Aristolochene synthase, crystal structure

CARUTHERS, J. M., KANG, I., RYNKIEWICZ, M. J., CANE, D. E., CHRISTIANSON, D. W., Crystal structure determination of aristolochene synthase from the blue cheese mold, Penicillium roqueforti, J. Biol. Chem., 2000, 275, 25533-25539. 

The recently published crystal structure of tobacco epi-aristolochene synthase (a sesquiterpene synthase) has provided the first look at the three-dimensional configuration of a plant terpene s)mthase (Starks et al, 1997). 

The three-dimensional structures of three non-plant sesquiterpene synthases reveals a single domain composed entirely of a-hehces and loops despite the low homology on amino acid sequence level [14,16,17]. The secondary elements of 5-epi-aristolochene synthase, a plant sesquiterpene synthase, conform to this pattern with the exception of two domains solely composed of a-helices and loops. It is reasonable, but still a matter of debate, to extrapolate these data to the case of amorpha-4,11-diene synthase, which will probably only display a-hehces and loops once the crystal structme has been solved. 

A further element shared by all sesquiterpene synthases is the need for a divalent metal ion as cofactor. The metal ion is essential for substrate binding but also for product specificity. The metal ions stabihze the negatively charged pyrophosphate group of farnesyl diphosphate as illustrated by the crystal structure of 5-epz-aristolochene synthase [9]. The highly conserved sequence (I, L, V)DDxxD(E) serves to bind the metal ions in all known terpene and prenyl synthases (Fig. 5) [18-22]. A further interesting property among terpene synthases is that the active sites are enriched in relatively inert amino acids, thus it is the shape and dynamic of the active site that determines catalytic specificity [23]. 

Because of the striking similarities in properties, mechanisms and primary structures between the plant monoterpene, sesquiterpene, and diterpene synthases [21], the recent crystallization and X-ray diffraction analysis of 5-epi-aristolochene synthase (Nt-EAS) [62] is of central importance in providing the first three-dimensional model with which fundamental and common structural themes may be proposed. This model provides a convenient starting point for discussing general structure/function relationships of all terpene cyclases. 

FIGURE 7.4 Crystal structure of (a) bisabolene synthase active center bound to farnesyl diphosphate inhibitor (b) aristolochene synthase cocrystallized with FDP (metal coordination and hydrogen bonds are shown as dashed lines magnesium anions and water molecules are shown as spheres) and the proposed catalytic mechanisms for each cyclization Images created with PyMOL. (See insert for color representation of the figure.)... 

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