This becomes important when the enzyme concentration is large, as is usually the case in studies of fast reactions. The rate of reaction as defined here is an intensive quantity. This means that its value does not change with the total U0126 amount of material considered, so a concentration of 1 mM glucose in a solution is the same whether we are concerned with 1 ml or with 1 µl, whereas the amount of glucose, an extensive quantity is not. IUPAC recommendations older than those of 1981 defined the rate of reaction as an extensive quantity with dimensions of amount of substance divided by time, but this definition is obsolete

in chemistry and has hardly ever been used in biochemistry. Most biochemists, indeed, would be surprised to learn that it had ever been suggested. An elementary reaction was defined as one with no reaction intermediates in the chemical mechanism; such a reaction is said to occur in a single step. Few if any complete

enzyme catalysed reactions are selleck compound of this type, but are instead composite, consisting of two or more elementary steps, which are, however, themselves elementary reactions. This section noted that the term molecularity should only be applied to elementary reactions, and then defines bimolecular and unimolecular in the ways universally used in biochemistry, so no discussion is required here. The document stated that “the term order of reaction can be applied to any elementary reaction considered in one direction only, and to certain composite reactions”. This is certainly the meaning that applies in chemical kinetics, but it is too restrictive for enzyme-catalysed reactions, for which the idea is well established that saturation of an enzyme implies a gradual decrease (through fractional values) of the order of reaction from 1 at zero substrate concentration to 0 at saturation. I see no objection to saying that a reaction has an

order i with respect to a concentration a in conditions where the derivative dlnvdlna=iis applicable, with no implication medroxyprogesterone that i is a constant independent of a. In a later paragraph the 1981 recommendations admit this possibility, and suggest the term apparent order. For an elementary reaction occurring in one direction the order of reaction is equal to the molecularity, but it describes the kinetics not the mechanism. When two or more reactants are considered there is an overall order for the whole set of reactant, and separate orders with respect to the different reactants. The 1981 recommendations define the orders with respect to the individual reactants as partial orders, but this term is virtually unknown in the biochemical literature.