4. Effect of pH
What is the optimum pH of an enzyme catalysed reaction?
Enzymes perform best in their 'optimum' ambient conditions
investigating the effect of pH on enzyme activity, three factors must be
kept constant, (i) the temperature and the concentrations of both the
(ii) substrate and (iii) enzyme must all be kept constant (see
General description of graphs: For any enzyme,
initially, as you increase the pH, the rate of reaction increases.
Then as the pH increases, the reaction rate reaches
a maximum at the optimum pH and then decreases with further increase
The optimum pH varies quite a bit from one enzyme to
pH effect: The structure of the protein enzyme
can depends on how acid or alkaline the reaction medium is, that is, it is
If it is too acid (very low pH) or
too alkaline (very high pH), the structure of the protein is changed and
it is 'denatured' affecting the shape of the 'active site' which becomes less effective.
The acid or alkali may chemically react with the enzyme at or near the
active site affecting the shape of the active site or the ability of
the substrate molecule to 'dock in' to the active site - a
denaturing effect due to interfering with the bonds holding the enzyme
together in its unique 3D shape.
The two graphs above illustrate how the rate of
reaction varies with pH for many enzymes.
The enzyme catalase breaks down
harmful hydrogen peroxide into water and oxygen, with an optimum pH
range around pH 7 (top left graph).
Another enzyme may have an optimum pH range of
3.5 to 4.5 (lower-right graph).
Note that an enzyme is active over a pH range of
perhaps several pH units, but beyond this range it is relatively
optimum pH range, the enzyme catalysis is at its most efficient. In the
denaturing process the 'active site' (see 'key and lock'
mechanism details above) may be damaged by highly acid (low pH) or
alkaline (high pH) conditions, and changed in such
a way that the
enzyme cannot perform its catalytic function on the substrate molecules
- they don't fit in the active site.
If the enzyme does not have the correct 'lock' structure
in the protein (the 'active site'), it
cannot function efficiently by accepting the 'key' substrate molecule to
form the substrate-enzyme complex
Most enzymes have an optimum pH of between 4
and 9, and quite frequently near the neutral point of pH 7. Our own body
fluids e.g. in blood or cells have a pH of ~pH 7.2 to 7.4, so its
no coincidence that many of our enzymes have an optimum operating pH ~7,
but it does depend on where you are in your body!
enzyme pepsin has a peak at pH 2 (graph on right) and can operate in the very acid
(hydrochloric) conditions of the stomach to help breakdown proteins for
digestion in the small intestine.
Examples of optimum pH values for
Increase in acidity or
alkalinity creating a pH well away from the optimum, can affect the protein
structure of the enzyme and so affecting the active site, and, the substrate
molecule can no longer readily lock into place into the active site and
cannot be transformed into the product molecules.
The first diagram is typical of many enzymes
operating in near neutral solutions (~pH 7)
The other two diagrams shows the wide range of
pH that different enzymes can operate in
e.g pepsin breaks down proteins in the very
acid conditions of the stomach.
Blood has a pH of ~7.4 and carbonic
anhydrase (optimum pH ~7) is found in red blood cells. This enzyme enables
the efficient conversion of carbon dioxide and water into the carbonic acid
and the hydrogen carbonate ion ('bicarbonate ion') and operates in near
Trypsin is a protease enzyme from the
pancreas that breaks down proteins (peptides) in the alkaline conditions
(~pH 8.5) of the smaller intestine, so its optimum rate of reaction is
around that value.