Scheme 1. Mechanism of Quinone Reduction at the Flavin Active Site, Incorporating both the Ping-Pong and Ping-Pong-Pong Reactions, and Double-Substrate Inhibition.

The ping-pong and ping-pong-pong reactions diverge at [Fl_Red Q]: the ping-pong reaction follows the dotted arrow to produce QH₂ and returns directly to [Fl_Ox], and the ping-pong-pong reaction follows the outside pathway to form two semiquinones (QH^•). Abbreviations: Fl_Ox/Red, oxidized or reduced state of the flavin; terms in brackets, enzyme-bound species; k_cat, first-order rate constant for substrate transformation and product dissociation; K_M, Michaelis−Menten constant; K_Ox, K_Semi, and K_Red, dissociation constants, referring to the oxidized, semireduced, and reduced enzyme states, respectively.