Dual-substrate enzyme reactions

It is not always immediately apparent from an enzymatic reaction whether it should be treated with single- or dual-substrate enzyme kinetic expressions. Lactose hydrolase, which catalyzes the hydrolysis of lactose according to... 

Many dual-substrate reactions can be represented by a sequence of elementary steps involving a ternary complex comprised of the enzyme and the two reactants, and application of the SSA to these systems can be quite complicated [10]. If a RDS is assumed to exist, then the derivation of a rate expression can be markedly simplified, as shown next. Assume that substrates A and B interact with an enzyme E to form a product P according to the following series of elementary steps, where the first four steps are quasi-equilibrated and the last step is the RDS ... 

The first synthetic bisubstrate analog inhibitor 18 was successfully designed for al,2-FucT II by Palcic et al., based on the proposed ion-pair mechanism shown in Scheme 8 [22]. Analog 18, where the Gal unit is attached to the terminal phosfor of GDP through a flexible ethylene linkage, was found to be a competitive inhibitor with respect to both donor and acceptor substrates with K values of 16 and 2.3 pM, respectively. Inhibition studies with the bisubstrate analog also helped establish the kinetic mechanism of the enzyme reaction. These dual competitive inhibition patterns are only consistent with a random kinetic mechanism where either substrate can bind to free enzyme. 

Banko et al. [11] speculated that the putative dual-enzyme process was instead catalyzed by a single, multifunctional enzyme. Their data clearly showed that in cell-free extracts of C. acremonium the rate of ACV synthesis was significantly greater when the precursor amino acids were provided than it was when AC and valine were used as substrates for the reaction [11], Jensen and her coworkers [12] demonstrated a similar result using cell-free extracts of S. clavuli-gerus. 

In reactions catalyzed by DHAP-aldolases, hydroxylated aldehydes are generally superior to unsubstituted aldehydes presumably because of their higher reactivity (electrophilicity), higher affinity to the enzyme active site (lower values), and the fact that the products are stabilized by the formation of cyclic isomers [42].Accordingly,substrates with dual 2- or 3-hydroxyaldehyde termini seemed to be a logical choice for potential tandem aldolizations. 

Regulation of liver 6-phosphofructokina.se and fructose-1,6-bisphosphatase. These multimodulated enzymes catalyze nonequilibrium reactions, the former in glycolysis and the latter in gluconeogenesis. Note the dual action of fructose-2,6-bisphosphate (F-2.6-BP), which activates phosphofnictokinase (PFK-1) and inactivates fructose-1,6-bisphosphatase. The activity of F-2.6-BP is under hormonal and substrate regulation (Figure 15-6). = positive effectors 0 = negative effectors. 

Single and parallel dual-membrane structures are successfully prepared by using multilayer flow such as organic/aqueous two-layer flow and organic/aqueous/organic three-layer flow inside a microchannel, and this method can be applied to the preparation of surface-modified polymer membranes (Fig. 42). For example, horseradish peroxidase is immobilized on one side of the membrane surface, and this enzyme-modified membrane realizes substrate permeation and a subsequent reaction. 

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