Activity assays of isobutanol pathway enzymes for C. cellulolyticum strains expressing the empty vector, *alsS ilvCD kivd adhA, and kivd yqhD alsS ilvCD, determining the activity for AlsS (one specific unit of Als activity corresponds to the formation of 1 nmol of acetoin per min per mg of soluble protein at 34°C) (A), IlvC (one specific unit of IlvC activity corresponds to the oxidation of 1 nmol of NADPH per min per mg of soluble protein at 34°C) (B), IlvD (one specific unit of IlvD activity corresponds to the formation of 1 nmol of 2-ketoisovalerate per min per mg of soluble protein at 34°C) (C), Kivd (one specific unit of Kivd activity corresponds to the oxidation of 1 nmol of NADPH per min per mg of soluble protein at 34°C) (D), and AdhA and YqhD activity [one specific unit of ADH activity corresponds to the oxidation of 1 nmol of NAD(P)H per min per mg of soluble protein at 34°C] (E).