Crystallization and preliminary X-ray analysis of maltose O-acetyltransferase

Maltose O-acetyltransferase (Mac) is a member of the hexapeptide-repeat fam ily of enzymes, which contains proteins with left-handed parallel beta -hel ix architecture forming homotrimers. Diffraction data for four well diffrac ting crystal forms were collected. Crystal form I diffracted beyond 1.53 An gstrom resolution but was perfectly merohedrally twinned with an apparent s pace group P622. Crystal forms II and III (space groups R3 and C2, respecti vely) could be obtained under very similar conditions by adjusting the buff er pH differently. Crystal forms II and III had several monomers in the asy mmetric unit and were difficult to derivatize. However, during soaking with trimethyl lead acetate, the form III crystals dissolved and crystals with a different habit and space group grew in their place (form IV). In three o f the crystal forms, a ladder of peaks was visible in the native Patterson maps along the c axis. These peaks were interpreted as corresponding to the vectors between the beta -strands in the turns of the beta -helix. Crystal form IV is suitable for structure determination of Mac exploiting the anom alous scattering of lead.