Site-directed mutagenesis and substrate compatibility to reveal the structure-function relationships of plant oxidosqualene cyclases

Covering: up to May 2020Oxidosqualene cyclases (OSCs) catalyze one of the most complex polycyclization reactions in nature, using the linear 2,3-oxidosqualene to generate an array of triterpene skeletons in plants. Despite the structural diversity of the products, the protein sequences of plant OSCs are highly conserved, where a few key amino acids could govern the product selectivity. Due to the absence of crystal structures, site-directed mutagenesis and substrate structural modification become key approaches to understand the cyclization mechanism. In this review, 98 mutation sites in 25 plant OSCs have been summarized, and the conserved key residues have been identified by sequence alignment. Structure-function relationships are further discussed. Meanwhile, the substrate selectivity has been summarized to probe the active site cavity of plant OSCs. A total of 77 references are included.

Bioactive terpenoid constituents from Eclipta prostrata

Chemical fractionation of the ethanolic extract of Eclipta prostrata yielded a series of unreported terpenoid constituents, including a rare 6/6/6/6-fused tetracyclic triterpenoid, a pentacyclic triterpenoid, two pentacyclic triterpenoid saponins, a diterpenoid and a sesquiterpenoid. Structures were assigned to these compounds on the basis of comprehensive spectroscopic analyses, with the absolute configurations of the tetracyclic triterpenoid, the diterpenoid and the sesquiterpenoid being determined via explanation of electronic circular dichroism data. Screening of these isolates in an array of bioassays revealed antibacterial, cytotoxic and α-glucosidase inhibitory activities for selective compounds. Of particular interest, the tetracyclic triterpenoid showed very strong inhibition against α-glucosidase with an IC₅₀ of 0.82 ± 0.18 μM, being 10³-fold as active as the positive control acarbose.

β-Amyrin biosynthesis: catalytic mechanism and substrate recognition

In the past five years, there have been remarkable advances in the study of β-amyrin synthase. This review outlines the catalytic mechanism and substrate recognition in β-amyrin biosynthesis, which have been attained by the site-directed mutagenesis and substrate analog experiments.

β-Amyrin synthase from Euphorbia tirucalli L. functional analyses of the highly conserved aromatic residues Phe413, Tyr259 and Trp257 disclose the importance of the appropriate steric bulk, and cation-π and CH-π interactions for the efficient catalytic action of the polyolefin cyclization cascade

Many of the functions of the active site residues in β-amyrin synthase and its catalytic mechanism remain unclear. Herein, we examined the functions of the highly conserved Phe413, Tyr259, and Trp257 residues in the β-amyrin synthase of Euphorbia tirucalli. The site-specific mutants F413V and F413M [corrected] showed nearly the same enzymatic activities as the wild type, indicating that π-electrons are not needed for the catalytic reaction. However, the F413A [corrected] mutant yielded a large amount of the tetracyclic dammarane skeleton, with decreased production of β-amyrin. This indicates that the Phe413 [corrected] residue is located near the D-ring formation site and works to position the oxidosqualene substrate correctly within the reaction cavity. On the other hand, the major catalysis-related function of the Tyr259 and Trp257 residues is to yield their π-electrons to the cationic intermediates. The Y259F variant showed nearly equivalent activity to that of the wild type, but aliphatic mutants such as the Ala, Val, and Leu variants showed significantly decreased the activity and yielded the tetracyclic dammarane scaffold, strongly demonstrating that the Tyr259 residue stabilizes the baccharenyl secondary cation via cation-π interaction. The aliphatic variants of Trp257 exhibited remarkably decreased enzymatic activity, and lupeol was produced in a high production ratio, indicating that Trp257 stabilizes the oleanyl cation via cation-π interaction. The aromatic Phe and Tyr mutants exhibited high activities owing to their more increased π-electron density relative to that of the aliphatic mutants, but lupeol was produced in a significantly high yield besides β-amyrin. The Trp residue is likely to be responsible for the robust binding of Me-30 through CH-π interaction. The decreased π-electron density of the Phe and Tyr mutants compared to that of Trp would have resulted in the high production of lupeol.