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Numbering of Reactions
The elementary reactions in a composite mechanism should be numbered as systematically as possible and in such a way that reverse processes are easily recognized. The reaction numbers should be used as subscripts to k, for rate constants, or v, for the individual rates (chemical fluxes). The preferred scheme for ordinary use is: 1) k 1, k -1, k 2, k -2....; v 1, v -1, v 2, v -2,... In this scheme positive subscripts may be prefixed by a + sign, i.e. k 1 may be written as k +1, if it helps avoid ambiguity resulting from confusion with scheme 3 (below), or to emphasize that it refers to a step in the forward direction of the reaction. For some kinds of computer application and for theoretical discussions of enzyme mechanisms it is sometimes convenient to number the different forms of the enzyme rather than the elementary steps and then to denote the step from (e.g.) E3 to E4 as 34, etc. With this scheme the numbering of enzyme forms must be given explicity and the rate constants and rates listed above might become 2) k 12, k 21, k 23, k 32,...; v 12, v 21, v 23, v 32,... If there are more than nine enzyme forms in the mechanism the subscripts should be separated by a comma, e.g. k 10,11 but this can be omitted when it is not required for clarity. The following scheme, in which odd subscripts refer to forward steps and even subscripts to reverse steps: 3) k 1, k 2, k 3, k 4....; v 1, v 2, v 3, v 4,... is less satisfactory, both because it conflicts with IUPAC Recommendations [3] and because it makes it more difficult to recognize the forward and reverse rate constants for particular steps. It is unrealistic to expect any universal system of numbering rate constants to be equally satisfactory in all circumstances. For example, in a mechanism where enzyme forms in different states of protonation can undergo analogous reactions it may be clearer to assign the same numbers to the analogous steps and distinguish between them by the use of primes, etc., rather than try to apply any of the above schemes in a rigid way. Regardless of what system is used, rate constants should never be referred to except in explicit relation to a mechanism or to a kinetic equation. Дата добавления: 2015-09-27 | Просмотры: 553 | Нарушение авторских прав |