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.