Thermodynamics of reactions catalysed by branched-chain-amino-acid transaminase
1397
the dependency of the apparent equilibrium constant K0 of reaction (2) on temperature, pH,
and ionic strength, respectively, are shown. Essentially identical trends of K0, on T , pH, and
Im as seen for reaction (2) in these figures are also exhibited by reactions (1) and (3). If one
desires to manufacture l-leucine, a lower value of K0 translates to an increase in the product
yield. Interestingly, there is a very significant decrease in the value of K0 if this reaction
is carried out at high ionic strengths. This could be accomplished by addition of a salt to
the reaction mixture; this assumes that this does not interfere adversely with the kinetics
or the chemistry of the reaction. There is also a decrease in the value of K0 as the pH is
lowered—particularly below ≈6.0. However, the activity of the enzyme may not be optimal
at such pHs. Additionally, if the enzyme can still maintain sufficient activity, there is a clear
indication that operating at lower temperatures will lead to higher product yields. Thus, an
appropriate combination of shifts in the temperature, pH, and ionic strength could lead to
improved product yields for these branched-chain amino acids. While the relative trends
with pH and ionic strength (see figures 3 and 4) can be predicted without a knowledge
of the equilibrium constants for the chemical references reactions (7), (8), and (9), one
does need the values of 1r Hmo for these reference reactions in order to predict the effect of
temperature on K0. The equilibrium constants for the three reference reactions are needed
to calculate the optimal product yields.
Also, the possibility of using Le Chatelier’s principle to derive these reactions to
completion should not be overlooked. In fact, this principle has been used(10, 27–29)
in transamination reactions catalysed by aspartate aminotransferase. In this case, the
oxaloacetate that is produced is converted to (pyruvate + carbon dioxide) thus increasing
the product yield of the desired l-amino acid. The predicted shifts in the position of
equilibrium (see figure 3) with pH can also be viewed in terms of Le Chatelier’s principle.
We thank Dr Andreas Bommarius of Degussa-Hu¨ls A. G. for providing a sample of 3,3-
dimethyl-2-oxobutanoic acid.
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