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Flavin-containing monooxygenases (FMOs) are ubiquitous in all domains of life and metabolize a myriad of xenobiotics, including toxins, pesticides and drugs. However, despite their pharmacological importance, structural information remains bereft. To further our understanding behind their biochemistry and diversity, we used ancestral-sequence reconstruction, kinetic and crystallographic techniques to scrutinize three ancient mammalian FMOs: AncFMO2, AncFMO3-6 and AncFMO5. Remarkably, all AncFMOs could be crystallized and were structurally resolved between 2.7- and 3.2-Å resolution. These crystal structures depict the unprecedented topology of mammalian FMOs. Each employs extensive membrane-binding features and intricate substrate-profiling tunnel networks through a conspicuous membrane-adhering insertion. Furthermore, a glutamate–histidine switch is speculated to induce the distinctive Baeyer–Villiger oxidation activity of FMO5. The AncFMOs exhibited catalysis akin to human FMOs and, with sequence identities between 82% and 92%, represent excellent models. Our study demonstrates the power of ancestral-sequence reconstruction as a strategy for the crystallization of proteins.Ancestral reconstruction leads to characterization and crystallization of three ancient mammalian flavin-containing monooxygenases, offering insights into their mechanisms of membrane binding, catalytic activity and substrate selection.

The Baeyer-Villiger Monooxygenases (BVMOs) are enzymes belonging to the \"Class B\" of flavin monooxygenases and are capable of performing exquisite selective oxidations. These enzymes have been studied from a biotechnological perspective, but their physiological substrates and functional roles are widely unknown. Here, we investigated the origin, taxonomic distribution and evolutionary history of the BVMO genes. By using in silico approaches, 98 BVMO encoding genes were detected in the three domains of life: Archaea, Bacteria and Eukarya. We found evidence for the presence of these genes in Metazoa (Hydra vulgaris, Oikopleura dioica and Adineta vaga) and Haptophyta (Emiliania huxleyi) for the first time. Furthermore, a search for other \"Class B\" monooxygenases (flavoprotein monooxygenases--FMOs--and N-hydroxylating monooxygenases--NMOs) was conducted. These sequences were also found in the three domains of life. Phylogenetic analyses of all \"Class B\" monooxygenases revealed that NMOs and BVMOs are monophyletic, whereas FMOs form a paraphyletic group. Based on these results, we propose that BVMO genes were already present in the last universal common ancestor (LUCA) and their current taxonomic distribution is the result of differential duplication and loss of paralogous genes.

Metabolons are protein assemblies that perform a series of reactions in a metabolic pathway. However, the general importance and aptitude of metabolons for enzyme catalysis remain poorly understood. In animals, biosynthesis of coenzyme Q is currently attributed to ten different proteins, with COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9 forming the iconic COQ metabolon. Yet several reaction steps conducted by the metabolon remain enigmatic. To elucidate the prerequisites for animal coenzyme Q biosynthesis, we sought to construct the entire metabolon in vitro. Here we show that this approach, rooted in ancestral sequence reconstruction, reveals the enzymes responsible for the uncharacterized steps and captures the biosynthetic pathway in vitro. We demonstrate that COQ8, a kinase, increases and streamlines coenzyme Q production. Our findings provide crucial insight into how biocatalytic efficiency is regulated and enhanced by these biosynthetic engines in the context of the cell. Coenzyme Q has several important biological functions, but the understanding of the biosynthesis of coenzyme Q in humans remains incomplete. Now, by constructing the entire COQ metabolon in vitro, the enzymes and reactions underlying coenzyme Q biosynthesis are characterized.

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Photosynthetic organisms, fungi, and animals comprise distinct pathways for vitamin C biosynthesis. Besides this diversity, the final biosynthetic step consistently involves an oxidation reaction carried out by the aldonolactone oxidoreductases. Here, we study the origin and evolution of the diversified activities and substrate preferences featured by these flavoenzymes using molecular phylogeny, kinetics, mutagenesis, and crystallographic experiments. We find clear evidence that they share a common ancestor. A flavin-interacting amino acid modulates the reactivity with the electron acceptors, including oxygen, and determines whether an enzyme functions as an oxidase or a dehydrogenase. We show that a few side chains in the catalytic cavity impart the reaction stereoselectivity. Ancestral sequence reconstruction outlines how these critical positions were affixed to specific amino acids along the evolution of the major eukaryotic clades. During Eukarya evolution, the aldonolactone oxidoreductases adapted to the varying metabolic demands while retaining their overarching vitamin C-generating function. Photosynthetic organisms, fungi, and animals contain distinct pathways for vitamin C biosynthesis, but the final biosynthetic step consistently involves an oxidation reaction catalysed by the aldonolactone oxidoreductases. Here, the authors investigate the origin and evolution of the diversified activities and substrate preferences featured by these enzymes using different methods and find evidence that they share a common ancestor.

Among the molecular mechanisms of adaptation in biology, enzyme functional diversification is indispensable. By allowing organisms to expand their catalytic repertoires and adopt fundamentally different chemistries, animals can harness or eliminate new-found substances and xenobiotics that they are exposed to in new environments. Here, we explore the flavin-containing monooxygenases (FMOs) that are essential for xenobiotic detoxification. Employing a paleobiochemistry approach in combination with enzymology techniques we disclose the set of historical substitutions responsible for the family’s functional diversification in tetrapods. Remarkably, a few amino acid replacements differentiate an ancestral multi-tasking FMO into a more specialized monooxygenase by modulating the oxygenating flavin intermediate. Our findings substantiate an ongoing premise that enzymatic function hinges on a subset of residues that is not limited to the active site core. Detoxification enzymes are crucial for the survival of animals in new environments. Here, the authors study the molecular mechanism behind the catalytic diversification of a major family of tetrapod detoxification enzymes—the FMOs—during evolution.

The F420 deazaflavin cofactor is an intriguing molecule as it structurally resembles the canonical flavin cofactor, although biochemically behaves as a nicotinamide cofactor. Since its discovery, numerous enzymes relying on it have been described. The known deazaflavoproteins are taxonomically restricted to Archaea and Bacteria. The biochemistry of the deazaflavoenzymes is diverse and they exhibit some degree of structural variability as well. In this study a thorough sequence and structural homology evolutionary analysis was performed in order to generate an overarching classification of all known F420-dependent oxidoreductases. Five different superfamilies are described: Superfamily I, TIM-barrel F420-dependent enzymes; Superfamily II, Rossmann fold F420-dependent enzymes; Superfamily III, β-roll F420-dependent enzymes; Superfamily IV, SH3 barrel F420-dependent enzymes and Superfamily V, 3 layer ββα sandwich F420-dependent enzymes. This classification aims to be the framework for the identification, the description and the understanding the biochemistry of novel deazaflavoenzymes. Competing Interest Statement The authors have declared no competing interest.

The presence of several putative Baeyer-Villiger Monooxygenases (BVMOs) encoding genes in Aspergillus fumigatus Af293 was demonstrated for the first time. One of the identified BVMO-encoding genes was cloned and successfully overexpressed fused to the cofactor regenerating enzyme phosphite dehydrogenase (PTDH). The enzyme named BVMO Af1 was extensively characterized in terms of its substrate scope and essential kinetic features. It showed high chemo-, regio- and stereoselectivity not only in the oxidation of asymmetric sulfides, ( S )-sulfoxides were obtained with 99% ee , but also in the kinetic resolution of bicyclo[3.2.0]hept-2-en-6-one. This kinetic resolution process led to the production of (1 S ,5 R ) normal lactone and (1 R ,5 S ) abnormal lactone with a regioisomeric ratio of 1:1 and 99% ee each. Besides, different reaction conditions, such as pH, temperature and the presence of organic solvents, have been tested, revealing that BVMO Af1 is a relatively robust biocatalyst.

Type II NADH dehydrogenases (NDH-2s) are accessory enzymes of the bacterial electron transport chain (ETC). While they functionally overlap with Complex I, their main role is not proton translocation but maintaining the intracellular NADH/NAD+ balance. Although often non-essential, NDH-2 become crucial in species lacking complex I, serving as the primary electron entry point into the ETC. Their virtual absence in mammals makes these enzymes attractive targets for antimicrobial drug development and mitochondrial functional restoration. NDH-2s catalyze electron transfer from NADH to quinones, yet two distinct catalytic mechanisms have been described for members of the family: a classical ping-pong mechanism and an atypical ternary mechanism involving the formation of a charge transfer complex (CTC). The molecular basis of these mechanisms remains unclear. Also, their occurrence among NDH-2s from different bacterial lineages in unknown. Here we combined molecular phylogenetics, ancestral sequence reconstruction, expression and biochemical characterization of ancestral and modern enzymes and, molecular dynamics simulations to explore the mechanistic versatility of NDH-2s across Bacteria. Our results show the atypical ternary mechanism is restricted to the Firmicutes (Bacillota) lineage and it is defined by the presence of a single substitution located at the bottom of the active site. This work provides an evolutionary framework for understanding NDH-2 mechanistic versatility. Besides, it establishes a basis for drug discovery targeting pathogenic strains and opens avenues to develop innovative strategies to complement dysfunctional mitochondria.