Differential inhibition in vivo of ammonia monooxygenase, soluble methane monooxygenase and membrane‐associated methane monooxygenase by phenylacetylene

Phenylacetylene was investigated as a differential inhibitor of ammonia monooxygenase (AMO), soluble methane monooxygenase (sMMO) and membrane‐associated or particulate methane monooxygenase (pMMO) in vivo . At phenylacetylene concentrations > 1 µM, whole‐cell AMO activity in Nitrosomonas europaea was completely inhibited. Phenylacetylene concentrations above 100 µM inhibited more than 90% of sMMO activity in Methylococcus capsulatus Bath and Methylosinus trichosporium OB3b. In contrast, activity of pMMO in M. trichosporium OB3b, M. capsulatus Bath, Methylomicrobium album BG8, Methylobacter marinus A45 and Methylomonas strain MN was still measurable at phenylacetylene concentrations up to 1000 µM. AMO of Nitrosococcus oceanus has more sequence similarity to pMMO than to AMO of N. europaea . Correspondingly, AMO in N. oceanus was also measurable in the presence of 1000 µM phenylacetylene. Measurement of oxygen uptake indicated that phenylacetylene acted as a specific and mechanistic‐based inhibitor of whole‐cell sMMO activity; inactivation of sMMO was irreversible, time dependent, first order and required catalytic turnover. Corresponding measurement of oxygen uptake in whole cells of methanotrophs expressing pMMO showed that pMMO activity was inhibited by phenylacetylene, but only if methane was already being oxidized, and then only at much higher concentrations of phenylacetylene and at lower rates compared with sMMO. As phenylacetylene has a high solubility and low volatility, it may prove to be useful for monitoring methanotrophic and nitrifying activity as well as identifying the form of MMO predominantly expressed in situ .