Biochemical characterization of chloromethane emission from the wood-rotting fungus phellinus pomaceus

Many wood-rotting fungi, including Phellinus pomaceus, produce chloromethane (CH3Cl). P. pomaceus can be cultured in undisturbed glucose mycological peptone liquid medium to produce high amounts of CH3Cl. The biosynthesis of CH3Cl is catalyzed by a methyl chloride transferase (MCT), which appears to be membrane bound. The enzyme is labile upon removal from its natural location and upon storage at low temperature in its bound state. Various detergents failed to solubilize the enzyme in active form, and hence it was characterized by using a membrane fraction. The enzyme had a sharp pH optimum between 7 and 7.2. Its apparent Km for Cl- (ca. 300 mM) was much higher than that for I- (250 &mgr;M) or Br- (11 mM). A comparison of these Km values to the relative in vivo methylation rates for different halides suggests that the real Km for Cl- may be much lower, but the calculated value is high because the CH3Cl produced is used immediately in a coupled reaction. Among various methyl donors tested, S-adenosyl-L-methionine (SAM) was the only one that supported significant methylation by MCT. The reaction was inhibited by S-adenosyl-L-homocysteine, an inhibitor of SAM-dependent methylation, suggesting that SAM is the natural methyl donor. These findings advance our comprehension of a poorly understood metabolic sector at the origin of biogenic emissions of halomethanes, which play an important role in atmospheric chemistry.