Chemistry Reference
In-Depth Information
(
K
m
) and catalytic rate constant (
k
cat
) for free and immobilised systems. The ad-
vantage of this approach is that the maximum rate is determined by performing
analyses over a range of substrate concentrations, and the activity is therefore evalu-
ated from a broader analysis, rather than a single-point condition. The majority of
approaches, however, tend to favour the former, and only a small number of papers
reviewed reported kinetic parameters [
16
-
20
]. Another difficulty in comparing en-
zyme activity is the use of normalised activity and a lack of absolute values of rates
of product formation being reported.
Enzyme stability can be estimated in various ways. One way is to measure the
enzyme performance/initial rates under varying conditions of temperature or pH for
example [
16
,
21
-
24
]. An alternative approach is to compare the half-life: the time
it takes for enzyme activity to reduce to half of its original activity. The half-life
is determined by monitoring the enzyme's activity at various durations, at a given
condition, such as a temperature, which will result in a decrease in enzyme activity.
Thermal enzyme denaturation typically follows an exponential decay, therefore al-
lowing the half-life to be deduced, and big half-lives are indicative of better thermal
stability. This approach allows a numerical increase in stability (the stabilisation
factor) to be estimated [
7
,
25
-
27
].
4.3.2 Lipase
Lipases are a technologically important family of enzymes within the esterase fam-
ily. They are primarily associated with catalysing the hydrolysis of triglycerides
(fats) into fatty acids and glycerol utilising a histidine-aspartate-serine catalytic
triad. They are found in animals and plants, as well as micro-organisms such as
bacteria and yeast [
28
]. Due to the ability to produce them on a large scale through
micro-organism proliferation, they have been widely implemented on an industrial
level in biotechnological applications such as the food processing industries (par-
ticularly dairy), pharmaceuticals industries (primarily to assist with producing en-
antiopureproducts) and chemicals industries (with significant use seen in producing
synthetic polymers and detergents) [
3
-
6
,
11
,
29
,
30
]. More recently, lipases have
been implemented in the renewable energy industry to aid the conversion of oils to
fuels, particularly in biodiesel production [
31
,
32
]. This area is likely to see consid-
erable growth in coming years.
The use of lipases on an industrial scale is, however, often limited due to rea-
sons such as instability at higher temperatures and pH extremes. These drawbacks
are particularly highlighted in the foods and pharmaceutical industries, where the
production of pure, non-contaminated (enzyme-free) products is a necessity. The
use of high temperatures is also often desirable in these industries, for reasons
including improved reaction rates, improved mass transfer and higher levels of
solubility [
33
,
34
].
Due to the popularity of lipases in biotranformations, there are number of im-
mobilisation methods which have been tested. Perhaps the most common form of
