Figure 2.19: Enzymes lower the activation energy, thus making reactions occur fasterEnzymes are not consumed by the reactions they catalyse: they do not alter the equilibrium
of reactions, thus they catalyse both forward and reverse reactions. The direction in which
a reaction proceeds is determined by concentration of the substrates and the products of the
reactions.
Enzymes may be involved in reactions that break down or build up molecules. The break-
down reactions are known ascatabolicreactions. The building up reactions are known as
anabolicreactions.
The ‘lock and key’ model of enzyme action
Enzymes are highly specific regarding the reactions they catalyse. The specificity depends on
the bonds formed between theactive siteof an enzymes and its substrate. Active sites have
a specific shape that allows binding of a very specific substrate. The highly specific nature
of the enzyme-substrate binding has been compared to a ”lock and key” with the enzyme
as the ’lock’ and the substrate as the ’key’ (Figure 2.20). The substrate binds the active site
to form an enzyme-substrate complex. The reaction takes place, then the product leaves the
active site as it no longer fits the ’lock’ in the same way as the substrate did. The enzyme
remains unchanged.
Figure 2.20: This diagram illustrates the ‘lock-and-key’ model of enzyme action.Enzymes in every day life
Chapter 2. The chemistry of life 43