Spectral and kinetic characterization of the Michaelis charge transfer complex in monomeric sarcosine oxidase

Monomeric sarcosine oxidase is a flavoenzyme that catalyzes the oxidation of the methyl group in sarcosine ( N- methylglycine). Rapid reaction kinetic studies under anaerobic conditions at pH 8.0 show that the enzyme forms a charge transfer Michaelis complex with sarcosine ( E- FAD(ox), sarcosine) that exhibits an intense long- wavelength absorption band ( lambda(max)) 516 nm, epsilon 516) 4800 M-1 cm(-1)). Since charge transfer interaction with sarcosine as donor is possible only with the anionic form of the amino acid, the results indicate that the pK(a) of enzyme- bound sarcosine must be considerably lower than the free amino acid ( pK(a)) 10.0). No redox intermediate is detectable during sarcosine oxidation, as judged by the isosbestic spectral course observed for conversion of E- FADox, sarcosine to reduced enzyme at 25 or 5 degrees C. The limiting rate of the reductive half- reaction at 25 degrees C ( 140 +/- 3 s(-1)) is slightly faster than turnover ( 117 +/- 3 s(-1)). The kinetics of formation of the Michaelis charge transfer complex can be directly monitored at 5 degrees C where the reduction rate is 4.5- fold slower and complex stability is increased 2- fold. The observed rate of complex formation exhibits a hyperbolic dependence on sarcosine concentration with a finite Y- intercept, consistent with a mechanism involving formation of an initial complex followed by isomerization to yield a more stable complex. Similar results are obtained for charge transfer complex formation with methylthioacetate. The observed kinetics are consistent with structural studies which show that a conformational change occurs upon binding of methylthioacetate and other competitive inhibitors.