Soluble expression and purification of the oxidoreductase component of toluene 4-monooxygenase

Toluene 4-monooxygenase (T4MO) is a member of the bacterial multicomponent monooxygenases, an enzyme family that utilizes a soluble diiron hydroxylase to oxidize a variety of hydrocarbons as the initial step in their metabolism. The hydroxylases obtain reducing equivalents from NAD(P)H via an electron transfer chain that is initiated by an oxidoreductase containing an N-terminal ferredoxin domain and C-terminal flavin- and NAD-binding domains. T4moF, the NADH oxidoreductase of T4MO, was expressed as a soluble protein in Escherichia coli BL21(DE3) from the pUC-derived expression vector pRS205. This vector contains a lac promoter instead of a T7 promoter. A three step purification from the soluble cell lysate yielded ∼1 mg of T4moF per gram of wet cell paste with greater than 90% purity. The purified protein contained 1 mol of FAD and 2 mol of Fe per mol of T4moF; quantitative EPR spectroscopy showed ∼1 mol of the S = 1/2 signal from the reduced [2Fe–2S] cluster per mol of T4moF. Steady state kinetic analysis of p-cresol formation activity treating T4moF as the variable substrate while all other proteins and substrates were held constant gave apparent KM- and apparent kcat-values of 0.15 μM and 3.0 s−1, respectively. This expression system and purification allows for the recovery of the soluble oxidoreductase in yields that facilitate further biochemical and structural characterizations.