Through the looking glass: Chiral recognition of substrates and products at the active sites of racemases and epimerases.

Unlike most enzymes, which exhibit stereospecific substrate binding, racemases and epimerases bind and catalyze the reversible interconversion of enantiomeric and epimeric pairs of substrates. Over the past 15 years, a growing number of racemase and epimerase structures have been solved, furnishing insights into the nature of chiral recognition of substrates by these enzymes. Those enzymes catalyzing stereoinversion of a carbon acid substrate via a direct 1,1-proton transfer mechanism all bind their substrates in a mirror-image packing orientation. This does not apply generally to epimerases and racemases that use other mechanisms, such as NADH-dependent epimerases that employ a "flipping" mechanism. In general, polar groups are bound and fixed at the three binding determinants on the protein defining a pseudo-mirror plane, while nonpolar groups may be mobile. The hydrogen atoms on each stereocenter are positioned antipodal with respect to the pseudo-mirror plane, making a two-base mechanism imperative. Recognition that mirror-image packing is the common binding mode for enantiomeric or epimeric substrates of these enzymes should inform modelling/docking studies and protein engineering.