Biomedical Engineering Reference
In-Depth Information
which are remarkable taking into consideration these reactions normally occur at ambient
temperature and pressure. For example, 1 kg of immobilized aspartate ammonia lyase can
produce more than 100 000 kg of L-aspartic acid, making it one of the most efficient
biocatalytic processes known (Rozzell, 1999).
Enzymes were once dictated as requiring water to maintain their active conformation
for catalytic activity. Hence, enzymatic reactions were initially conducted in aqueous
solution. This has greatly undermined the capabilities of enzymes and limited its
applications. In the 1960s, certain enzymes, namely chymotrypsin (Dastoli et al ., 1966 ),
xanthine oxidase (Dastoli and Price, 1967) and porcine pancreatic lipase (Zaks and
Klibanov, 1985) were found to be catalytically active even when suspended in organic
solvents. This has led to the finding that only a minute amount of water is required to
maintain the activity of enzyme. Hence, it is now possible to apply enzymes as catalysts
in non-aqueous reactions such as organic synthesis (Zaks and Klibanov, 1985; Schmid
et al ., 2001 ).
In addition, enzymes are able to act on a broad range of substrates (Rozzell, 1999; Schmid
et al ., 2001). Most importantly, unlike chemical catalysts, enzymes function at mild reaction
conditions, ambient temperature, atmospheric pressure, and neutral pH.
14.3 ENZYME KINETICS IN INDUSTRIAL APPLICATIONS
Principles of enzyme kinetics have been reviewed in textbooks by Marangoni (2003) and
Cornish-Bowden (2004). The rate of any chemical reaction follows strict mathematical
rules, and understanding of the rules is important for correct predictions of the process
timing, particularly in industrial scale processing. The following section briefly introduces
some basic aspects of the enzyme kinetics with particular focus on lipases.
Enzyme ( E ) accelerates conversion between a substrate ( S ) and a product ( P ) by formation
of the intermediate binding complexes ES and EP .
E S
+↔
ES
EP
↔ +
E
P
(14.1)
Initially, the reaction proceeds forward in the absence of product. The rate of the reaction
at this stage ( v = v + ) depends only on the substrate concentration and follows the Michaelis-
Menten equation.
v
v
+
+
v
=
;
v
s s
(
<<
K
);
v
V
(
s
>>
K
)
(14.2)
K
mS
+
mS
K
1
+
mS
mS
s
where V + = k + ·e 0 is the maximal v + at the current enzyme concentration e 0 ; s is the concentra-
tion of free substrate (usually equated to the total substrate s 0 = s
e 0 ); K mS is
the Michaelis constant or s ½ (i.e. the concentration of substrate giving a half-maximal rate).
The Michaelis constant is often identical to the dissociation constant K s , which describes the
equilibrium E
+
es
s if s 0
>>
ES .
The most common biochemical reactions typically involve two substrates ( A and B ) and
one or two product ( P and Q ):
+
S
ABE PQE
++↔++
(14.3)
 
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