Thermodynamic Activity-Based Michaelis Constants

Abstract

The classical approach towards analysing the influence of co-solvents (i.e., cellular mole- cules that are chemically inert and do not act as reacting agents) on the Michaelis constants of enzyme-catalysed reactions is empirical. More precisely, reaction kinetics is usually mathematically modelled by fitting empirical parameters to experimental concentration vs. time data. In this chapter, a thermodynamic approach is presented that replaces substrate concentrations by thermodynamic activities of the substrates. This approach allows determining activity-based Michaelis constants. The advantage of such activity-based constants K aM over their concentration-based pendants K obsM is twofold: First, K aM is independent of any co-solvent added (while K obsM is not) as long as it does not directly interfere with the reaction mechanism (e.g., inhibitor or activator). Second, known K aM values allow predictions of Michalis constants for different enzymes and reactions under co-solvent influence. This is demonstrated for a pseudo-one-substrate peptide hydrolysis reaction as well as for more complex two-substrate alcohol dehydrogenase reactions.