In carrying out their metabolic processes, microorganisms interconvert diverse organic compounds. These “biotransformations” are catalyzed with high specificity and efficiency by enzymes. The active site of an enzyme, where substrate binding and catalysis are carried out, is an asymmetric surface whose special geometry frequently guarantees that the enzymecatalyzed reaction will yield a particular stereoisomer as the sole product. Such stereospecific, or enantioselective, reactions may be difficult or impossible to achieve by purely chemical means. The terms used to describe the stereochemistry of organic compounds are defined in Box 11.1, and the determination of enantioselectivity is described in Box 11.2.

Even when an organic compound can be synthesized chemically, the process may require many steps, whereas a single enzyme-catalyzed reaction can often achieve the same end. Also, enzymes can catalyze reactions at ambient temperature, away from extremes of pH, and at atmospheric pressure. Undesired isomerization, racemization, epimerization, and rearrangement reactions that are frequently encountered during chemical processes are generally avoided. The absence of such side reactions is a particular advantage when the desired product is rather labile. Finally, enzymes can accelerate the rates of chemical reactions by factors of 108 to 1012. For all these reasons, biotransformations by microorganisms, or by enzymes purified from microorganisms, are highly useful in preparative organic chemistry.

Certain disadvantages do limit the use of enzymes in organic chemical processes, but these limitations have frequently proved to be surmountable challenges rather than impenetrable barriers.