Figure 1. Reaction mechanisms relevant to NDH-2 catalysis.

The binding sites for NADH/NAD⁺ and Q/QH₂ are shown shaded in red and blue, respectively. (A) Ping-pong mechanism in which NADH and Q (quinone) react sequentially and are never bound to the enzyme at the same time. (B) Classical ternary mechanism in which both substrates bind to form a ternary complex, the reaction occurs, and both products dissociate. (C) Atypical ternary mechanism that better reflects proposed mechanisms for NDH-2, in which NADH binds and reacts but NAD⁺ does not dissociate, then Q binds and reacts in a ternary complex with the enzyme-product complex.