Enzymes Enzymes are catalysts. Most are proteins. (A few ribonucleoprotein
enzymes have been discovered and, for some of these, the catalytic
activity is in the RNA part rather than the protein part. Link to
discussion of these ribozymes.)
Enzymes bind temporarily to one or more of the reactants of the
reaction they catalyze. In doing so, they lower the amount of
activation energy needed and thus speed up the reaction.
Most of these interactions are weak and especially so if the atoms
involved are farther than about one angstrom from each other. So
successful binding of enzyme and substrate requires that the two
molecules be able to approach each other closely over a fairly broad
surface. Thus the analogy that a substrate molecule binds its enzyme
like a key in a lock.
This requirement for complementarity in the configuration of substrate
and enzyme explains the remarkable specificity of most enzymes.
Generally, a given enzyme is able to catalyze only a single chemical
reaction or, at most, a few reactions involving substrates sharing the
same general structure.
[IMAGE]The necessity for a close, if brief, fit between enzyme and
substrate explains the phenomenon of competitive inhibition.
One of the enzymes needed for the release of energy within the cell is
Link to illustrated
discussion of the
citric acid cycle.
It catalyzes the oxidation (by the removal of two hydrogen atoms) of
succinic acid (a). If one adds malonic acid to cells, or to a test
tube mixture of succinic acid and the enzyme, the action of the enzyme
is strongly inhibited. This is because the structure of malonic acid
allows it to bind to the same site on the enzyme (b). But there is no
oxidation so no speedy release of products. The inhibition is called
competitive because if you increase the ratio of succinic to malonic
acid in the mixture, you will gradually restore the rate of catalysis.
At a 50:1 ratio, the two molecules compete on roughly equal terms for
the binding (=catalytic) site on the enzyme.
Factors Affecting Enzyme Action
[IMAGE]The activity of enzymes is strongly affected by changes in pH
and temperature. Each enzyme works best at a certain pH (left graph)
and temperature (right graph), its activity decreasing at values above
and below that point. This is not surprising considering the
* tertiary structure (i.e. shape) in enzyme function and
* noncovalent forces, e.g., ionic interactions and hydrogen bonds,
in determining that shape.
* the protease pepsin works best as a pH of 1-2 (found in the