Electrophilic and nucleophilic enzymatic cascade reactions in biosynthesis

Abstract

The biosynthesis of cyclic terpenoids and polyethers involves enzyme-initiated cascade reactions for ring formation. While the former are obtained by electrophilic cascades through carbenium ions as intermediates, cyclic polyethers are formed by nucleophilic cascade reactions of (poly)epoxide precursors. These mechanistically complementary pathways follow common principles via (i) triggering of the cascade by forming a reactive intermediate ('initiation'), (ii) sequential 'proliferation' of the cyclization and finally (iii) 'termination' of the cascade. As analyzed in this concept paper, the multiplicity of precursors, combined with various initiation and termination routes and kinetically favored or disfavored cyclization modes accounts for the enormous diversity in cyclic terpenoid and polyether scaffolds. Although the essential role of enzymes in the triggering of these cascades is reasonably well understood, remarkably little is known about their influence in proliferation reactions, especially those implying kinetically disfavored (anti-Markovnikov and anti-Baldwin) routes. Mechanistic analysis of enzymatic cascade reactions provides biomimetic strategies for natural product synthesis.

Enzymkaskadenreaktionen in der Biosynthese

“…[32] Die umfangreichste Kategorie findet auf den Biosynthesewegen von Te rpenen statt, bei denen die Reaktionen von Carbokationen die enzymatischen Reaktionsfolgen dominieren.…”

3.10 Emerging Enzymes

“…[44,45] AacSHC catalyzes the polyene cyclization of squalene (19) to hopene (20) and hopanol (21). [46] SHC binds squalene (19) in the all-chair conformation and initiates the sequential ring formation by protonation of the terminal isoprene C=C bond. The polyene cyclization proceeds via a series of conformationally rigid, partially cyclized carbocationic intermediates within an at least partially concerted reaction.…”

“…Furthermore, cyclase enzymes shield the carbocationic intermediates from premature addition of water or deprotonation with the hydrophobic environment of aromatic amino acid residues, typically phenylalanine, tyrosine, or tryptophan, in the active site. [46] Scheme 12 Squalene Hopene Cyclase Catalyzed Cyclization of Squalene to Hopene and Hopanol [46] Enz-AH The protonation of C=C bonds requires a relatively strong Brønsted acid, which is provided by SHCs unique protonation machinery. Earlier site-directed mutagenesis studies established that the active site residue aspartic acid at position 376 (Asp376) of AacSHC is essential for catalytic activity.…”

“…As described above, the same mutant has been used in the synthesis of (+)-ambrein from squalene (19). [46] Moreover, the relaxed substrate and product specificities of mutant SHCs point to the possibility of generating novel terpenes and terpenoids by evolution of these enzymes. Therefore, a semi-rational approach to reshape the active site of AacSHC has been undertaken substituting every position within 15 of the catalytic aspartic acid Asp376.…”

Stability/activity features of the main enzyme components of rohapect 10L

“…Enzymes are extremely useful catalysts in many areas . In some instances, a sequence of reactions is necessary to get the final product, the so‐called cascade reactions, involving several enzymes . That is, the case of polysaccharides modifying enzymes, where the substrate is complex, having different bonds and several enzymes are required to reach the final product (e.g., glucose) .…”