Sarcosine Dehydrogenase - Mechanism

Mechanism

Sarcosine dehydrogenase, with sarcosine as its substrate, follows Michaelis-Menten kinetics and has a Km of 0.5 mM and a Vmax of 16 mmol/hr/mg protein. The enzyme is inhibited competitively by methoxyacetic acid, which has a Ki of 0.26 mM

The exact mechanism of sarcosine dehydrogenase is not available. However, according to the overall net reaction discussed in Honova.E, et al. paper:

• Sarcosine + H₂O + O₂ → glycine + formaldehyde + H₂O₂

the first step of the reaction might involve the transfer of a hydride on the N-methyl group of sarcosine onto FAD, allowing H₂O to attack the carbocation in order to form intermediate 1 (See figure 1). There is no deamination step. Instead, the demethylation of the N-methyl group on sarcosine occurs directly. The reduced FADH- from the first step then is oxidized by O₂ to form H₂O₂.

The demethylation of sarcosine catalyzed by sarcosine dehydrogenase can proceed with or without the presence of tetrahydrofolate. Under anaerobic condition and without tetrahydrofolate, however, a free formaldehyde is formed after the N-demethylation of sarcosine. The reaction with 1 mole of sarcosine and 1 mole of FAD, under this condition, yields 1 mole of glycine and 1 mole of formaldehyde (See figure 2 for mechanism).

Under the presence of tetrahydrofolate, sarcosine dehydrogenase binds to tetrahydrofolate and convert tetrahydrofolate to 5,10-methylenetetrahydrofolate. Tetrahydrofolate here serves as a 1-carbon acceptor during the demethylation process (See figure 3 for mechanism).