Definition: Imposes ordering on a step in a biochemical pathway. Retionale: A biochemical reaction can be reversible by itself, but can be physiologically directed in the context of a pathway, for instance due to flux of reactants and products. Usage: Only one conversion interaction can be ordered at a time, but multiple catalysis or modulation instances can be part of one step.
Source:http://www.biopax.org/release/biopax-level3.owl#BiochemicalPathwayStep