Page 258 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

234 10 Perspectives on Multienzyme Process Technology

A + B C + D P + Q

(a)

A B
x x′
F G

(b)

A B + C P + I
x x′
F G
Q

(c)

Figure 10.1 Enzyme classification accord- pathway toward the desired product P.
ing to the action in the reaction network.
is a secondary enzyme, which is
(a) Only primary enzymes (cascade). (b)
involved in the cofactor regeneration of
One primary enzyme and one secondary
enzyme (parallel reactions). (c) Combina- x. is a secondary enzyme, which is
tion of primary and secondary enzymes involved in the inhibitory co-product removal.
(network reactions), where the symbols For the reaction A, C, and G represent sub-
strates, B intermediate, and F, Q, and I
and represents the primary
co-products.
enzymes that are involved in the direct
perspective, there is no doubt that a whole-cell format looks the most attractive
in terms of simplicity and cost. Nevertheless, this will not always be the obvious
choice because of some of the potential limitations with whole-cell systems as listed
in Table 10.2. Therefore, although for some pathways it will be beneficial to operate
inside the cell [14], for others it will be better to operate outside the cell [15]. In
both situations, the yield will need to be focused on the desired product, with the
aim of reducing intermediate separation and losses to by-products. The options
are outlined in Table 10.3. The format (immobilized enzyme, whole cell, cell-free
extract, or soluble enzyme) determines the possibilities for recovery and recycle
of the biocatalyst. In many systems, this is the key to managing the cost of the
biocatalyst. It is interesting that similar considerations are in focus today in the area
of homogeneous catalysis [16]. Likewise today other possibilities concerning the
format exist including deliberately selecting the location of the enzyme(s) within
the catalytic entity [17].