Practicality of industrial enzymes.

In the late 1960s and early 1970s ‘enzyme technology’ was viewed with the optimism and enthusiasm currently devoted to ‘biotechnology’. With one or two spectacularly successful exceptions, industry as a whole has not welcomed enzyme technology with open arms. Consideration of successful and less-successful processes indicates what industry wants and what it has been able to achieve; consideration of the patent literature indicates the processes that may be employed industrially and how the relevant enzymes may have to be improved to make these processes operate economically. To be of value on an industrial scale an enzyme system must have the following attributes. (1) It must do the job required. This may not be the conversion which that enzyme does normally in Nature, e.g. most glucose isomerases are truly xylose isomerases, or it may be a conversion of materials that do not occur widely in Nature, e.g. pesticides, xenobiotics in general. In certain cases the enzyme may be required to operate in ‘non-physiological’ conditions, for instance in organic solvents, because the substrate or product, or both, are sparingly soluble in water or because use of such a solvent allows manipulation of the equilibrium reaction mixture. (2) It must be simple to use. An industrial enzyme must be stable to changes of temperature, pH and, ideally, microbial contamination. Immobilized enzymes must be robust: neither the enzyme nor the support material must find its way into the final product. Although sophisticated control equipment is available, ‘Murphy’s Law’ (anything that can go wrong will go wrong) will always apply, and simple systems offer the least scope for problems. (3) It must be cheap. ‘Cheap’ is a relative term: penicillin acylases are much more expensive than glucose isomerases, but that fact is tolerable to the antibiotic manufacturer because of the high value of his product. A frequently voiced criticism, usually by chemical engineers, of enzyme processes is that they work in dilute aqueous solution, in the cold. This is true for penicillin acylase but not for the enzymes used in glucose and fructose syrup manufacture. One of the primary advantages of immobilized enzymes is their overall cheapness: if their useful lifespan is sufficient they will be cheaper per se than soluble enzymes and, because far higher activities can be applied to the substrate, residence times in reactors are far shorter. This allows smaller, cheaper, reactors to be used. Hence immobilized enzymes will be used whenever practical. Once an immobilized enzyme system is installed, its cost is determined by the initial activity and by its longevity. The