Watching Enzymes Change Molecules’ Chirality

Some antibiotics kill bacteria by blocking the enzymes that the microbes use to build their cell walls. In certain types of bacteria, cell wall construction relies on an enzyme called alanine racemase, which converts the amino acid L-alanine to its mirror image, D-alanine. A team of researchers has now developed a nuclear magnetic resonance technique that can monitor the activity of this enzyme in real time (Anal. Chem., DOI: 10.1021/ac4004002). Learning more about the mechanism of alanine racemase could lead to new antibiotics, the researchers say.

Currently researchers study these enzymes by mixing them with one isomer, or enantiomer, and then measuring how much of the mirror-image molecule appears over time using a technique such as circular dichroism. The chemists must take samples from the reaction at certain time points and then analyze them.

NMR can follow the progress of reactions in real time, but the method, on its own, cannot distinguish one enantiomer from its mirror image. Philippe Lesot of the University of Paris-South, in Orsay, and his colleagues previously reported a way to discriminate between the L- and D-alanine enantiomers using NMR (Chem. Commun., DOI: 10.1039/C1CC15097A). They placed a mixture of the amino acids in an NMR sample tube that included short DNA strands. These strands form chiral liquid crystals that orient the two enantiomers differently within the sample, giving these compounds distinct chemical fingerprints. In typical NMR experiments, chemists dissolve compounds in solvents that wouldn’t produce these unique orientations for the two mirror images.

When she heard about Lesot’s work at a conference, chemist Monique Chan-Huot, a postdoctoral researcher at the Ecole Normale Supérieure, in Paris, wondered if the technique might be useful for studying alanine racemase. She and Lesot soon began working together to adapt Lesot’s technique to observing the enzyme in action. Their main question was whether the enzyme would retain its activity in this unusual liquid crystal environment.

The researchers found that as long as they mixed their samples using a centrifuge so that the enzyme was distributed evenly within the chiral liquid crystals, the conversion rates matched those measured by other techniques. The team next plans to use this NMR technique to study inhibitors of the enzyme as leads for potential new antibiotics.

The method is “a significant new application” of the use of chiral liquid crystals in NMR, says Stefan Berger of the University of Leipzig, in Germany. Chan-Huot and her colleagues are interested in extending this method to other enzymes that react with chiral molecules.