Rapid Equilibrium bisubstrate systems

However, the primary double reciprocal plots of some rapid equilibrium systems are identical. In rapid equilibrium systems, in the presence of the products of reaction, the primary reciprocal plots are very characteristic and depend on the number and type of enzyme-substrate and enzyme-product complexes that can form. Therefore, in order to distinguish between different t5q>es, one must revert to product inhibition patterns that can easily distinguish between aU types of rapid equilibrium bisubstrate systems (Plowman, 1972 Segel, 1975) (Table 2). 

In this chapter, the subject of bisubstrate enzyme reactions is introduced by examining the rapid equilibrium bireactant systems, with two substrates and two products of reaction. The main characteristics of this type of reactions is that both substrates, A and B, an both products of reaction, P and Q, are bound to enzyme very rapidly, and that aU binding and dissociation steps are, in most cases, much faster than the chemical transformations of substrates to products or, vice versa, the transformation of products to substrates. 

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... 

As a mle, a noncompetitive inhibition occurs only if there are more than one substrate or product (Todhunter, 1979 Fromm, 1995). For example, a noncompetitive inhibition will take place in a random bisubstrate reaction, when an inhibitor competes with one substrate while the other substrate is varied. Thus, the equilibria shown below describe a Rapid Equilibrium Random bisubstrate system in which an inhibitor competes with A but allows B to bind. 

Figure 2. Noncompetitive inhibition. Rapid Equilibrium Random bisubstrate system with an inhibitor noncompetitive with B. Graphical presentation of Eq. (S-io), with A as a constant and B as a variable substrate.

Dead-end Inhibition in a Rapid Equilibrium Ordered Bisubstrate System... 

In Section 5.4, a case of a dead-end inhibition in a Rapid Equilibrium Ordered bisubstrate system was described. One can compare this system with the following example. 

Note that Eqs. (9.15) and (9.16) are identical with the corresponding rate equations for the Rapid Equilibrium Random bisubstrate system (Chapter 8 Eqs. (8.7) and (8.8)). 

The Rapid Equilibrium Ordered Bi Bi system (Section 8.2) is a limiting case of the more realistic Steady-State Ordered Bi Bi system (Section 9.2). In bisubstrate mechanisms, the two approaches yield different velocity equations. As described... 

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