6
Enzyme Kinetics and Metabolic Control Analysis
There is little doubt that “Enzymes” was the word emblazoned over the gate through which most
chemists, physicists and chemical engineers entered the realm of bio-reactions.
Chemical reactions catalyzed by enzymes are not much different from dehydrogenation reactions
catalyzed by noble metal catalysts. There is only one reaction to be considered rather than the
myriad of reactions which take place in the metabolic network of cells, and the stoichiometry of
the reaction is known. As we have seen in Chapters 3 and 5 the study of stoichiometry and of the
steady state flux distribution in a metabolic network is a difficult task, and it must necessarily be
done before any meaningful investigation of the kinetics of cellular reactions can be started.
Enzymes are usually highly specific catalysts which allow only one form of the substrate
(reactant) to be treated - e.g. the L- but not the D-form of an enantiomer. Structural studies of the
enzymes reveal how the substrate docks on the catalyst, where the cofactor sneaks in and how the
product is detached. Poisoning (or “inhibition”) of enzymes follows the same rules as have been
developed for the inhibition (or reversible poisoning) of inorganic catalysts, and one can explain
why overloading of the enzyme with substrate, or blocking of the enzyme activity by a reaction
product or with a foreign molecule can reduce the rate of conversion of the substrate. In short:
the enzymatic reaction can if desired be treated by mechanistic models to obtain the true kinetics
of the reaction. This is in sharp contrast to the wholly empirical approach that has to be used for
cell reactions and where the outcome of the modeling exercise frequently has a very limited
predictive power.
Quite apart from the pedagogical qualities of enzyme reactions which open up new avenues of
studies for readers of classical textbooks on reaction engineering (e.g. Levenspiel (1999)) the
subject has of course enormous significance for modem biotechnology.
Enzymes (rather than living cells) are used in a multitude of processes, in industry as well as in
the daily household, and enzymatically based processes are replacing many classical processes
due to their mild reaction conditions and environmentally friendly outcome. Large companies
such as Novozymes and Genencor, the two dominant players on the enzyme market, are entirely
devoted to the production o f enzymes (by fermentation processes) for their various applications.
The market (approximately 1.5 billion USD in 2000) is expanding, and new applications of
enzymes are constantly being explored as can be seen in the yearly reports of the enzyme
producers.
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