Abortive complexes

Rudolph, F. B., Product inhibition and abortive complex formation. Methods Enzymol, 1979. 63 pp. 411-436. 

Nonproductive reversible complexes of an enzyme with various substrates and/or products. The International Union of Biochemistry distinguishes dead-end complex from abortive complex, and the latter term is regarded... 

Wong and Hanes ° pointed out that equihbrium exchange studies can be useful in detecting the presence of abortive species. Although abortive complexes can complicate exchange kinetic behavior, the Wedler-Boyer protocoT minimizes the influence of abortives on equilibrium exchange studies. 

Different abortives may be formed with alternative products or substrates. Such procedures can be useful in helping to distinguish Theorell-Chance mechanisms from ordered systems with abortive complexes . In the case of lactate dehydrogenase, the E-pyruvate-NAD+ and E-lactate-NADH abortive complexes may play a regulatory roles in aerobic versus anaerobic metabolism. 

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... 

An enzyme-catalyzed reaction involving two substrates and one product. There are two basic Bi Uni mechanisms (not considering reactions containing abortive complexes or those catagorized as Iso mechanisms). These mechanisms are the ordered Bi Uni scheme, in which the two substrates bind in a specific order, and the random Bi Uni mechanism, in which either substrate can bind first. Each of these mechanisms can be either rapid equilibrium or steady-state systems. 

Selected entries from Methods in Enzymology [vol, page(s)] Abortive complex formation, 63, 432 adenylate cyclase assay,... 

Rule 3. Both Rules 1 and 2 are applied if the reversible inhibitor binds to more than one enzyme form. Secondary plots can be nonlinear. See Abortive Complexes... 

A potential limitation encountered when one seeks to characterize the kinetic binding order of certain rapid equilibrium enzyme-catalyzed reactions containing specific abortive complexes. Frieden pointed out that initial rate kinetics alone were limited in the ability to distinguish a rapid equilibrium random Bi Bi mechanism from a rapid equilibrium ordered Bi Bi mechanism if the ordered mechanism could also form the EB and EP abortive complexes. Isotope exchange at equilibrium experiments would also be ineffective. However, such a dilemma would be a problem only for those rapid equilibrium enzymes having fccat values less than 30-50 sec h For those rapid equilibrium systems in which kcat is small, Frieden s dilemma necessitates the use of procedures other than standard initial rate kinetics. 

The steady-state determinants for each enzyme form (in the absence of abortive complexes) are ... 

The expressions are derived for systems at chemical equilibrium and in the absence of abortive complexes. All expressions, except for the Uni Uni and Rapid Equilibrium cases, were derived assuming only one central catalytic complex. 

Under initial rate conditions [P] and [Q] = 0) and no abortive complexes being formed, the initial-rate expression is... 

The steady-state initial rate expression, in the absence of products and abortive complexes, is ... 

An enzyme reaction scheme in which there are two substrates (A and B) and three products (P, Q, and R) and in which the substrates bind and the products are released in an ordered fashion. This reaction scheme is exemphfied by the malic enzyme The initial rate expression, in the absence of abortive complexes and products, is identical to the corresponding equation for the ordered Bi Bi mechanism. See Multisubstrate Mechanisms Ordered Bi Bi Mechanism... 

An enzyme reaction mechanism involving A binding before B and followed with the random release of products. In the absence of products and abortive complexes, the steady-state rate expression is identical to the rate expression for the ordered Bi Bi mechanism . A random on-ordered off Bi Bi mechanism has been proposed for a mutant form of alcohol dehydrogenase. ... 

A three-substrate (A, B, and C), two-product (P and Q) enzyme reaction scheme in which all substrates and products bind and are released in an ordered fashion. Glyceraldehyde-3-phosphate dehydrogenase has been reported to have this reaction scheme. The steady-state and rapid equilibrium expressions, in the absence of products and abortive complexes, are identical to the ordered Ter Ter mechanism. See Ordered Ter Ter Mechanism... 

Experiments designed to reach conclusions about an enzyme-catalyzed reaction by examining how one or more products of the reaction alter the kinetic behavior of the enzyme. The diagnostic value of these approaches can be limited by formation of E substrate product abortive complexes in multisubstrate mechanisms. 

Fromm and Rudolph have discussed the practical limitations on interpreting product inhibition experiments. The table below illustrates the distinctive kinetic patterns observed with bisubstrate enzymes in the absence or presence of abortive complex formation. It should also be noted that the random mechanisms in this table (and in similar tables in other texts) are usually for rapid equilibrium random mechanism schemes. Steady-state random mechanisms will contain squared terms in the product concentrations in the overall rate expression. The presence of these terms would predict nonhnearity in product inhibition studies. This nonlin-earity might not be obvious under standard initial rate protocols, but products that would be competitive in rapid equilibrium systems might appear to be noncompetitive in steady-state random schemes , depending on the relative magnitude of those squared terms. See Abortive Complex... 

ABORTIVE COMPLEXES DEADEND COMPLEXES ENZYME REGULATION FRIEDEN DILEMMA INHIBITION... 

ABORTIVE COMPLEXES ADENYLATE KINASE MAGNESIUM ION (INTRACELLULAR)... 

ALTERNATIVE PRODUCT INHIBITION ABORTIVE COMPLEXES ALTERNATIVE SUBSTRATES COMPETITIVE INHIBITOR ABORTIVE COMPLEXES MAPPING SUBSTRATE INTERACTIONS USING KINETIC DATA MEMBRANE TRANSPORT ENERGY OF ACTIVATION Old... 

Creatine kinase-MgADP-creatine abortive, ABORTIVE COMPLEXES CRiTiCAL CONCENTRATiON ACTIN ASSEMBLY KINETICS END-WISE DEPOLYMERIZATION MICROTUBULE ASSEMBLY KINETICS Critical energy,... 

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