Multiple alternative substrate kinetics

•The history of determining enzyme specificity by kinetic methods is briefly reviewed•The relative kcat/Km values for all of the substrates present in a combinatorial library in a single reaction is derived•Internal competition kinetic isotope effects on kcat/KM for 3 isotopologs at non-tracer concentrations are derived

The specificity of enzymes for their respective substrates has been a focal point of enzyme kinetics since the initial characterization of metabolic chemistry. Various processes to quantify an enzyme's specificity using kinetics have been utilized over the decades. Fersht's definition of the ratio kcat/Km for two different substrates as the “specificity constant” (ref [7]), based on the premise that the important specificity existed when the substrates were competing in the same reaction, has become a consensus standard for enzymes obeying Michaelis–Menten kinetics. The expansion of the theory for the determination of the relative specificity constants for a very large number of competing substrates, e.g. those present in a combinatorial library, in a single reaction mixture has been developed in this contribution. The ratio of kcat/Km for isotopologs has also become a standard in mechanistic enzymology where kinetic isotope effects have been measured by the development of internal competition experiments with extreme precision. This contribution extends the theory of kinetic isotope effects to internal competition between three isotopologs present at non-tracer concentrations in the same reaction mix. This article is part of a special issue titled: Enzyme Transition States from Theory and Experiment.

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