Figure 6.15 Multiphase membrane
(hydrophobic) reactor for the
transformation of lipids (S—lipidic
substrate, P 1 water soluble product, P 2
water insoluble product)
the membranes used in those cases were not capable of completely retaining the hydrated
micelles, which had to be continuously supplemented to the reactor.
Reactions with CofactorsMany enzymes which are involved in useful reactions such as covalent bond synthesis,
energy transfer, group transfer and redox reactions depend on the presence of freely
dissociated cofactors or bound prosthetic groups to perform their catalytic tasks. These
reactions are attractive from the point of view of industry because of the wide variety of
products that can be synthesised (Cheryan & Mehaia, 1986). Among the enzymes used
are for instance oxidoreductases, transferases and ligases which need cofactors such as
nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate
(NADP) and adenosine 5′-phosphate (ATP) or depend on prosthetic groups such as
flavine adenine dinucleotide (FAD) or methoxantine (PQQ) for their catalytic activity
(Miyawaki, 1993; Brielbeck et al., 1994). The development of a bioprocess based on
these reactions should consider the fact that cofactors and prosthetic groups are spent just
like substrates after each turnover. Unfortunately, the cost of cofactors is most of the
times so high that practical applications can only be developed if a regenerating system
for those compounds is designed. The application of membrane reactors is presently
considered the solution for this problem, constituting an interesting research topic and
having originated a significant number of publications in the past years (Table 6.6).
Membrane reactors can offer the possibility of regenerating cofactors by using a
coupled enzyme system or, alternatively coupled substrates approach (Kulbe et al.,
1990). In either case, the coenzyme is required only in catalytic amounts. The first
Multiphase bioreactor design 168