Page 267 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

10.6 Process Modeling 243

primary reactions, secondary reactions, competitive reactions, reversible reac-
tions, and undesired reactions. For a single enzyme, reaction mechanisms are
well developed and then they are included into the full model to describe the
multienzyme process by combining the effect of the individual enzymes. In this
way, the different possible reaction schemes are generated to give a cascade,
parallel, or network structure. In the model structure of a multienzyme process,
additional terms can appear because of inhibitory effects or can disappear as a
result of the fast consumption of intermediates. Furthermore, the importance of
all the reactions must be analyzed because they increase the complexity of the
model significantly.
• Interaction matrix: this matrix is suggested to identify the different interactions
that can exist between compounds and enzymes in the process. In this case,
the reaction structure defined in the previous step is useful to visualize and
classify those relationships that can happen with a higher degree of probability.
Similar ideas about the interaction between compounds can be found in the
scientific literature or from experimental experience in the laboratory. In order
to build the matrix, the compounds involved in the process (i.e., substrates,
intermediates, by-products, products, etc.) are arranged in rows (i.e., A, B, C, … ),
and the enzymes (E ) are arranged in columns (for i = 1, 2, 3, … ). In this way, the
i
matrix is filled defining the relationship between each compound and enzyme
in turn, that is, (S) for substrate, (P) for product, (I) for inhibitor, or (X) when
there is no interaction between one compound and one enzyme. This compiled
information is extremely useful to make decisions about the relevant terms
or kinetic parameters that must be added or removed from the reaction rate
expressions and process model. The position of the new term/parameter in the
final expression is defined by the enzyme kinetic mechanism which shows how
the compound inhibits the enzyme, for example, competitive, uncompetitive,
noncompetitive, or mixed inhibition.

Similarly, the process considerations describe the key characteristics that can
affect or modify the mass balances when formulating the model. In this way, a
preliminary idea, of which process configurations and further phenomena can be
expected in the model, is obtained. They may be described as

• Operating mode: the operating mode is related to the liquid flow exchange
characteristics in the process. The operating mode must be identified in order to
have a clear idea of the terms that must be considered in the mass balances of the
process. For multienzyme processes, operating modes such as batch, fed-batch,
or continuous modes are used according to the process characteristics (e.g.,
inhibitory effects and transport limitations). However, other operating modes
can also be considered such as the pH switching [42]. In this example, the pH
in the process was changed (increased/reduced) in order to activate or deactivate
certain enzymes in the mixture and thus prioritize some reactions in the media
over others at given conversion points.
• Type of reactor: the identification of the type of reactor is relevant to analyze the
physical characteristics and constraints that can be present in the process, such