Explain noncompetitive inhibitor

A noncompetitive inhibitor can combine with either the free enzyme or the enzyme- substrate  complex, interfering  both.  The  most  common  type  of noncompetitive inhibition  is affected by  the substances that combine with some functional group of the  enzyme (outside  the  catalytic  site)  that  is  essential for  maintaining  the conformation of the enzyme molecule required for its activity. For example, enzymes possessing the essential -SH  group are sometimes inhibited by metals like mercury or copper.