AN IMMOBILIZED ENZYME AS AN INDUSTRIAL CATALYST

While the oil crisis of the 1970s was front-page news, the soft drink industry was experiencing a less-heralded shock of its own. High sugar cane prices sparked a scramble to other sweeteners—not artificial sweeteners, mind you, but other forms of sugar. Table sugar—sucrose—is just one member of a family of several dozen closely related natural sugars. Other members include fructose, found in fruits; glucose, found in honey and grapes; lactose, found in milk; and maltose, found in malted grain. Fructose became the sweetener of choice, thanks to the adaptation of a catalyst to its large-scale industrial production. The catalyst, glucose isomerase, was derived from Streptomyces, a common, soil-dwelling bacterium best known as the source of many antibiotics, including streptomycin.

Glucose isomerase is an enzyme—one of nature's own catalysts. All living things use enzymes, each one tailored to carry out one of the multitude of reactions essential for life itself. An enzyme is essentially a protein molecule, although it may have other atoms or molecules attached that help it do its job. A protein molecule is a long chain made up of hundreds or thousands of smaller units called amino acids assembled in a very specific order. When dissolved in water, this chain naturally kinks and knots up. The sequence of amino acids making up the protein determines the shape that the protein knots itself into, and it is this shape that allows the protein to catalyze its reaction. The molecules that participate in the reaction fit into a crevice in the protein, like a key in its lock. Once inside the crevice, called the "active site," the molecules are held in just the right relative orientation for the reaction between them to proceed.

Adapting an enzyme to a continuous-flow industrial process requires that the soluble protein be immobilized somehow. (If the protein were left in its soluble form, it would be well-nigh impossible to separate it from the process stream, and it would all wash away in the flow.) To keep doing its job, the immobilized enzyme must retain its dissolved shape, yet it must also be firmly anchored to its solid support. In addition, the catalyst-support combination must be stable at the processing temperature and strong enough not to break up under processing conditions. Resin and polymer supports were tried first, because these molecules are chemically very similar to enzymes, which makes it easy to attach enzymes to them. Unfortunately, the resin and polymer beads were crushed into a gummy mass under the processing conditions and clogged the works.

One way around the problem is to attach the enzyme molecules to a ceramic material. Tiny ceramic particles have a high surface area, allowing a lot of catalyst to be attached to them and increasing the reaction's efficiency. Ceramics are also incompressible, and so the



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