Definition: A control interaction in which a physical entity modulates a catalysis interaction. Rationale: Biologically, most modulation interactions describe an interaction in which a small molecule alters the ability of an enzyme to catalyze a specific reaction. Instances of this class describe a pairing between a modulating entity and a catalysis interaction. Usage: A typical modulation instance has a small molecule as the controller entity and a catalysis instance as the controlled entity. A separate modulation instance should be created for each different catalysis instance that a physical entity may modulate, and for each different physical entity that may modulate a catalysis instance. Examples: Allosteric activation and competitive inhibition of an enzyme's ability to catalyze a specific reaction.
Source:http://www.biopax.org/release/biopax-level3.owl#Modulation