Reaction specificity
Some enzymes catalyze only one reaction acting on a specific substrate. Example: urease and catalase acts only on urea and hydrogen peroxide, respectively. This is also called absolute specificity.
Many enzymes can catalyze same type of reactions (phosphate transfer, oxidation-reduction, hydrolysis etc.) in several structurally-related compounds. Example: carboxypeptidase acts on protein chains and removes one amino acid at a time from the C-terminal, irrespective of the nature of zimino acid.
A substrate caa undergo many reactions but in a specific reaction, an enzyme will catitlyze only one of these reactions. Example: citrate synthase converts oxaloacetate to citrate in the presence of acetyl-CoA. But, in absence of acetyl- CoA, oxaloacetate is acted upon by a different enzyme malate dehydrogenase with the formation of malate.