The reaction was initiated by the addition of approx. 100 μM DQ or Q2 to the assay buffer containing NADH, complex I and asolectin phospholipids, and monitored by HPLC analysis (◆, DQ; ■, Q2; ◇, DQH2; □, Q2H2) and UV–visible spectroscopy (+,NADH). (A) Conversion of DQ into DQH2 modelled using Km=24 μM, Ki=26 μM and kcat=3.0 mol−1·dm3·s−1. (B) Conversion of Q2 into Q2H2 modelled using Km=20.4 μM, Ki=15.7 μM and kcat=1.2 mol−1·dm3·s−1.