Structure of the branched-chain aminotransferase from Streptococcus mutans.

The branched-chain amino-acid aminotransferase from Streptococcus mutans (SmIlvE) was recombinantly expressed in Escherichia coli with high yield. An effective purification protocol was established. A bioactivity assay indicated that SmIlvE had aminotransferase activity. The specific activity of SmIlvE towards amino-acid substrates was found to be as follows (in descending order): Ile > Leu > Val > Trp > Gly. The protein was crystallized using the hanging-drop vapour-diffusion method with PEG 3350 as the primary precipitant. The structure of SmIlvE was solved at 1.97 A resolution by the molecular-replacement method. Comparison with structures of homologous proteins enabled the identification of conserved structural elements that might play a role in substrate binding. Further work is needed to confirm the interaction between SmIlvE and its substrates by determining the structures of their complexes.