Dr. B. Manjari
Enzyme is a biological catalyst, i.e. a substance that
alters the rate of a reaction without itself becoming
permanently altered by its participation in the reaction.
The ability of an enzyme (particularly a proteinaceous
enzyme) to catalyze a reaction can be altered by binding
various small molecules to it,
sometimes at its active site, and sometimes at a site
distant from the active site.
Usually these alterations involve a reduction in the
enzyme's ability to accelerate the reaction; less
commonly, they give rise to an increase in the enzyme's
ability to accelerate a reaction.
Inhibitors are chemicals that reduce the rate of
They are usually specific and they work at low
They block the enzymatic action but they do not
usually destroy them.
Inhibitors are broadly be classified as reversible
Irreversible inhibitors bind permanently to their target
enzyme, often via a covalent bond that influences catalysis.
Irreversible inhibition can be rarely seen i.e, they are not as
common as Reversible inhibition.
Inhibitors can be washed off the body by the dialysis
They are of 2 categories based on the distinction related to
the location and characteristics of the inhibition.
Many inhibitors act at the active site by binding in the
same way that one of the substrates or intermediates binds.
Thus if the inhibitor is present, it is competing for the
active site of the enzyme- Competitive inhibition.
If the inhibitor binds more tightly to the active site than the
substrate does, it is an effective competitive inhibitor, if it
binds less strongly, it's a poor competitive inhibitor.
If instead the inhibitor binds at a site on the enzyme that is
distinct from the active site, then its presence will induce a
conformational change in the protein.
This reduces the ability of the enzyme to facilitate the
This kind of inhibitor does not interfere with the binding of
the substrate(s) to the active site, but it does interfere with
Such a competitor is termed non-competitive, since it isn't
competing for the same site as the substrate.
Uncompetitive inhibitors are those that bind to a site
distinct from the active site, but only in the presence of
When they bind, they reduce the reaction velocity, because
we convert some molecules of the enzyme-substrate
complex into the nonproductive enzyme-substrate-inhibitor
Mixed inhibitors bind in a way that is reflective of some of
the properties of a competitive and a noncompetitive, or a
noncompetitive and an uncompetitive, inhibitor.
Most commonly, these are inhibitors that bind at a site
distant from the active site, but in binding they influence
substrate binding as well as turnover.
The process of increasing the amount or the activity of a
protein – Enzyme induction.
A homeostatic mechanism for regulating enzyme
production in a barrier organ, such as the liver, intestine,
The inducer usually combines with and
deactivates/activates a regulatory protein which leads to
increased gene expression.
Many of the enzymes involved in drug metabolism may be
up-regulated by exposure to drugs and environmental
chemicals leading to increased rates of metabolism.