Q.What is feedback inhibition?

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.

A physiologically important special case is feedback inhibition, where the final product of a metabolic pathway inhibits an enzyme that acts early in that same pathway, automatically slowing production once enough end-product has built up. Feedback inhibition is usually carried out through allosteric inhibition - the end-product binds a regulatory site distinct from the active site, changing the enzyme's shape and thereby its activity, rather than physically blocking the active site the way a competitive inhibitor does. So while every case of feedback inhibition in a pathway is generally allosteric in mechanism, allosteric inhibition is the broader mechanism, and feedback inhibition is the specific instance of it that regulates a pathway by its own end-product.