Question

what is the role of enzyme in metabolic regulation?

Answer 1

The enzymes make the reactions happen at a much faster rate than the rate which would be seen if the enzymes were absent. Also, the products of the reaction can (in some cases) inhibit the action of the enzyme so the reaction slows down appropriately. An example might bephosphofructokinase (PFK), an enzyme involved in glycolysis which is less active in cells with high energy levels because it is inhibited by ATP, a high energy molecule which is produced by glycolysis.

Answer 2

Sorry, I forgot something - when an enzyme is inhibited by a molecule which isn't the same shape as it's substrate (the molecule it usually works on) that's called allosteric inhibition. You can read about it here:

http://en.wikipedia.org/wiki/Allosteric_inhibition