Random sequential mechanism

In the random sequential mechanism, the order of addition of substrates and release of products is random. Sequential random reactions are illustrated by the formation of phosphocreatine and ADP from ATP and creatine, a reaction catalyzed by creatine kinase (Section 14.1.5). 

With respect to mechanism of action, the most extensive kinetic and equilibrium exchange studies have been carried out on monofunctional 10-formyl-H4-folate synthetase from Cl. cylindrosporum [84]. The data support a random sequential mechanism that does not involve the formation of freely dissociable intermediates. The most likely mechanism, however, is not concerted but probably involves the formation of a formyl phosphate intermediate, since the synthetase catalyzes phosphate transfer from carbamyl phosphate but not acetyl phosphate to ADP with H 4-folate serving as an activator. Carbamyl phosphate is an inhibitor of 10-formyl-H 4-folate synthesis - an inhibition that can be eliminated only when both ATP and formate are present in accord with the concept that it spans both sites [85]. It would be of considerable interest to attempt to demonstrate positional isotope exchange employing [, y- 0]ATP for this enzyme in order to further implicate an enzyme-bound formyl phosphate species [86]. 

Figure. 6.9. Random sequential mechanism for bisubstrate reactions.

In case of the random sequential mechanism (Figure 6.17) there are two routes leading to the ternary complex. 

For the ordered and random mechanisms, as the concentration of the fixed substrate rises, the Michaelis constants may increase, decrease, or remain constant at their limiting values, depending on the ratio of Ki and Ka- The intersecting patterns, which can intersect above, below, or on the axis, represent either ordered or random sequential mechanism, or, exceptionally, a Ping Pong ones with unstable F form or with a product present. 

Figure 7.1. Fixed substrate concentration dependence for enzymes displaying random-sequential mechanisms (a) Dependence of V ax (b) dependence of K on [A] ...

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