Peering down the sink: A review of isoprene metabolism by bacteria

Abstract

Isoprene (2-methyl-1,3-butadiene) is emitted to the atmosphere each year in sufficient quantities to rival methane (>500 Tg C yr À1 ), primarily due to emission by trees and other plants. Chemical reactions of isoprene with other atmospheric compounds, such as hydroxyl radicals and inorganic nitrogen species (NO x ), have implications for global warming and local air quality, respectively. For many years, it has been estimated that soil-dwelling bacteria consume a significant amount of isoprene ($20 Tg C yr À1 ), but the mechanisms underlying the biological sink for isoprene have been poorly understood. Studies have indicated or confirmed the ability of diverse bacterial genera to degrade isoprene, whether by the canonical iso-type isoprene degradation pathway or through other less wellcharacterized mechanisms. Here, we review current knowledge of isoprene metabolism and highlight key areas for further research. In particular, examples of isoprene-degraders that do not utilize the isoprene monooxygenase have been identified in recent years. This has fascinating implications both for the mechanism of isoprene uptake by bacteria, and also for the ecology of isoprene-degraders in the environments.

Analysis of Essential Isoprene Metabolic Pathway Proteins in Variovorax sp. Strain WS11

“…This was also observed during a metabolomics experiment in wild-type Variovorax sp. WS11 grown with isoprene and in mutant strains deficient for the essential genes isoG , isoJ , and aldH after incubation with isoprene ( 42 ). These experiments illustrate the conserved nature of isoprene metabolism between the Gram-positive Rhodococcus sp.…”

“…Possible explanations include that AldH may catalyze a second essential reaction in the isoprene metabolic pathway, although no such reaction has been proposed, or IsoH may simply be unable to meet the metabolic demand of converting GMB to GMBA and generating reducing power in the form of NADH, necessitating the synergistic activities of IsoH and AldH. Alternatively, although isoprene monooxygenase produces a vast excess of ( R )-epoxyisoprene ( 42 ), some ( S )-epoxyisoprene is still formed which may accumulate if IsoI cannot form HGMB from this form of the epoxide. AldH may be responsible for the conversion of intermediates derived from ( S )-epoxyisoprene, thereby potentially preventing toxic accumulations of isoprene pathway intermediates.…”

“…HGMB, 1-hydroxy-2-glutathionyl-2-methyl-3-butene; GMB, 2-glutathionyl-2-methyl-3-butenal; GMBA, 2-glutathionyl-2-methyl-3-butenoic acid; SG, glutathione; GSH, reduced glutathione; X-CoA, donor ( 25 ). Adapted from 42 .…”

Whole‐cell studies of substrate and inhibitor specificity of isoprene monooxygenase and related enzymes

“…Cultivation-dependent and Leanne Sims and Chloe Wright contributed equally to to this study. cultivation-independent studies have highlighted that isoprene degradation is a widespread trait across particular representatives of many bacterial genera, including Rhodococcus, Mycobacterium, Nocardioides, Gordonia, Sphingopyxis, and Variovorax, and isoprene degraders have been isolated from terrestrial, freshwater, coastal and marine environments including soils and the leaves of trees, reviewed by Dawson et al (2023). Rhodococcus sp.…”

“…Isoprene‐degrading bacteria can grow aerobically on isoprene as sole carbon and energy source and are a substantial sink for isoprene, removing an estimated 20 Tg of isoprene from the atmosphere per year (Cleveland & Yavitt, 1997; Gray et al, 2015; Pegoraro et al, 2005). Cultivation‐dependent and cultivation‐independent studies have highlighted that isoprene degradation is a widespread trait across particular representatives of many bacterial genera, including Rhodococcus , Mycobacterium , Nocardioides , Gordonia , Sphingopyxis , and Variovorax , and isoprene degraders have been isolated from terrestrial, freshwater, coastal and marine environments including soils and the leaves of trees, reviewed by Dawson et al (2023). Rhodococcus sp.…”