Enzyme kinetics
Dihydrofolate reductase from E. coli with its
two substrates, dihydrofolate (right) and
NADPH (left), bound in the active site. The
protein is shown as a ribbon diagram, with
alpha helices in red, beta sheets in yellow
and loops in blue. Generated from 7DFR.
Enzyme kinetics is the study of the chemic-
al reactions that are catalysed by enzymes,
with a focus on their reaction rates. The
study of an enzyme’s kinetics reveals the
catalytic mechanism of this enzyme, its role
in metabolism, how its activity is controlled,
and how a drug or a poison might inhibit the
enzyme.
Enzymes are usually protein molecules
that manipulate other molecules — the en-
zymes’ substrates. These target molecules
bind to an enzyme’s active site and are trans-
formed into products through a series of
steps known as the enzymatic mechanism.
These mechanisms can be divided into single-
substrate and multiple-substrate mechan-
isms. Kinetic studies on enzymes that only
bind one substrate, such as triosephosphate
isomerase, aim to measure the affinity with
which the enzyme binds this substrate and
the turnover rate.
When enzymes bind multiple substrates,
such as dihydrofolate reductase
(shown
right), enzyme kinetics can also show the se-
quence in which these substrates bind and
the sequence in which products are released.
An example of enzymes that bind a single
substrate and release multiple products are
proteases, which cleave one protein sub-
strate into two polypeptide products. Others
join two substrates together, such as DNA
polymerase linking a nucleotide to DNA. Al-
though these mechanisms are often a com-
plex series of steps, there is typically one
rate-determining step that determines the
overall kinetics. This rate-determining step
may be a chemical reaction or a conforma-
tional change of the enzyme or substrates,
such as those involved in the release of
product(s) from the enzyme.
Knowledge of the enzyme’s structure is
helpful in interpreting the kinetic data. For
example, the structure can suggest how sub-
stra