Identification of Critical Steps Governing the Two-Component Alkanesulfonate Monooxygenase Catalytic Mechanism

The alkanesulfonate monooxygenase enzyme (SsuD) catalyzes the oxygenolytic cleavage of a carbon–sulfur bond from sulfonated substrates. A mechanism involving acid–base catalysis has been proposed for the desulfonation mechanism by SsuD. In the proposed mechanism, base catalysis is involved in abstracting a proton from the alkane peroxyflavin intermediate, while acid catalysis is needed for the protonation of the FMNO^– intermediate. The pH profiles of k_cat indicate that catalysis by SsuD requires a group with a pK_a of 6.6 ± 0.2 to be deprotonated and a second group with a pK_a of 9.5 ± 0.1 to be protonated. The upper pK_a value was not present in the pH profiles of k_cat/K_m. Several conserved amino acid residues (His228, His11, His333, Cys54, and Arg226) have been identified as having potential catalytic importance due to the similar spatial arrangements with close structural and functional relatives of SsuD. Substitutions to these amino acid residues were generated, and the pH dependencies were evaluated and compared to wild-type SsuD. Although a histidine residue was previously proposed to be the active site base, the His variants possessed similar steady-state kinetic parameters as wild-type SsuD. Interestingly, R226A and R226K SsuD variants possessed undetectable activity, and there was no detectable formation of the C4a-(hydro)­peroxyflavin intermediate for the Arg226 SsuD variants. Guanidinium rescue with the R226A SsuD variant resulted in the recovery of 1.5% of the wild-type SsuD k_cat value. These results implicate Arg226 playing a critical role in catalysis and provide essential insights into the mechanistic steps that guide the SsuD desulfonation process.