Proposed scheme for backflow of electrons in cytochrome <i>bd</i> in MV state.

<p>Each rhombus represents one of the three hemes in cytochrome <i>bd</i>. Filled or semi-filled rhombus denotes that site is in the reduced state. Empty rhombus denotes that site is in the oxidized state. During the backflow reaction (observed in ∼4% of the enzyme at 1 mM CO) the electron from heme <i>d</i> transfers sequentially to heme <i>b</i>₅₉₅ (A→B transition, τ<1.5 µs) and heme <i>b</i>₅₅₈ (B→C transition, τ∼16 µs). Finally, the electron equilibrates between the redox sites with respect to their redox potentials. A→D and C→D transitions describe recombination of CO to 96% and 4% of heme <i>d</i>, respectively.