Alcohol Theorell—Chance mechanism

Rate experiments that are typically carried out in the presence of different concentrations of an alternative product (or product analog) while using the normal substrates . This approach can be particularly useful when the normal product cannot be used because it is unstable, insoluble, or ineffective (the latter indicated by a very high Ki value). Moreover, the normal product may be consumed as an essential substrate in a coupled assay system for the primary enzyme. Fromm and Zewe used the alternative product inhibition approach in their study of hexokinase. Wratten and Cleland later applied this procedure to exclude the Theorell-Chance mechanism for liver alcohol dehydrogenase. See Abortive Complexes... 

The Theorell-Chance mechanism is an ordered mechanism in which the ternary complex does not accumulate under the reaction conditions, as is found for horse liver alcohol dehydrogenase ... 

ADH features another catalytic triad, Ser-Tyr-Lys. Whereas the liver ADH kinetic mechanism is highly ordered, coenzyme associating first and dissociating last, the yeast ADH mechanism is largely random. In both cases, the actual chemical reaction is a hydride transfer. In the oxidation of secondary alcohols by Drosophila ADH (DADH), the release of NADH from the enzyme-NADH complex is the rate-limiting step, so vmax is independent of the chemical nature of the alcohol. With primary alcohols, as vmax is much lower and depends on the nature of alcohol, Theorell-Chance kinetics are not observed and the rate-limiting step is the chemical interconversion from alcohol to aldehyde. 

The Theorell-Chance mechanism describes the predominating pathway for the conversion of a wide range of primary and also some secondary alcohols by HL-ADH. The same kind of mechanism is also valid for the reaction of DADH with secondary alcohols. As mentioned in the previous section, ternary complexes are kinetically insignificant for the compulsory ordered Theorell-Chance mechanism. 

LADHee and that the activity disappeared after carboxymethylation of a cysteine residue at the active site of LADH s [145]. In a recent study by Okuda and Okuda it was demonstrated that the -hydroxysteroid dehydrogenase activity in human liver was associated with a major isoenzyme of liver alcohol dehydrogenase (/82, 2) that the activity was inhibited by a chelating agent for Zn, which resides in the active site of the enzyme [146], Kinetic studies with the highly purified isoenzyme showed that neither a Theorell-Chance mechanism nor a simple ordered BiBi mechanism applied to the reaction. Evidence was obtained that the reaction was asymmetric in both directions. It has been established by Fukuba that the 4A-hydro-gen in NADH is involved [147]. 

What this really means is that the ternary complex has such a transitory existence that it never makes up a significant fraction of the total amount of enzyme. Steady-state kinetics concerns itself only with those complexes which, by their existence, detectably alter the pattern of dependence of reaction rate on substrate concentration. The Theorell-Chance mechanism may be seen perhaps as a manifestation of highly effective catalysis. Certainly, in the case of the enzyme for which it was first described, horse liver alcohol dehydrogenase, the mechanism is obeyed for good substrates i.e. short-chain primary alcohols with secondary alcohols, which are poor substrates, the ternary complex becomes kinetically significant - because it works less well [44]. 

From these and the corresponding, symmetrically related parameters for the reverse direction of reaction it may be seen that, as one would expect, all the relationships listed above for the more general case also apply to the Theorell-Chance mechanism. It does, however, have some diagnostic tests that are uniquely its own. First, not only, as before, is independent of the nature of B, but now also < o> d therefore the maximum rate, is independent of B. This is because in the Theorell-Chance mechanism the maximum rate is determined solely by the rate of dissociation of the outer product, P. It has been shown, for instance, that horse liver alcohol dehydrogenase gives essentially the same maximum rate for a series of primary alcohols [44] and this may be equated with the rate of dissociation of NADH. Conversely, of course, is independent of the nature of Q. From Eqn. 21, which is still obeyed, one may now see that the Dalziel maximum-rate relationships become equalities for the Theorell-Chance mechanism ... 

The following mechanism, known as the Theorell-Chance mechanism, was originally proposed in 1951 by H, Theorell and B. Chance to explain the kinetics of alcohol dehydrogenase ... 

Theorell-Chance mechanism is a simplified version of an Ordered Bi Bi mechanism where the steady-state level of central complexes is very low. TheoieU and Chance (1951) have proposed an Ordered Bi Bi mechanism without the central ternary complexes for alcohol dehydrogenase from equine liver. The hit-and-mn reaction sequence can be written ... 

Theorell-Chance This special case of the ordered bi-bi mechanism occurs if the first product P dissociates from the enzyme very rapidly and an EAB-, EPQ-complex does not occur in a significant concentration. (Example alcohol-dehydrogenase)... 

Early evidence for the sequential mechanism proposed by Theorell and Chance 332) has been reviewed in a previous chapter on LADH (1). The requirements of this mechanism are satisfied under certain conditions for primary alcohols and aldehydes but not for secondary alcohols 295,322,333-336). Thus, the first step is the binding of the coenzyme and the last and rate limiting step dissociation of the coenzyme. Formation of the productive ternary complexes E NAD Alc and E NADH Ald have been demonstrated 333,334,337-339). The interconversion of these complexes are not kinetically important for the above-mentioned conditions as required by the mechanism. It has been suggested, however, that this step is rate limiting during different conditions 324,336). Substrate dissociation constants for the ternary complexes have been estimated 340). 

Theorell (79) has suggested an alternative mechanism based on the combination of substrate and acceptor molecules with two hematin groups. Chance (78) has pointed out that this mechanism cannot apply to the primary alkyl hydroperoxide complexes reacting with an alcohol or hydrogen peroxide (as in reactions ii and iii) because all hematin groups are attached to hydroperoxide molecules in these complexes however, it is applicable to the catalytic decomposition of hydrogen peroxide. The mechanism may be represented ... 

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