Compulsory ordered ternary complex mechanism

Figure 4.9 Basic compulsory-order ternary-complex mechanism. The basic ordered mechanism for the general reaction A + B P + Q, with a = [A], b = [B], p = [P], and q = [Q] is illustrated. The four states are unbound enzyme (E), enzyme-substrate A complex (E-A), enzyme-substrate A-substrate B complex (E-AB), and enzyme-product Q complex (E-Q).

This nomenclature has been introduced by Cleland (1963), but other descriptions of bisubstrate mechanisms are also found in the biochemical literature. For example, a sequential addition in bisubstrate reactions, an Ordered Bi Bi mechanism is also called a compulsory-order ternary-complex mechanism whereas a Random Bi Bi mechanism is called a random-order ternary-complex... 

The products of the ER reaction, butyryl-CoA and NAD, were utihzed as inhibitors in an attempt to determine whether the reaction proceeds via a random or compulsory order ternary-complex mechanism using the rules of Cleland[5]. Assays were carried out in triplicate. The data was analyzed by linear regression and statistical t-tests were employed to determine if lines intersected at the same or significantly different point on the vertical axis. 

Figure 2.13 Reaction pathway for a bi-bi compulsory ordered ternary complex reaction mechanism.

A second ternary complex reaction mechanism is one in which there is a compulsory order to the substrate binding sequence. Reactions that conform to this mechanism are referred to as bi-bi compulsory ordered ternary complex reactions (Figure 2.13). In this type of mechanism, productive catalysis only occurs when the second substrate binds subsequent to the first substrate. In many cases, the second substrate has very low affinity for the free enzyme, and significantly greater affinity for the binary complex between the enzyme and the first substrate. Thus, for all practical purposes, the second substrate cannot bind to the enzyme unless the first substrate is already bound. In other cases, the second substrate can bind to the free enzyme, but this binding event leads to a nonproductive binary complex that does not participate in catalysis. The formation of such a nonproductive binary complex would deplete the population of free enzyme available to participate in catalysis, and would thus be inhibitory (one example of a phenomenon known as substrate inhibition see Copeland, 2000, for further details). When substrate-inhibition is not significant, the overall steady state velocity equation for a mechanism of this type, in which AX binds prior to B, is given by Equation (2.16) ... 

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. 

This mechanism is known as the ordered bi-bi mechanism ( bi-bi denotes a bi-substrate bi-product reaction), or the compulsory-order ternary mechanism , where the term ternary refers to the three-species complex formed by the binding of two substrates to the enzyme. 

ATP -I- 2 -deoxynucleoside = ADP + 2 -deoxynucleoside 5 -phosphate (<1>, compulsory ordered steady-state reaction mechanism with formation of a ternary complex with the phosphate donor and acceptor [2] the enzyme from embryonic cells of Drosophila melanogaster differs from other deoxynucleoside kinases [EC 2.7.1.76 (deoxyadenosine kinase) and EC 2.7.1.113 (deoxyguanosine kinase)] in its broad specificity for all four common deoxynucleosides)... 

The classical steady-state studies of Theorell and Chance showed that the increased affinity for substrate by the NADH-bound enzyme leads to a distinct sequence of the binding of coenzyme and substrate and subsequent reaction.1442. The binding of coenzyme is a compulsory step prior to substrate binding. Release of products from the enzyme site occurs via reversal of the sequence. This mechanism, known as an ordered bi-bi mechanism because of the required order of association and dissociation of the coenzyme and substrate with ternary complex formation is summarized in Scheme 6, where E, S and P represent enzyme, substrate and product respectively. 

The donor and acceptor can be bound in compulsory order or random order, the main point being that both must be bound at adjacent sites before group transfer can occur. This mechanism is kinetically indistinguishable from one in which an additional covalent phosphoryl-enzyme or nucleotidyl-enzyme exists and connects the two ternary complexes, as in Eq. (3). In this mechanism the enzyme mediates group transfer by nucleophilic catalysis, utilizing an enzymic nucleophile as the catalytic functional group. 

In a sequential mechanism, on the other hand, such as is followed by most NAD(P) -linked dehydrogenases [42], the enzyme forms a ternary complex i.e. a complex containing the enzyme itself and both substrates. This allows for several further possibilities. There may be either random-order or compulsory-order binding. If there is a compulsory order of substrate addition and product release, there are 4 possible sequences ... 

It should be re-emphasised that for a given enzyme the compulsory-order mechanism (with or without kinetically significant ternary complexes) is not just a single mechanism, because for a real enzyme the symbols denote real substrates so that there is the question of which is the leading substrate in each direction. Some of the... 

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