of one of the unique properties of enzymes: they were designed by nature to function together in complex synthetic or degradative pathways. Because of this property, whole cells and microorganisms can be used as catalytic entities that carry out multiple reactions for the complete synthesis of complex chiral molecules. A patent was recently issued for a genetically engineered Escherichia coli that synthesizes the molecule D-biotin directly from glucose. Biotin has three chiral centers, and the current chemical synthesis requires 13-14 steps with low yields. Similarly, researchers are constructing a microorganism that directly catalyzes the synthesis of a vitamin C precursor from glucose. Combining genes from various organisms results in a process that uses a microbially synthesized intermediate with a final chemical conversion to vitamin C. Whole cells of microorganisms are also used in the synthesis of antibiotics from carbohydrate starting materials, and whole cells are used in the biocatalysis of certain steroids. A number of specialty chemicals with complex synthetic schemes can be produced most efficiently by intact microorganisms utilizing a series of enzyme-catalyzed reactions designed by nature to work together.
The biotechnology field also has a growing number of examples of reactions of industrial significance catalyzed by isolated enzymes. The conversion of cornstarch into corn syrup by the enzymes alpha- and gluco-amylase and glucose isomerase is a large industrial process, generating corn sweetener for soft drinks and other uses. The enzymatic conversion of acrylonitrile to acrylamide has recently been commercialized in Japan. Japanese companies and researchers have been very diligent in developing enzymatic processes for the synthesis of fine chemicals. Enzyme-catalyzed reactions are used by the Japanese for the synthesis of monosodium glutamate, L-tryptophan, and phenylalanine.
The stereospecificity of enzyme-catalyzed reactions has been used to advantage in polymer synthesis as well. Workers at ICI have developed a combined enzymatic and chemical process for the synthesis of polyphenylene from benzene. Benzene is oxidized to a cis-dihydrodiol by an enzyme-catalyzed oxygenation. The diol is derivatized, polymerized, and rearomatized to give polyphenylene in a reaction that cannot be carried out by classic chemical methods because of solubility problems. This new route to polyphenylene is an excellent example of combined enzyme and classic chemical synthesis to make a product that is otherwise too expensive for practical use. Other biological polymers are also finding their way into the catalyst field in various applications. Microorganisms are used to synthesize materials such as poly(beta-hydroxybutyrate), a biodegradable plastic, and researchers are exploring a series of synthetic silklike materials that may have uses in high-tensile-strength applications.