Formaldehyde degradation in Corynebacterium glutamicum involves acetaldehyde dehydrogenase and mycothiol-dependent formaldehyde dehydrogenase

Corynebacterium glutamicum, a Gram-positive soil bacterium belonging to the actinomycetes, is able to degrade formaldehyde but the enzyme(s) involved in this detoxification process were not known. Acetaldehyde dehydrogenase Ald, which is essential for ethanol utilization, and FadH, characterized here as NAD-linked mycothiol-dependent formaldehyde dehydrogenase, were shown to be responsible for formaldehyde oxidation since a mutant lacking ald and fadH could not oxidize formaldehyde resulting in the inability to grow when formaldehyde was added to the medium. Moreover, C. glutamicum ΔaldΔfadH did not grow with vanillate, a carbon source giving rise to intracellular formaldehyde. FadH from C. glutamicum was purified from recombinant Escherichia coli and shown to be active as a homotetramer. Mycothiol-dependent formaldehyde oxidation revealed K_m values of 0.6 mM for mycothiol and 4.3 mM for formaldehyde and a V_max of 7.7 U mg⁻¹. FadH from C. glutamicum also possesses zinc-dependent, but mycothiol-independent alcohol dehydrogenase activity with a preference for short chain primary alcohols such as ethanol (K_m = 330 mM, V_max = 9.6 U mg⁻¹), 1-propanol (K_m = 150 mM, V_max = 5 U mg⁻¹) and 1-butanol (K_m = 50 mM, V_max = 0.8 U mg⁻¹). Formaldehyde detoxification system by Ald and mycothiol-dependent FadH is essential for tolerance of C. glutamicum to external stress by free formaldehyde in its habitat and for growth with natural substrates like vanillate, which are metabolized with concomitant release of formaldehyde.