Enzyme specificity: its meaning in the general case.

If an enzyme catalyses two competing reactions, their relative rates are determined by the concentrations of the competing substrates and the two specificity constants, i.e. the catalytic constants for the two substrates divided by the corresponding Michaelis constants. The concept of a specificity constant can be extended to reactions that require two or more substrates: in such cases the specificity for any competing substrate is determined by the apparent specificity constant measured at whatever concentrations of co-substrates, inhibitors, etc., exist under the conditions of competition. The partitioning between two competing substrates is independent of the concentration of any species, such as co-substrate, inhibitor, etc., that reacts only in the part of the mechanism that is common between the competing substrates.