A monodomain class II terpene cyclase assembles complex isoprenoid scaffolds.

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Class II terpene cyclases catalyze complex polycyclization reactions, typically featuring a β,γ-didomain architecture.
• This architecture is thought to have evolved from fused monodomain proteins, but no monodomain class II cyclases have been identified.
• Class I terpene cyclases with single domains have been previously characterized.

Purpose of the Study:

• To present the high-resolution X-ray structures of a novel monodomain class II cyclase, merosterolic acid synthase (MstE).
• To elucidate the cyclization mechanism of this enzyme and its role in producing cytotoxic meroterpenoids.
• To explore the evolutionary implications of monodomain class II cyclases.

Main Methods:

• High-resolution X-ray crystallography of merosterolic acid synthase (MstE).
• Structural analysis of the enzyme with bound substrate, product, and inhibitor.
• Biochemical characterization of the enzyme's catalytic activity.

Main Results:

• The first high-resolution structures of a monodomain class II cyclase (MstE) were determined.
• MstE, with a minimalistic β-domain, synthesizes four-ring cytotoxic meroterpenoids with sterol-like topology.
• Structural data revealed a cyclization mechanism governed by noncanonical enzyme regions.

Conclusions:

• Monodomain class II cyclases exist and can catalyze complex polycyclization reactions, challenging previous architectural assumptions.
• Archaic monodomain cyclases may have been capable of intricate reaction cascades.
• The findings provide a structural basis for understanding terpene biosynthesis and evolution.