40
Bioprocess Engineering
the enzyme–substrate complex (uncom-
petitive inhibition). All these cases result
in reduction of the enzyme-catalyzed reac-
tion rate. In addition, the substrate itself or
the product formed may be inhibitory, also
resulting in a decrease in reaction rate.
When immobilized reactors are utilized,
the reaction may be diffusion limited. A
concentration gradient causes the sub-
strate molecule to move, and this move-
ment may be slower than the enzyme-
catalyzed reaction rate. The diffusion rate
is a function of temperature, pressure, and
the composition of the system. If a porous
support is used, the following equation is
used to describe the enzyme kinetics:
D
e
µ
d
2
S
d
r
2
+
2dS
r
d
r
=
V
m
S
K
m
+
S
(
2
)
where
D
e
is the effective diffusivity of
the substrate within the porous support
matrix, and
r
is the radius of the porous
support. This expression assumes the
following:
The system is at steady state.
Flux is by diffusion only, not convection.
Diffusion is governed by Fick’s law.
The enzyme-catalyzed reaction follows
Michaelis–Menten kinetics.
The substrate does not adsorb to the
support.
The support matrix has a constant void
volume.
The enzyme is evenly distributed within
the support matrix.
Enzyme technology is exploited in the area
of biosensors. Enzyme-coated electrodes
provide a highly selective and sensitive
method for determining the amount of
a given substrate. Examples of electrodes
include sensors for fermentation prod-
ucts and intermediates (e.g. amino acids,
lactic acid, penicillin, alcohols). Current
research involves coupling enzyme reac-
tions to improve sensitivity by eliminating
interfering signals, as well as improving
enzyme stability by using enzymes iso-
lated from organisms that live in extreme
environments like hot springs or oceans.
The use of these speci±c electrodes facili-
tates the control of bioprocesses, resulting
in higher productivity. Biosensors are also
playing an increased role in medical ap-
plications like the detection of glucose in
blood and urine and detection of scarce
enzymes such as tissue plasminogen acti-
vator. Future use will also involve detection
of biological and chemical contaminants in
the environment and in water supplies as
security tightens around the world.
For everyday use, the food and detergent
industries purchase the largest amounts
of enzymes. Proteases that hybridize pro-
teins into smaller peptide units constitute
the largest part of the enzyme market.
They are used in cheese making, bak-
ing, meat tenderizing, brewing, tanning,
and detergents for the hydrolysis of pro-
tein stains. Other enzyme uses include
medicine (penicillinase removes penicillin
from an allergic individual and trypsin is
used as an anti-inflammatory agent); pulp
and paper manufacture (lignases biopulp
wood); and the textile and leather indus-
t
r
ie
s
.Thei
s
suebe
come
soneo
fhowto
process or cheaply make large quantities
of the enzymes. This leads to the next
area of the bioprocessing industry – live
cell bioreactors.
3
Whole-cell Bioreactors
Living cells can be viewed as small bio-
chemical reactors of great complexity.
They are utilized in most aspects of
biotechnology, and most biotechnology
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