Heat shapes handedness of enzyme products.

Many organic molecules come in right- and left-handed mirror-image versions. Because one form may smell like mint and the other like caraway, or one act as a sedative and the other as a poison, chemists try to control which of these chemical isomers they make. Often, they accomplish this with the help of enzymes, which in the body prefer to catalyze chemical conversions of just one isomer.

Outside of cells, however, enzymes can pull some unexpected tricks. Robert S. Phillips of the University of Georgia in Athens has found that enzymes can flip their preferences from one isomer to the other. Furthermore, heat can sometimes boost an enzyme's selectivity.

"It was surprising to us that by increasing heat, which increases disorder, or entropy, you can actually get greater selectivity," Phillips says. As temperature increases, ordinary chemical reactions tend to generate equal proportions of the two isomers, producing so-called racemic mixtures.

Phillips studied secondary alcohol dehydrogenase, an enzyme taken from a bacterium. When the enzyme converted the substrate 2-butanone into 2-butanol at 15 degrees C, it favored the left-handed form, which made up 55 percent of the product. At room temperature, the product became a racemic mixture. At 50 degrees C, the balance shifted to 90 percent right-handed 2-butanol.

Phillips' message to drug and chemical manufacturers: "By changing the temperature, you might make an impractical reaction nearly practical."