Also, and though more complex, the coupling of different enzymes in multi
enzymatic systems seems to be a more efficient alternative than using a single
enzyme. Within these systems, one of the first reports was the use of FDH coupled
with methanol dehydrogenase (MDH) to convert CO 2 into methanol, in a system
where enzymes were both in solution together with mediators such as MV and
pyrroloquinolinequinone (PQQ) [80]. A bi-enzymatic system using phosphoenolpyruvate carboxylase (PEPCase) and CA was attained by the encapsulation of
the enzymes in polymeric microbeads. This cascade CA/PEPCase enzymatic system allows the production of oxaloacetate by CO 2 conversion using a compartmentalized structure where PEPCase is crosslinked in an organic polymer bead and
CA crosslinked in inorganic silica on the surface of the polymer [56]. The system
reveals good stabilization of the enzymes and the method may be adapted to other
enzymes. Interesting multi enzymatic systems were developed by immobilizing
different dehydrogenases and its co-factors in multilayers of polyelectrolytes in a
sandwich-type design [81, 82]. A system with a scaffold prepared with poly(allylamine hydrochloride) (PAH) and polyurethane (PU) nanofibers was used to
immobilize FDH (from C. boidinii), formaldehyde dehydrogenase (AldDH, from
Pseudomonas sp), alcohol dehydrogenase (ADH) together with the cofactor NADH
aiming to build a cascade multi enzymes for methanol synthesis from CO 2
reduction. Also, glutamate dehydrogenase (GDH) and carbonic anhydrase
(CA) were co-assembled in the system to enhance the co-factor regeneration and
CO 2 hydration [81]. This complex system presented a methanol yield of 98%,
higher than the synthesis attained with the free-enzymes in solution as already
observed also in other studies. A previous cascade system using FDH, AldDH and
ADH in solution, where a poly(neutral red) modified electrode was used to
regenerate NADH, had already shown that was possible to attain methanol. This
study, however, shown that the slowest step was the initial conversion of CO 2 to
formate and that CA addition improved the reaction rate [58].
The complexity and difficulty in the systems control seems directly proportional
to the efficiency of the conversions and the use of CO 2 towards added-value
compounds production. A step forward on complexity is the successful use of
whole cells’ systems. An example is the use of a BES for the direct production of
methane and acetate from CO 2 using a methanogenesis and acetogenic bacteria
biofilm on the cathode compartment. Although the attained efficiency could not be
considered high due to requirement of the application of considerable overpotentials, this was an important proof-of-concept for future developments [83]. A system using sulfate-reducing bacteria (from Desulfovibrio species) that contain
several FDH and hydrogenases (Hase), with no mediators added, was tested in a
bioreactor where continuous CO 2 and H 2 gas was added for formate production
through the hydrogenation of CO 2 [84]. Recently, a novel approach, using CO 2 as
intermediate, was reported aiming to convert carbon monoxide into a polyhydroxyalkanoate biopolymer (poly-3-hydroxybutyrate, PHB) considered as a viable
alternative for petroleum-based nondegradable plastic products. The system
92
C. M. Cordas et al.
enzymatic systems seems to be a more efficient alternative than using a single
enzyme. Within these systems, one of the first reports was the use of FDH coupled
with methanol dehydrogenase (MDH) to convert CO 2 into methanol, in a system
where enzymes were both in solution together with mediators such as MV and
pyrroloquinolinequinone (PQQ) [80]. A bi-enzymatic system using phosphoenolpyruvate carboxylase (PEPCase) and CA was attained by the encapsulation of
the enzymes in polymeric microbeads. This cascade CA/PEPCase enzymatic system allows the production of oxaloacetate by CO 2 conversion using a compartmentalized structure where PEPCase is crosslinked in an organic polymer bead and
CA crosslinked in inorganic silica on the surface of the polymer [56]. The system
reveals good stabilization of the enzymes and the method may be adapted to other
enzymes. Interesting multi enzymatic systems were developed by immobilizing
different dehydrogenases and its co-factors in multilayers of polyelectrolytes in a
sandwich-type design [81, 82]. A system with a scaffold prepared with poly(allylamine hydrochloride) (PAH) and polyurethane (PU) nanofibers was used to
immobilize FDH (from C. boidinii), formaldehyde dehydrogenase (AldDH, from
Pseudomonas sp), alcohol dehydrogenase (ADH) together with the cofactor NADH
aiming to build a cascade multi enzymes for methanol synthesis from CO 2
reduction. Also, glutamate dehydrogenase (GDH) and carbonic anhydrase
(CA) were co-assembled in the system to enhance the co-factor regeneration and
CO 2 hydration [81]. This complex system presented a methanol yield of 98%,
higher than the synthesis attained with the free-enzymes in solution as already
observed also in other studies. A previous cascade system using FDH, AldDH and
ADH in solution, where a poly(neutral red) modified electrode was used to
regenerate NADH, had already shown that was possible to attain methanol. This
study, however, shown that the slowest step was the initial conversion of CO 2 to
formate and that CA addition improved the reaction rate [58].
The complexity and difficulty in the systems control seems directly proportional
to the efficiency of the conversions and the use of CO 2 towards added-value
compounds production. A step forward on complexity is the successful use of
whole cells’ systems. An example is the use of a BES for the direct production of
methane and acetate from CO 2 using a methanogenesis and acetogenic bacteria
biofilm on the cathode compartment. Although the attained efficiency could not be
considered high due to requirement of the application of considerable overpotentials, this was an important proof-of-concept for future developments [83]. A system using sulfate-reducing bacteria (from Desulfovibrio species) that contain
several FDH and hydrogenases (Hase), with no mediators added, was tested in a
bioreactor where continuous CO 2 and H 2 gas was added for formate production
through the hydrogenation of CO 2 [84]. Recently, a novel approach, using CO 2 as
intermediate, was reported aiming to convert carbon monoxide into a polyhydroxyalkanoate biopolymer (poly-3-hydroxybutyrate, PHB) considered as a viable
alternative for petroleum-based nondegradable plastic products. The system
92
C. M. Cordas et al.