Figure 3.

(A) Two mechanisms have been proposed for the conversion of N-methylguanosine to 4-demethylwyosine catalyzed by Tyw1. (top) A lysine stabilizes intermediates by forming a Schiff base with pyruvate; the auxiliary cluster functions as a redox center.⁴³ (bottom) Coordination of the auxiliary cluster promotes the reaction of the substrate radical with pyruvate facilitates cyclization.⁴⁴ (B) Proposed mechanism for aminofutalosine synthesis catalyzed by MqnE. The key first step involves a radical Michael addition of Ado• to the chorismate-derived cosubstrate; subsequent rearrangement and oxidative decarboxylation produces aminofutalosine. (C) Proposed mechanism for the reaction of tyrosine and diaminouracil catalyzed by F₀ synthase. The mechanism involves the abstraction of hydrogen from tyrosine and the release of a dehydroglycine, producing a reactive 4-oxidobenzyl radical, which adds to diaminouracil. A second round of radical chemistry initiates formation of the tricyclic deazaflavin, F₀, that in further steps is converted to F₄₂₀.