It is a quantitative value of the ability of a pathway enzyme (or group of enzymes) to change its rate or activity when changes in its ligands (substrates, products, activators, inhibitors) are varied during the pathway function. It is written as ε^ai_X where a is the rate or activity of a pathway enzyme i and X is any of its ligands. The elasticities are positive for ligands that increase the enzyme rate whereas they are negative for those that decrease it. The elasticity coefficients are intrinsic properties of the enzymes since they only depend on its particular kinetic capabilities.

 Metabolic Control Theory