Thousands of terpenes have been identified to date. However, only two classes of enzymes are known to be involved in their biosynthesis, and each class has characteristic amino-acid motifs. We recently identified a novel large-terpene (C25/C30/C35) synthase, which shares no motifs with known enzymes. To elucidate the molecular mechanism of this enzyme, we determined the crystal structure of a large-β-prene synthase from B. alcalophilus (BalTS). Surprisingly, the overall structure of BalTS is similar to that of the α-domain of class I terpene synthases although their primary structures are totally different from each other. Two novel aspartate-rich motifs, DYLDNLxD and DY(F,L,W)IDxxED, are identified, and mutations of any one of the aspartates eliminate its enzymatic activity. The present work leads us to propose a new subclass of terpene synthases, class IB, which is probably responsible for large-terpene biosynthesis.
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