Team:UC Davis/Protein Engineering MM

Now that we had identified three aldehyde dehydrogenases with unique specificity profiles, we proceeded to experimentally determine the kinetic constants (kcat and KM) of these enzymes on each substrate in our assay conditions.

Key: N/A: Not applicable
ND: No detection of activity

Stock Solutions (Unless stated, all solutions were prepared in 11.1mM Potassium Phosphate, 111.1mM Potassium Chloride solution)
Base: 1.54% Tween 20 (v/v), 1.11mM Dithiothreitol (DTT)
Substrate: 10x final concentration of aldehyde in isopropyl alcohol
Enzyme: 0.004mg/mL Aldehyde Dehydrogenase enzyme, 4mM NAD+, 1.11mM Dithiothreitol (DTT)

Order of addition to initiate assay: (1) 130uL substrate, (2) 20uL aldehyde, (3) 50uL enzyme. Assays were conducted in a Costar 96 well flat bottom plate. The activities of all aldehyde dehydrogenases were measured by the accumulation of NADH resulting from the oxidation of the assayed substrates. All assays were performed by incubating 18.72nM aldehyde dehydrogenase enzyme with substrate in a 200µL reaction containing 10mM phosphate buffer containing 100mM potassium chloride (pH 7.4), 1mM NAD+, 1mM DTT, 1% Tween 20, 10% isoproyl alcohol, and substrate. All aldehyde dehydrogenases were assayed against substrate concentrations of 1000uM, 500uM, 100uM, 50uM, 10uM, and 5uM. The change in absorbance of NADH at a wavelength of 340nm was measured using Biotek Epoch and Synergy microplate spectrophotometers. Kinetic constants were derived from the steady-state velocity curves using R. Curves were fit linearly [V=(kcat/KM)S], Michaelis-Menten [V=Vmax*S/(Km+S)], or substrate inhibition [V=Vmax*S/(Km+S*(1+S/Ki))].