Q.What are non-competitive inhibitors? Give an example.

Many enzymes need more than their protein chain alone to catalyse a reaction. The protein portion by itself, called the apoenzyme, is catalytically inactive; it becomes active only once it combines with a non-protein partner called a co-factor, and the resulting active combination of apoenzyme plus co-factor is called the holoenzyme. Co-factors fall into three kinds. Prosthetic groups are organic co-factors bound tightly, often covalently, to the enzyme on a near-permanent basis, such as the haem group in catalase. Co-enzymes are also organic but associate with the enzyme only loosely and transiently, during the act of catalysis itself, and are frequently built from vitamins - NAD+, NADP+ and coenzyme A are typical examples. Metal-ion co-factors (activators) are inorganic ions, such as Zn2+, Mg2+ or Mn2+, that form co-ordination bonds essential to the enzyme's structure or its binding of substrate.

Enzyme activity can also be reduced deliberately by inhibitors. Competitive inhibitors resemble the normal substrate closely enough in shape to occupy the active site themselves, competing directly with the substrate for that site - malonate blocking succinate dehydrogenase, because of its structural similarity to succinate, is the classic example - and because the inhibitor and substrate are competing for the same site, raising the substrate concentration can outcompete the inhibitor. Non-competitive inhibitors instead bind at a separate site on the enzyme, distorting its shape so the active site no longer works properly; because they do not compete for the active site, adding more substrate cannot reverse their effect, and binding of heavy-metal ions at such a site is a typical example. …