Activation-independent cyclization of monoterpenoids

The biosynthesis of cyclic monoterpenes (C(10)) generally requires the cyclization of an activated linear precursor (geranyldiphosphate) by specific terpene cyclases. Cyclic triterpenes (C(30)), on the other hand, originate from the linear precursor squalene by the action of squalene-hopene cyclases (SHCs) or oxidosqualene cyclases (OSCs). Here, we report a novel terpene cyclase from Zymomonas mobilis (ZMO1548-Shc) with the unique capability to cyclize citronellal to isopulegol. To our knowledge, ZMO1548-Shc is the first biocatalyst with diphosphate-independent monoterpenoid cyclase activity. A combinatorial approach using site-directed mutagenesis and modeling of the active site with a bound substrate revealed that the cyclization of citronellal proceeds via a different mechanism than that of the cyclization of squalene.

Characterization of the channel constriction allowing the access of the substrate to the active site of yeast oxidosqualene cyclase

In oxidosqualene cyclases (OSCs), an enzyme which has been extensively studied as a target for hypocholesterolemic or antifungal drugs, a lipophilic channel connects the surface of the protein with the active site cavity. Active site and channel are separated by a narrow constriction operating as a mobile gate for the substrate passage. In Saccharomyces cerevisiae OSC, two aminoacidic residues of the channel/constriction apparatus, Ala525 and Glu526, were previously showed as critical for maintaining the enzyme functionality. In this work sixteen novel mutants, each bearing a substitution at or around the channel constrictions, were tested for their enzymatic activity. Modelling studies showed that the most functionality-lowering substitutions deeply alter the H-bond network involving the channel/constriction apparatus. A rotation of Tyr239 is proposed as part of the mechanism permitting the access of the substrate to the active site. The inhibition of OSC by squalene was used as a tool for understanding whether the residues under study are involved in a pre-catalytic selection and docking of the substrate oxidosqualene.

Squalene-hopene cyclases

Hopanoids and sterols are members of a large group of cyclic triterpenoic compounds that have important functions in many prokaryotic and eukaryotic organisms. They are biochemically synthesized from linear precursors (squalene, 2,3-oxidosqualene) in only one enzymatic step that is catalyzed by squalene-hopene cyclase (SHC) or oxidosqualene cyclase (OSC). SHCs and OSCs are related in amino acid sequences and probably are derived from a common ancestor. The SHC reaction requires the formation of five ring structures, 13 covalent bonds, and nine stereo centers and therefore is one of the most complex one-step enzymatic reactions. We summarize the knowledge of the properties of triterpene cyclases and details of the reaction mechanism of Alicyclobacillus acidocaldarius SHC. Properties of other SHCs are included.

An Oxidosqualene Cyclase Makes Numerous Products by Diverse Mechanisms: A Challenge to Prevailing Concepts of Triterpene Biosynthesis

The genome of the model plant Arabidopsis thaliana encodes 13 oxidosqualene cyclases, 9 of which have been characterized by heterologous expression in yeast. Here we describe another cyclase, baruol synthase (BARS1), which makes baruol (90%) and 22 minor products (0.02-3% each). This represents as many triterpenes as have been reported for all other Arabidopsis cyclases combined. By accessing an extraordinary repertoire of mechanistic pathways, BARS1 makes numerous skeletal types and deprotonates the carbocation intermediates at 14 different sites around rings A, B, C, D, and E. This undercurrent of structural and mechanistic diversity in a superficially accurate enzyme is incompatible with prevailing concepts of triterpene biosynthesis, which posit tight control over the mechanistic pathway through cation-pi interactions, with a single proton acceptor in a hydrophobic active site. Our findings suggest that mechanistic diversity is the default for triterpene biosynthesis and that product accuracy results from exclusion of alternative pathways.

A synchronized substrate-gating mechanism revealed by cubic-core structure of the bovine branched-chain alpha-ketoacid dehydrogenase complex

The dihydrolipoamide acyltransferase (E2b) component of the branched-chain alpha-ketoacid dehydrogenase complex forms a cubic scaffold that catalyzes acyltransfer from S-acyldihydrolipoamide to CoA to produce acyl-CoA. We have determined the first crystal structures of a mammalian (bovine) E2b core domain with and without a bound CoA or acyl-CoA. These structures reveal both hydrophobic and the previously unreported ionic interactions between two-fold-related trimers that build up the cubic core. The entrance of the dihydrolipoamide-binding site in a 30-A long active-site channel is closed in the apo and acyl-CoA-bound structures. CoA binding to one entrance of the channel promotes a conformational change in the channel, resulting in the opening of the opposite dihydrolipoamide gate. Binding experiments show that the affinity of the E2b core for dihydrolipoamide is markedly increased in the presence of CoA. The result buttresses the model that CoA binding is responsible for the opening of the dihydrolipoamide gate. We suggest that this gating mechanism synchronizes the binding of the two substrates to the active-site channel, which serves as a feed-forward switch to coordinate the E2b-catalyzed acyltransfer reaction.