Definition: An interaction in which one or more entities is physically transformed into one or more other entities. Comment: This class is designed to represent a simple, single-step transformation. Multi-step transformations, such as the conversion of glucose to pyruvate in the glycolysis pathway, should be represented as pathways, if known. Since it is a highly abstract class in the ontology, instances of the conversion class should never be created. Examples: A biochemical reaction converts substrates to products, the process of complex assembly converts single molecules to a complex, transport converts entities in one compartment to the same entities in another compartment.
Source:http://www.biopax.org/release/biopax-level2.owl#conversion