industry. One of the earliest and most famous is the biotransformation of progesterone to 11
a-hydroxyprogesterone. The reactions are stereospecific, the ultimate in specificity being
emplified by such steroid bioconversions. This specificity is exploited in the resolution of
racemic mixtures, when a specific isomer rather than a racemic mixture is desired. Bioconversion
is becoming essential to the fine chemical industry in that their customers demand singleisomer
intermediates (Rogers, 1999). Bioconversions are characterized by extremely high
yields (i.e. 90–100%). Other attributes include mild reaction conditions and the coupling of reactions
using a microorganism containing several enzymes working in series. There is a tremendous
interest in immobilized cells to carry out such processes. These are usually much
more stable than either free cells or enzymes and are more economical than immobilized enzymes.
Recombinant DNA techniques have been useful in developing new bioconversions.
For example, the cloning of the fumarase-encoding gene in S. cerevisiae improved the bioconversion
of malate to fumarate from 2 g per L to 125 g per L in a single manipulation (Neufeld
et al., 1991). The conversion yield using the constructed strain was near 90%.