Pentalenene synthase

Pentalenene synthase fi-om Streptomyces sp. was cloned and overexpressed in Escherichia coli getting pentalenene, a known sesquiterpene site-directed mutants yielded other known sesquiterpenes, A -protoilludene and germacrene A (Seemann 1999). 

LESBURG, C. A., ZHAI, G. Z., CANE, D. E., CHRISTIANSON, D. W., Ciystal structure of pentalenene synthase Mechanistic insights on terpenoid cyclization reactions in biology, Science, 1997,277,1820-1824. 

Lesburg, C.A., Zhai, G.Z., Gane, D.E. and Ghristianson, D.W. (1997) Grystal structure of pentalenene synthase mechanistic insights on terpenoid cyclization reactions in biology. Science, Til, 1820-4. 

Terpene cyclase enzymes catalyze the cychzation of allylic pyrophosphate substrates to form carbocyclic products via car-bocation reaction intermediates. One well-studied example is pentalenene synthase (11, 12), which catalyzes the cychzation of farnesyl pyrophosphate to give pentalenene, whose reaction mechanism is shown in Fig. 15. Cychzation of farnesyl pyrophosphate is proposed to form an 11-membered intermediate, humulene, which is followed by a five-membered ring closure to form a bicychc tertiary carbocation. 1,2-Hydride migration followed by an additional five-membered ring closure gives a tricyclic carbocation, which gives pentalenene, at elimination. 

How do these cyclase enzymes control the precise regiochem-istry and stereochemistry of these multistep cyclizations The active site of pentalenene synthase consists of a hydrophobic cleft, which is lined with aromatic and nonpolar residues. It is thought that the carbocation intermediates might be stabilized by the formation of tt-cation interactions, with aromatic residues such as phenylalanine, tyrosine, and tryptophan. In pentalenene synthase, replacement of Phe-76 or Phe-77 by Ala gave > 10-fold reduction in activity, which suggests that they may stabilize carbocationic intermediates through Jt-cation interactions. 

Blake CCF, lohnson LN, Mair GA, North ACT, Philhps DC, Sharma VR. Lysozyme. Proc. Roy. Soc. Ser. B 1967 167 378-381. Lesburg CA, Zhai G, Cane DE, Christianson DW. Crystal structure of pentalenene synthase mechanistic insights on terpenoid cychzation reactions in biology. Science 1997 277 1820-1824. Seemann M, Zhai G, de Kraker IW, Paschall CM, Christianson DW, Cane DE. Pentalenene synthase. Analysis of active site residues by site-directed mutagenesis. 1. Am. Chem. Soc. 2002 124 7681-7689. 

The biosynthesis of T. can proceed directly from fame-syl diphosphate to the tricyclic skeleton, e.g., to pen-talenene under the influence of pentalenene synthase . 

This a-domain is common to all class I terpene synthases and is the only domain in bacterial and fungal class I enzymes such as the pentalenene synthase from Streptomyces ey oliatus or the trichodiene synthase from Fusarium sporotri-chioides (Fig. 87.14a) [191,194]. Plant hemi-, mono-, and sesquiterpene synthases exhibit a second helical p-domain that resembles a barrel structure. An example of this class is given by the (+)-5-cadinene synthase from G. arboreum (Fig. 87.14b) [195]. In plant diterpene cyclases and in the exceptional case of the ( )-a-bisabolene synthase from A. grandis (Fig. 87.14c), a third helical y-domain with a barrel-like structure is present [197, 200, 202]. The p- and y-domains in the plant enzymes are nonfunctional but are required for correct enzyme folding. 

Fig. 87.14 Three different structure types for class I terpene synthases, (a) The a-type pentalenene synthase from S. exfoliatus (IPSl), (b) a(3-type (+)-5-cadinene synthase from G. arboreum (3G4D), and (c) a(3y-type ( )-a-bisabolene synthase from A. grandis (3SAE)...

---