative to the interconversions of the ternary complexes (32). This means that in at least one direction of reaction the dissociation of products in the overall reaction is essentially ordered for all these enzymes, the coenzymes dissociating last, as in the preferred pathway mechanism (Section I,B,4). With malate, lactate, and liver alcohol dehydrogenases, the NAD/NADH exchange rate increased to a... 

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