5-Epi-aristolochene synthase

The 5-epi-aristoIochene synthase of tobacco makes the 5-epimer of aristolochene (Fig. 22-4). It binds the diphosphate group within a central cavity using two Mg2+ ions held by carboxylates, some of which are in the DDXXD sequence found also in polyprenyl diphosphate synthases. The enzyme active sites of both the epi-aristolochene synthase and a pentale-nene synthase from Streptomyces96 are rich in polar groups that form hydrogen-bonded networks and which participate in proton abstraction and donation during the rearrangement reactions that must occur. 

The formation of the sesquiterpene (+)-5-epi-aristolochene (2) represents, from a biosynthetic point of view, the transformation requiring the fewest steps among those discussed in this review. A single enzyme, tobacco 5-epi-aristolochene synthase (TEAS), converts the biosynthetic precursor, farnesyl diphosphate (6), to... 

Figure 10.11 A proposed pathway for the biosynthesis of capsidiol in elicitor-treated tobacco cell cultures. Earlier work had not resolved how 5-epi-aristolochene, synthesized from FPP by the action of 5-epi-aristolochene synthase, was converted to capsidiol.

STARKS, C. M., BACK, K. W., CHAPPELL, J., NOEL, J. P, Structural basis for cyclic terpene biosynthesis by tobacco 5- epi-aristolochene synthase, Science, 1997, 111, 1815-1820. 

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. 

Nt-EAS Nicotiana tabacum 5-epi-aristolochene synthase tps terpene synthase... 

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. 

Neither capsidiol nor glutinosone could be detected in healthy tissues. According to Bohlmann et al. (2002) . it is conceivable that capsidiol contributes not only to an inducible defense against pathogens, but also to a constitutive defense in an organ-specific manner in some species of Nicotiana Constitutive expression of 5-epi-aristolochene synthase (EAS) and capsidiol accumulation could not be observed in shoots of N. attenuata, but enzyme transcripts were induced by feeding of Manduca sexta, the tobacco homworm. However, EAS was expressed constitutively in roots of this N. species and N. sylvestris Spec. Comes. 

O Maille, RE. et al. (2006) Biosynthetic potential of sesquiterpene synthases alternative products of tobacco 5-epi-aristolochene synthase. Arch. Biochem. Biophys. 448, 73-82... 

Picaud et al. purified recombinant AMDS and determined its pH optimum to 6.5 [24]. Several sesquiterpene synthases show maximum activity in this range examples are tobacco 5-epz-aristolochene synthase [25,26], germa-crene A synthase from chickory [26] and nerolidol synthase from maize [27]. Terpenoid synthases are, however, not restricted to a pH optimum in this range. Intriguing examples are the two (-l-)-(5-cadinene synthase variants from cotton, which exhibit maximum activity at pH 8.7 and 7-7.5, respectively [28] and 8-epi-cedrol synthase from A. annua [29] with the pH optimum around... 

Protonation-induced cychzation of germacrene A generates the cis-decalin skeleton of the eudesmyl cation, an intermediate in the biosynthesis of the phytoalexin capsidiol (79). The eudesmane skeleton is converted to the 5-epi-aristolochene (80) skeleton by Wagner-Meerwein rearrangements of a hydride and a methyl group (Cane 1990). Hydroxylation of 80 then produces capsidiol. The entire process of formation of epi-aristolochene from FPP is mediated by a single synthase which has been purified from tobacco (Starks et al. 1997). The intermediacy of germacrene A in the biosynthesis was demonstrated by mutation of... 

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