it is possible to increase efficiently the half-life of the enzyme by the addition of enzymes
that decompose H2O22 (catalase or peroxidase) to the biocatalyst preparation (Barzana et
al., 1989). According to the authors, the reaction seems to occur by direct interaction of
the gaseous substrate with the enzyme.
The use of alcohol dehydrogenases in solution has already received attention but
serious limitations still exist:
− the operational instability of enzymes and cofactors (NAD+ or NADH, H+);
− the insolubility of most of the substrates and products in water which requires working
in emulsions or organic solvents;
− the product inhibition of the enzyme;
− the lack of stability of some substrates and products in aqueous solutions.
In order to overcome these problems, a solid-gas bioreactor has been used for alcohol
and/or aldehyde production (Pulvin et al., 1986, 1988). A methodology has been
elaborated using alcohol dehydrogenase and NAD+ (or NADH, H+) co-immobilised into
albumin-glutaraldehyde porous particles in batch and continuous fed column reactors. An
aldehyde reduction was coupled to a second alcohol oxidation in order to regenerate the
cofactor.
Other types of enzymes have been tested in solid-gas reactions such as lipases, which
are known to need interfacial activation. Candida rugosa lipase and other esterolytic
enzymes have been coated on glass beads and suspended in mixtures of substrates and
water vapors over a relative humidity range of 56 to 100% (Ross and Schneider, 1991).
Since the relative humidities of the system were quite high, the enzyme was in a thin
liquid film of concentrated buffer on the surface of the glass beads. Under these
conditions, the extent of reaction can be classified in the following order, for decreasing
aw values:
With this system it has been possible to obtain hydrolysis of a wide variety of substrates
even at 30°C with substrate vapor pressures as low as 0.08 mm Hg and with substrate
boiling points as high as 206°C. This has served to demonstrate that it is possible to work
in the gas phase with such compounds.
In all cases, what influences the activity and the stability of a gas/solid system is the
combination between water activity and the applied temperature. When the water activity
is high, that is to say, when free solvent water is present in the system, it cannot be
considered a solid-gas biphasic medium but rather a solid-liquid medium. Solid-gas
biocatalysis exists only when the biocatalyst has no free solvent surrounding the protein.
This has to be determined by the isotherm sorption curve of the catalytic preparation.
More research concerning the kinetics of gas/solid systems is needed in order to
understand precisely the catalysis in the gas phase. Nevertheless, for a single step
biotransformation, the gas/solid system appears to be an interesting field for the
development of new biotechnological purposes. The use of alcohol oxidases and
dehydrogenases for the synthesis of aldehydes could be a novel approach for the
production of precursors of flavours or fragrances, as well as the synthesis or the
modification of esters with esterolytic enzymes. Furthermore, compounds obtained with
Solid/gas systems, theory and applications 263