A tighter fit between an active site and the substrate molecule is believed to increase efficiency of a reaction.

The enzyme is unchanged by the reaction and can move on to combine with more substrate molecules.

To be specific, it is involved in the cofactor-assisted proton abstraction from the substrate molecule.

It is not easy to find out which substrate molecule is implicated in a particular phenotypic or functional change, if any.

Consider a substrate molecule undergoing a reaction to a product molecule.

Isotopes can also be used to reveal the fate of various parts of the substrate molecules in the final products.

This causes the substrate molecule to adopt a strained conformation similar to that of the transition state.

The probability of a given reaction to be chosen is proportional to the number of substrate molecules.

In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site.

Another useful constant is k, which is the number of substrate molecules handled by one active site per second.