something that an enzyme binds & transforms into “product.” remember Substrate->Start. It’s the “S” in E + S <-> [ES] <-> P (where E is enzyme, S is substrate, and P is product)

For example, in our stick-snapper enzyme example (where an enzyme binds and snaps sticks), the stick-snapper is the enzyme, the unsnapped stick is the substrate, and the snapped stick is the product. In a more realistic example, the substrates of protein kinases are the proteins which the kinase (enzymes) adds phosphates onto to form phosphoproteins (products).