Organization of the multiple coenzymes and subunits and role of the covalent flavin link in the complex heterotetrameric sarcosine oxidase

Heterotetrameric (alpha beta gamma delta) sarcosine oxidase from Corynebacterium sp. P-l (cTSOX) contains noncovalently bound FAD and NAD(+) and covalently bound FMN, attached to beta (His173). The beta (His173Asn) mutant is expressed as a catalytically inactive, labile heterotetramer. The beta and delta subunits are lost during mutant enzyme purification, which yields a stable alpha gamma complex. Addition of stabilizing agents prevents loss of the delta but not the beta subunit. The covalent flavin link is clearly a critical structural element and essential for TSOX activity or preventing FMN loss. The alpha subunit was expressed by itself and purified by affinity chromatography. The alpha and beta subunits each contain an NH2-terminal ADP-binding motif that could serve as part of the binding site for NAD(+) or FAD. The alpha subunit and the alpha gamma complex were each found to contain 1 mol of NAD(+) but no FAD, Since NAD(+) binds to alpha, FAD probably binds to beta. The latter could not be directly demonstrated since it was not possible to express beta by itself. However, FAD in TSOX from Pseudomonas maltophilia (pTSOX) exhibits properties similar to those observed for the covalently bound FAD in monomeric sarcosine oxidase and N-methyltryptophan oxidase, enzymes that exhibit sequence homology with P. A highly conserved glycine in the ADP-binding motif of the alpha (Gly139) or beta (Gly30) subunit was mutated in an attempt to generate NAD(+)- or FAD-free cTSOX, respectively. The alpha (Gly139Ala) mutant is expressed only at low temperature (t(optimum) = 15 degreesC), but the purified enzyme exhibited properties indistinguishable from the wild-type enzyme. The much larger barrier to NAD(+) binding in the case of the alpha (Gly139Val) mutant could not be overcome even by growth at 3 degreesC, suggesting that NAD(+) binding is required for TSOX expression. The beta (Gly30Ala) mutant exhibited subunit expression levels similar to those of the wild-type enzyme, but the mutation blocked subunit assembly and covalent attachment of FMN, suggesting that both processes require a conformational change in beta that is induced upon FAD binding. About half of the covalent FMN in recombinant preparations of cTSOX or pTSOX is present as a reversible covalent 4a-adduct with a cysteine residue. Adduct formation is not prevented by mutating any of the three cysteine residues in the beta subunit of cTSOX to Ser or Ala. Since FMN is attached via its 8-methyl group to the beta subunit, the FMN ring must be located at the interface between beta and another subunit that contains the reactive cysteine residue.