A novel, divergent alkane monooxygenase (alkB) clade involved in crude oil biodegradation

Structure and Function of Alkane Monooxygenase (AlkB)

Every year, perhaps as much as 800 million tons of hydrocarbons enters the environment; alkanes make up a large percentage of it. Most are transformed by organisms that utilize these molecules as sources of energy and carbon. Both aerobic and anaerobic alkane transformation chemistries exist, capitalizing on the presence of alkanes in both oxic and anoxic environments. Over the past 40 years, tremendous progress has been made in understanding the structure and mechanism of enzymes that catalyze the transformation of methane. By contrast, progress involving enzymes that transform liquid alkanes has been slower with the first structures of AlkB, the predominant aerobic alkane hydroxylase in the environment, appearing in 2023. Because of the fundamental importance of C-H bond activation chemistries, interest in understanding how biology activates and transforms alkanes is high.In this Account, we focus on steps we have taken to understand the mechanism and structure of alkane monooxygenase (AlkB), the metalloenzyme that dominates the transformation of liquid alkanes in the environment (not to be confused with another AlkB that is an α-ketogluturate-dependent enzyme involved in DNA repair). First, we briefly describe what is known about the prevalence of AlkB in the environment and its role in the carbon cycle. Then we review the key findings from our recent high-resolution cryoEM structure of AlkB and highlight important similarities and differences in the structures of members of class III diiron enzymes. Functional studies, which we summarize, from a number of single residue variants enable us to say a great deal about how the structure of AlkB facilitates its function. Next, we overview work from our laboratories using mechanistically diagnostic radical clock substrates to characterize the mechanism of AlkB and contextualize the results we have obtained on AlkB with results we have obtained on other alkane-oxidizing enzymes and explain these results in light of the enzyme's structure. Finally, we integrate recent work in our laboratories with information from prior studies of AlkB, and relevant model systems, to create a holistic picture of the enzyme. We end by pointing to critical questions that still need to be answered, questions about the electronic structure of the active site of the enzyme throughout the reaction cycle and about whether and to what extent the enzyme plays functional roles in biology beyond simply initiating the degradation of alkanes.

Untapped talents: insight into the ecological significance of methanotrophs and its prospects

The deep ocean is a rich reservoir of unique organisms with great potential for bioprospecting, ecosystem services, and the discovery of novel materials. These organisms thrive in harsh environments characterized by high hydrostatic pressure, low temperature, and limited nutrients. Hydrothermal vents and cold seeps, prominent features of the deep ocean, provide a habitat for microorganisms involved in the production and filtration of methane, a potent greenhouse gas. Methanotrophs, comprising archaea and bacteria, play a crucial role in these processes. This review examines the intricate relationship between the roles, responses, and niche specialization of methanotrophs in the deep ocean ecosystem. Our findings reveal that different types of methanotrophs dominate specific zones depending on prevailing conditions. Type I methanotrophs thrive in oxygen-rich zones, while Type II methanotrophs display adaptability to diverse conditions. Verrumicrobiota and NC10 flourish in hypoxic and extreme environments. In addition to their essential role in methane regulation, methanotrophs contribute to various ecosystem functions. They participate in the degradation of foreign compounds and play a crucial role in cycling biogeochemical elements like metals, sulfur, and nitrogen. Methanotrophs also serve as a significant energy source for the oceanic food chain and drive chemosynthesis in the deep ocean. Moreover, their presence offers promising prospects for biotechnological applications, including the production of valuable compounds such as polyhydroxyalkanoates, methanobactin, exopolysaccharides, ecotines, methanol, putrescine, and biofuels. In conclusion, this review highlights the multifaceted roles of methanotrophs in the deep ocean ecosystem, underscoring their ecological significance and their potential for advancements in biotechnology. A comprehensive understanding of their niche specialization and responses will contribute to harnessing their full potential in various domains.

Genome-Wide Expression Analysis Unravels Fluoroalkane Metabolism in Pseudomonas sp. Strain 273

Pseudomonas sp. strain 273 grows with medium-chain terminally fluorinated alkanes under oxic conditions, releases fluoride, and synthesizes long-chain fluorofatty acids. To shed light on the genes involved in fluoroalkane metabolism, genome, and transcriptome sequencing of strain 273 grown with 1,10-difluorodecane (DFD), decane, and acetate were performed. Strain 273 harbors three genes encoding putative alkane monooxygenases (AlkB), key enzymes for initiating alkane degradation. Transcripts of alkB-2 were significantly more abundant in both decane- and DFD-grown cells compared to acetate-grown cells, suggesting AlkB-2 catalyzes the attack on terminal CH3 and CH2F groups. Coordinately expressed with alkB-2 was an adjacent gene encoding a fused ferredoxin-ferredoxin reductase (Fd-Fdr). Phylogenetic analysis distinguished AlkB that couples with fused Fd-Fdr reductases from AlkB with alternate architectures. A gene cluster containing an (S)-2-haloacid dehalogenase (had) gene was up-regulated in cells grown with DFD, suggesting a possible role in the removal of the ω-fluorine. Genes involved in long-chain fatty acid biosynthesis were not differentially expressed during growth with acetate, decane, or DFD, suggesting the bacterium's biosynthetic machinery does not discriminate against monofluoro-fatty acid intermediates. The analysis sheds first light on genes and catalysts involved in the microbial metabolism of fluoroalkanes.

Medium-chain alkane biodegradation and its link to some unifying attributes of alkB genes diversity

Hydrocarbon footprints in the environment, via biosynthesis, natural seepage, anthropogenic activities and accidents, affect the ecosystem and induce a shift in the healthy biogeochemical equilibrium that drives needed ecological services. In addition, these imbalances cause human diseases and reduce animal and microorganism diversity. Microbial bioremediation, which capitalizes on functional genes, is a sustainable mitigation option for cleaning hydrocarbon-impacted environments. This review focuses on the bacterial alkB functional gene, which codes for a non-heme di‑iron monooxygenase (AlkB) with a di‑iron active site that catalyzes C₈-C₁₆ medium-chain alkane metabolism. These enzymes are ubiquitous and share common attributes such as being controlled by global transcriptional regulators, being a component of most super hydrocarbon degraders, and their distributions linked to horizontal gene transfer (HGT) events. The phylogenetic approach used in the HGT detection suggests that AlkB tree topology clusters bacteria functionally and that a preferential gradient dictates gene distribution. The alkB gene also acts as a biomarker for bioremediation, although it is found in pristine environments and absent in some hydrocarbon degraders. For instance, a quantitative molecular method has failed to link alkB copy number to contamination concentration levels. This limitation may be due to AlkB homologues, which have other functions besides n-alkane assimilation. Thus, this review, which focuses on Pseudomonas putida GPo1 alkB, shows that AlkB proteins are diverse but have some unifying trends around hydrocarbon-degrading bacteria; it is erroneous to rely on alkB detection alone as a monitoring parameter for hydrocarbon degradation, alkB gene distribution are preferentially distributed among bacteria, and the plausible explanation for AlkB affiliation to broad-spectrum metabolism of hydrocarbons in super-degraders hitherto reported. Overall, this review provides a broad perspective of the ecology of alkB-carrying bacteria and their directed biodegradation pathways.

A novel alkane monooxygenase (alkB) clade revealed by massive genomic survey and its dissemination association with IS elements

Background

Alkanes are important components of fossil energy, such as crude oil. The alkane monooxygenase encoded by alkB gene performs the initial step of alkane degradation under aerobic conditions. The alkB gene is well studied due to its ubiquity as well as the availability of experimentally functional evidence. The alkBFGHJKL and alkST clusters are special kind of alkB-type alkane hydroxylase system, which encode all proteins necessary for converting alkanes into corresponding fatty acids.

Methods

To explore whether the alkBFGHJKL and alkST clusters were widely distributed, we performed a large-scale analysis of isolate and metagenome assembled genome data (>390,000 genomes) to identify these clusters, together with distributions of corresponding taxonomy and niches. The set of alk-genes (including but not limited to alkBGHJ) located near each other on a DNA sequence was defined as an alk-gene cluster in this study. The alkB genes with alkGHJ located nearby on a DNA sequence were picked up for the investigation of putative alk-clusters.

Results

A total of 120 alk-gene clusters were found in 117 genomes. All the 117 genomes are from strains located only in α- and γ-proteobacteria. The alkB genes located in alk-gene sets were clustered into a deeply branched mono-clade. Further analysis showed similarity organization types of alk-genes were observed within closely related species. Although a large number of IS elements were observed nearby, they did not lead to the wide spread of the alk-gene cluster. The uneven distribution of these elements indicated that there might be other factors affecting the transmission of alk-gene clusters.

Conclusions

We conducted systematic bioinformatics research on alk-genes located near each other on a DNA sequence. This benchmark dataset of alk-genes can provide base line for exploring its evolutional and ecological importance in future studies.

A Structural View of Alkyl-Coenzyme M Reductases, the First Step of Alkane Anaerobic Oxidation Catalyzed by Archaea

Microbial anaerobic oxidation of alkanes intrigues the scientific community by way of its impact on the global carbon cycle, and its biotechnological applications. Archaea are proposed to degrade short- and long-chain alkanes to CO2 by reversing methanogenesis, a theoretically reversible process. The pathway would start with alkane activation, an endergonic step catalyzed by methyl-coenzyme M reductase (MCR) homologues that would generate alkyl-thiols carried by coenzyme M. While the methane-generating MCR found in methanogens has been well characterized, the enzymatic activity of the putative alkane-fixing counterparts has not been validated so far. Such an absence of biochemical investigations contrasts with the current explosion of metagenomics data, which draws new potential alkane-oxidizing pathways in various archaeal phyla. Therefore, validating the physiological function of these putative alkane-fixing machines and investigating how their structures, catalytic mechanisms, and cofactors vary depending on the targeted alkane have become urgent needs. The first structural insights into the methane- and ethane-capturing MCRs highlighted unsuspected differences and proposed some explanations for their substrate specificity. This Perspective reviews the current physiological, biochemical, and structural knowledge of alkyl-CoM reductases and offers fresh ideas about the expected mechanistic and chemical differences among members of this broad family. We conclude with the challenges of the investigation of these particular enzymes, which might one day generate biofuels for our modern society.

Assessment of Hydrocarbon Degradation Potential in Microbial Communities in Arctic Sea Ice

The anthropogenic release of oil hydrocarbons into the cold marine environment is an increasing concern due to the elevated usage of sea routes and the exploration of new oil drilling sites in Arctic areas. The aim of this study was to evaluate prokaryotic community structures and the genetic potential of hydrocarbon degradation in the metagenomes of seawater, sea ice, and crude oil encapsulating the sea ice of the Norwegian fjord, Ofotfjorden. Although the results indicated substantial differences between the structure of prokaryotic communities in seawater and sea ice, the crude oil encapsulating sea ice (SIO) showed increased abundances of many genera-containing hydrocarbon-degrading organisms, including Bermanella, Colwellia, and Glaciecola. Although the metagenome of seawater was rich in a variety of hydrocarbon degradation-related functional genes (HDGs) associated with the metabolism of n-alkanes, and mono- and polyaromatic hydrocarbons, most of the normalized gene counts were highest in the clean sea ice metagenome, whereas in SIO, these counts were the lowest. The long-chain alkane degradation gene almA was detected from all the studied metagenomes and its counts exceeded ladA and alkB counts in both sea ice metagenomes. In addition, almA was related to the most diverse group of prokaryotic genera. Almost all 18 good- and high-quality metagenome-assembled genomes (MAGs) had diverse HDGs profiles. The MAGs recovered from the SIO metagenome belonged to the abundant taxa, such as Glaciecola, Bermanella, and Rhodobacteracea, in this environment. The genera associated with HDGs were often previously known as hydrocarbon-degrading genera. However, a substantial number of new associations, either between already known hydrocarbon-degrading genera and new HDGs or between genera not known to contain hydrocarbon degraders and multiple HDGs, were found. The superimposition of the results of comparing HDG associations with taxonomy, the HDG profiles of MAGs, and the full genomes of organisms in the KEGG database suggest that the found relationships need further investigation and verification.