The Michaelis constants ratio for two substrates with a series of fungal (mould and yeast) β-galactosidases

MCR=KM(lactose)KM(NPG)Thus, MCR provides a simple and fast method for identification of related enzymes. MCR for a group of enzymes with a pair of standard substrates may serve as an identification number of this group. A substantial deviation of MCR for a particular enzyme from the group parameter might be an indication of possible errors in KM determination, or in assignment of the enzyme to this group. An important advantage of this approach is that the experimental KM values may be obtained for enzymes with different purity and concentration. Within the framework of classical Michaelis–Menten kinetics and in accordance with the customary use of KM for a comparison of enzyme–substrate affinity, the MCR parameter may be interpreted as a relative substrate affinity. This proportion remains nearly the same for different mould (or yeast) β-galactosidases, though the affinity to a particular substrate may vary essentially from enzyme to enzyme. The constancy of MCR may be a manifestation of structural similarity of binding sites for these enzymes.