isSubstrateOf

Given a predicate that represents a relationship or action between entities, generate a one-sentence description explaining its meaning.  
# Instructions
Focus on describing the relationship, not the entities themselves. 
# Response Format
Begin the description with \' Indicates...\'

Predicate: isSubstrateOf

Indicates that one entity serves as the material or reactant upon which another entity (typically an enzyme or process) acts in a chemical or biochemical reaction.