Methylmalonyl-CoA epimerase (MCE) from the hyperthermophilic archaeon, Pyrococcus horikoshii, was expressed at high levels in Escherichia coli, purified, and partially characterized. The P. horikoshii MCE enzyme was a homodimer with an apparent molecular mass of 31,700 Da. The K-m of the enzyme for methylmalonyl-CoA was 79 muM and the k(cat) was 240 s(-1). The P. horikoshii enzyme was extremely heat-stable and withstood boiling for 60 min without detectable loss in activity.