equisetin and decalin

Enzymes catalyzing [4+2] cycloaddition have attracted increasing attention because of their key roles in natural product biosynthesis. Here, we solved the X-ray crystal structures of a pair of decalin synthases, Fsa2 and Phm7, that catalyze intramolecular [4+2] cycloadditions to form enantiomeric decalin scaffolds during biosynthesis of the HIV-1 integrase inhibitor equisetin and its stereochemical opposite, phomasetin. Computational modeling, using molecular dynamics simulations as well as quantum chemical calculations, demonstrates that the reactions proceed through synergetic conformational constraints assuring transition state-like substrates folds and their stabilization by specific protein-substrate interactions. Site-directed mutagenesis experiments verified the binding models. Intriguingly, the flexibility of bound substrates is largely different in two enzymes, suggesting the distinctive mechanism of dynamics regulation behind these stereoselective reactions. The proposed reaction mechanism herein deepens the basic understanding how these enzymes work but also provides a guiding principle to create artificial enzymes.

Topics: Models, Molecular; Molecular Conformation; Naphthalenes; Pyrrolidinones; Stereoisomerism; Tetrahydronaphthalenes

Tetramic acid containing a decalin ring such as equisetin and phomasetin is one of the characteristic scaffolds found in fungal bioactive secondary metabolites. Polyketide (PKS)-nonribosomal peptide synthetase (NRPS) hybrid enzyme is responsible for the synthesis of the polyketide scaffold conjugated with an amino acid. PKS-NRPS hybrid complex programs to create structural diversity in the polyketide backbone have begun to be investigated, yet mechanism of control of the stereochemistry in a decalin formation via a Diels-Alder cycloaddition remains uncertain. Here, we demonstrate that fsa2, which showed no homology to genes encoding proteins of known function, in the fsa cluster responsible for equisetin and fusarisetin A biosynthesis in Fusarium sp. FN080326, is involved in the control of stereochemistry in decalin formation via a Diels-Alder reaction in the equisetin biosynthetic pathway.

Topics: Fusarium; Genes, Fungal; Multigene Family; Naphthalenes; Polyketide Synthases; Pyrrolidinones; Stereoisomerism; Tetrahydronaphthalenes

A late stage Diels-Alder reaction is used to prepare a mixture of JBIR-22, a natural product from the Equisetin family of tetramic acids, and one of its diastereomers. This is achieved in just 8 steps from pyruvate. The success of the late stage DA approach is discussed in the context of the biosynthesis of JBIR-22 (and perhaps related natural products).

Topics: Cyclization; Molecular Structure; Naphthalenes; Pyrrolidinones; Stereoisomerism; Tetrahydronaphthalenes

Three new decalin-type tetramic acid analogues, pyrrolocins A (1), B (2), and C (3), were defined as products of a metabolic pathway from a fern endophyte, NRRL 50135, from Papua New Guinea. NRRL 50135 initially produced 1 but ceased its production before chemical or biological evaluation could be completed. Upon transfer of the biosynthetic pathway to a model host, 1-3 were produced. All three compounds are structurally related to equisetin-type compounds, with 1 and 3 having a trans-decalin ring system, while 2 has a cis-fused decalin. All were active against Mycobacterium tuberculosis, with the trans-decalin analogues 1 and 3 exhibiting lower MICs than the cis-decalin analogue 2. Here we report the isolation, structure elucidation, and antimycobacterial activities of 1-3 from the recombinant expression as well as the isolation of 1 from the wild-type fungus NRRL 50135.

Topics: Anti-Bacterial Agents; Bacillus subtilis; Endophytes; Ferns; Microbial Sensitivity Tests; Models, Molecular; Molecular Structure; Mycobacterium tuberculosis; Naphthalenes; Nuclear Magnetic Resonance, Biomolecular; Papua New Guinea; Pyrrolidinones; Staphylococcus aureus; Stereoisomerism; Streptococcus pneumoniae; Tetrahydronaphthalenes