In search of lost time constants and of non-Michaelis–Menten parameters

Upon completing 100 years since it was published, the work Die Kinetik der Invertinwirkung by Michaelis and Menten (MM) was celebrated during the 6th Beilstein ESCEC Symposium 2013. As the 7th Beilstein ESCEC Symposium 2015 debates enzymology in the context of complex biological systems, a post-MM approach is required to address cell-like conditions that are well beyond the steady-state limitations. The present contribution specifically addresses two hitherto ambiguous constants whose interest was, however, intuited in the original MM paper: (i) the characteristic time constant τ_∞, which can be determined using the late stages of any progress curve independently of the substrate concentration adopted; and (ii) the dissociation constant K_S, which is indicative of the enzyme–substrate affinity and completes the kinetic portrayal of the Briggs–Haldane reaction scheme. The rationale behind τ_∞ and K_S prompted us to revise widespread concepts of enzyme's efficiency, defined by the specificity constant k_cat/K_M, and of the Michaelis constant K_M seen as the substrate concentration yielding half-maximal rates. The alternative definitions here presented should help recovering the wealth of published k_cat/K_M and K_M data from the criticism that they are subjected. Finally, a practical method is envisaged for objectively determining enzyme's activity, efficiency and affinity – (EA)² – from single progress curves. The (EA)² assay can be conveniently applied even when the concentrations of substrate and enzyme are not accurately known.