Abstract
Biocatalysis has emerged in the last decade as a pre-eminent technology for enabling the envisaged transition to a more sustainable bio-based economy. For industrial viability it is essential that enzymes can be readily recovered and recycled by immobilization as solid, recyclable catalysts. One method to achieve this is via carrier-free immobilization as cross-linked enzyme aggregates (CLEAs). This methodology proved to be very effective with a broad selection of enzymes, in particular carbohydrate-converting enzymes. Methods for optimizing CLEA preparations by, for example, adding proteic feeders to promote cross-linking, and strategies for making the pores accessible for macromolecular substrates are critically reviewed and compared. Co-immobilization of two or more enzymes in combi-CLEAs enables the cost-effective use of multiple enzymes in biocatalytic cascade processes and the use of “smart” magnetic CLEAs to separate the immobilized enzyme from other solids has raised the CLEA technology to a new level of industrial and environmental relevance. Magnetic-CLEAs of polysaccharide-converting enzymes, for example, are eminently suitable for use in the conversion of first and second generation biomass.
Highlights
One of the great technological challenges of the 21st century is to implement the transition from an unsustainable fossil resources-based economy to a greener and more sustainable one based on renewable biomass and utilizing manufacturing processes that minimize, or preferably avoid, the generation of waste and the use of toxic and/or hazardous materials
Turner and coworkers [40] recently outlined the requirements that an immobilized enzyme should meet for commercial viability of a process in the pharmaceutical industry: (i) high enzyme loading (>10 wt%), (ii) high activity recovery (>50%), (iii) no leaching under reaction conditions, (iv) tolerance to organic solvents, (v) recyclable (>20 cycles), (vi) good substrate mass transport and (vii) mechanically stable in batch and flow reactors and (viii) last but not least, it must result in a lower cost of goods compared with alternatives
Fischer and coworkers [155] showed that by using a Proline-specific X-prolyl-dipeptidyl aminopeptidase (PepX)/PepN combi-cross-linked enzyme aggregates (CLEAs) the degree of hydrolysis of casein was increased by 52%
Summary
One of the great technological challenges of the 21st century is to implement the transition from an unsustainable fossil resources-based economy to a greener and more sustainable one based on renewable biomass and utilizing manufacturing processes that minimize, or preferably avoid, the generation of waste and the use of toxic and/or hazardous materials. In the last two decades biocatalysis has emerged as an important technology for meeting the growing demand for green and sustainable processing [2,3,4] This embraces the whole gamut of chemicals manufacture, from the enantioselective synthesis of chiral drugs [5,6,7,8,9,10,11,12] to the conversion of renewable biomass to liquid fuels and commodity chemicals [13,14,15]. More enzymes are available, the enzymes are better, and, thanks to recombinant DNA technology, they can be economically produced on an industrial scale, enabling the design of improved industrial biocatalysts [19] Notwithstanding these numerous benefits of using enzymes, their industrial application is often hampered by a lack of long-term stability and difficult recovery and re-use. For both economic and environmental viability, these valuable catalysts need to be efficiently recovered and recycled and this is accomplished using immobilization technologies [20,21]
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