Kinetic Studies of the Mechanism of Carbon−Hydrogen Bond Breakage by the Heterotetrameric Sarcosine Oxidase of Arthrobacter sp. 1-IN

Abstract

The reaction of heterotetrameric sarcosine oxidase (TSOX) of Arthrobactor sp. 1-IN has been studied by stopped-flow spectroscopy, with particular emphasis on the reduction of the enzyme by sarcosine. Expression of the cloned gene encoding TSOX in Escherichia coli enables the production of TSOX on a scale suitable for stopped-flow studies. Treatment of the enzyme with sulfite provides the means for selective formation of a flavin−sulfite adduct with the covalent 8α-(N³-histidyl)-FMN. Formation of the sulfite−flavin adduct suppresses internal electron transfer between the noncovalent FAD (site of sarcosine oxidation) and the covalent FMN (site of enzyme oxidation) and thus enables detailed characterization of the kinetics of FAD reduction by sarcosine using stopped-flow methods. The rate of FAD reduction displays a simple hyperbolic dependence on sarcosine concentration. Studies in the pH range 6.5−10 indicate there are no kinetically influential ionizations in the enzyme−substrate complex. A plot of the limiting rate of flavin reduction/the enzyme−substrate dissociation constant (k_lim/K_d) versus pH is bell-shaped and characterized by two macroscopic pK_a values of 7.4 ± 0.1 and 10.4 ± 0.2: potential candidates for the two ionizable groups are discussed with reference to the structure of monomeric sarcosine oxidase (MSOX). The kinetic data are discussed with reference to potential mechanisms for the oxidation of amine molecules by flavoenzymes. Additionally, kinetic isotope effect studies of the rate of C−H bond breakage suggest that a ground-state quantum tunneling mechanism for H-transfer, facilitated by the low-frequency thermal motions of the protein molecule, accounts for C−H bond cleavage by TSOX. TSOX thus provides another example of C−H bond breakage by ground-state quantum tunneling, driven by protein dynamics [vibrationally enhanced ground-state quantum tunneling (VEGST)], for the oxidation of amines by enzymes.