Action of Enzymes
|Figure 4-4 How an enzyme works. This space-filling model shows that the
enzyme lysozyme bears a pocket containing the active site. When a chain
(substrate) enters the pocket, the protein enzyme changes shape
slightly so that the pocket enfolds the substrate and conforms to its shape.
the active site (an amino acid in the protein) next to a bond
between adjacent sugars in the chain, causing the sugar chain to break.
An enzyme functions by associating in
a highly specific way with its substrate,
the molecule whose reaction it
catalyzes. The enzyme bears an active
site located within a cleft or pocket
and contains a unique molecular configuration.
The active site has a flexible
surface that enfolds and conforms to
the substrate (Figure 4-4). The binding
of enzyme to substrate forms
an enzyme-substrate complex (ES
, in which the substrate is
secured by covalent bonds to one or
more points in the active site of the
enzyme. The ES complex is not strong
and will quickly dissociate, but during
this fleeting moment the enzyme provides
a unique chemical environment
that stresses certain chemical bonds in
the substrate so that much less energy
is required to complete the reaction.
If the formation of an enzyme-substrate
complex is so rapidly followed by dissociation,
how can biochemists be certain that
an ES complex exists? The original evidence
offered by Leonor Michaelis in 1913 is that,
when the substrate concentration is
increased while the enzyme concentration
is held constant, the reaction rate reaches a
maximum velocity.This saturation effect is
interpreted to mean that all catalytic sites
become filled at high substrate concentration.
It is not seen in uncatalyzed reactions.
Other evidence includes the observation
that the ES complex displays unique spectroscopic
characteristics not displayed by
either the enzyme or the substrate alone.
Furthermore, some ES complexes can be
isolated in pure form, and at least one kind
(nucleic acids and their polymerase
enzymes) has been directly visualized with
the electron microscope.
Enzymes that engage in important
main-line sequences—such as the crucial
energy-providing reactions of the
cell that proceed constantly—seem to
operate in sets rather than in isolation.
For example, conversion of glucose to
carbon dioxide and water proceeds
through 19 reactions, each requiring a
specific enzyme. Main-line enzymes are
found in relatively high concentrations
in the cell, and they may implement
quite complex and highly integrated
enzymatic sequences. One enzyme
carries out the first step, then passes
the product to another enzyme that
catalyzes another step, this process
continuing until the end of the enzymatic
pathway is reached. The reactions
may be said to be coupled. Coupled
reactions will be explained in a
following section on chemical energy
transfer by ATP.