Figure 6.

Rates of SQOR-catalyzed oxidation of H₂S in the presence of a single acceptor (X) were simulated by using a rate equation derived for a mechanism proposed by Libiad et al. (Scheme 3, eq. 5). The simulations shown in the solid line curves were generated with H₂S as the variable substrate in the presence of nearly saturating concentrations of CoQ₁ (10 × K_{m CoQ1}) and acceptor (100 × K_{m, x}), using K_m values for H₂S (13 μM) and CoQ₁ (19 μM) observed for the reaction with sulfite as acceptor, as described in the text. The red, blue, green, magenta and cyan lines (curves 1–5) were generated for hypothetical acceptors with K_m,x = 22 mM and k_cat,x = 37, 100, 176, 370 and 1000 s⁻¹, respectively. The dashed black line shows a simulated trace generated using an equation for a sequential mechanism (eq. 1a, A = H₂S; B = X), K_{m H²S} = 13 μM, K_m,x = 22 mM and k_cat,x = 37 or 3700 s⁻¹, [X] = 100 × K_{m, x}). To compare simulations generated using different k_cat,x values, predicted rates (k_sim) are divided by the k_cat,x value used in the simulation. Apparent K_m values for H₂S were obtained by fitting an hyperbola (y = (k_{cat app}[H₂S])/(K_{m app} + [H₂S]) to each of the simulated traces and are indicated by correspondingly colored diamond symbols.