Specific Examples of Sequential Mechanisms

One problem with isotope exchange studies is that there are very few general rules about isotope exchange, so that one has the impression of an avalanche of arbitrarily chosen special cases. Therefore, it is useful to survey the most common cases in a qualitative fashion. 

1 Qualitative Difference Between an Ordered Bi Bi and a Random Bi Bi Mechanism 

The most common application of exchange experiments is the determination of the order of substrate binding. Let us examine die usual Bi Bi mechanism, which can proceed via an ordered or a random pathway. 

The A - Q exchange velocity, as a function of the absolute levels of the [B]/[P] pair, is the diagnostic for an ordered mechanism, where one substrate. A, must precede the other s binding and one product, P, must precede the other product s release. 

On the other hand, in the random mechanism, we have alternative routes for exchange processes, and no exchange wUl be inhibited by high levels of any substrate-product pair. This is tme, since A may bind to E and EB, and raising the [B]/[P] pair will not inhibit the combination of A to the EB species. Because the mechanism is fuUy S5mimetrical, a similar statement is tme for all exchanges. 

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