Unearthing the roots of the terpenome

Abstract

SummaryAlthough terpenoid synthases catalyze the most complex reactions in biology, these enzymes appear to play little role in the chemistry of catalysis other than to trigger the ionization and chaperone the conformation of flexible isoprenoid substrates and carbocation intermediates through multi-step reaction cascades. Fidelity and promiscuity in this chemistry, i.e., whether a terpenoid synthase generates one or several products, depends on the permissiveness of the active site template in chaperoning each step of an isoprenoid coupling or cyclization reaction. Structure-guided mutagenesis studies of terpenoid synthases such as farnesyl diphosphate synthase, 5-epiaristolochene synthase, and γ-humulene synthase suggest that the vast diversity of terpenoid natural products is rooted in the facile evolution of α-helical folds shared by terpenoid synthases in all forms of life.

Identification of amino acid networks governing catalysis in the closed complex of class I terpene synthases

“…S3B). This strongly supports the model that terpene synthases mainly dictate reaction outcome by sterically restricting their substrate and discrete reaction intermediates to a subset of possible productive conformations (template model) (22,24). Moreover, it demonstrates differing binding modes followed by nonnative cyclization characteristics for substrate analogs that hamper the correct prediction of important structure-function relationships in class I terpene synthases.…”

“…The lack of essential Y residues results in hydroxylated products as in the case of CBTS, which suggests an amino acid-assisted mechanism that provide control for water-assisted quenching products. We could further demonstrate by moleculardocking calculations that, in the initial reaction phase, the acyclic cation A can only assume a single conformation, which strongly argues for the template model (22). The data confirm the existence of a monocyclic cation B, specifically as in TXS-W753H deprotonation of C17 results in formation of CM.…”

“…Although initial excess of spatial freedom allows GGPP misfolding, spatial freedom is shown to lead to predominant carbocation tumbling and imprecise barrier crossings at later stages of catalysis resulting in alternative product formation (21,22). Therefore, tumbling or imprecise barrier crossing in an oversized active site cavity may also be the reason for alternative products observed during the catalytic cascade, as alternative products in TXS and mutants seem exclusively to be derived from later catalytic stages, i.e., cations C-E ( Fig.…”

“…1). However, structural data and mechanistic considerations explaining this promiscuity on the molecular level are very limited (8,(21)(22)(23)(24)(25). The role of the protein scaffold has been widely thought to chaperone the cyclization cascade by merely conformational control (22,26).…”

Nature as organic chemist

“…82 These studies confirmed the universal all-helical fold of the sesquiterpene and monoterpene synthases, and have provided detailed pictures of the organization of the active sites of these remarkable, synthetically versatile cyclases. 76,[83][84][85][86] Complementing and enriching these studies, Prof Dean J Tantillo (U. C. Davis) has deepened the understanding of the mechanisms and energetics of terpene cyclization reactions through detailed quantum chemical calculations of the reaction pathways linking carbocationic intermediates and transition state structures for a wide variety of terpene synthase-catalyzed reactions. [87][88][89][90][91] In spite of the substantial progress in cloning terpene synthases, the search for additional terpene synthase genes was seriously hindered by the exceptionally low levels of mutual sequence similarity among known terpene synthases, thus thwarting the routine application of homology-based cloning methods to this class of microbial proteins.…”