Axel J. Scheidig

The human mitochondrial amidoxime-reducing component 1 (mARC1) is a molybdenum-dependent enzyme whose protein-coding variants confer protection against common metabolic liver diseases. Whereas the frequent A165T variant acts largely through accelerated cellular degradation, the basis of protection by the rarer M187K variant remains obscure, as it has been reported that there are no differences between the wild-type and M187K-variant protein in terms of cellular protein levels and localization. Here, the crystal structure of the human mARC1 M187K variant, crystallized as a T4 lysozyme fusion after iterative micro-seeding, was determined at 1.63 Å resolution. The variant structure is essentially superimposable with the previously reported wild-type and A165T structures, with pairwise root-mean-square deviations of 0.3–0.4 Å, and the pentacoordinated molybdenum cofactor is fully intact. Differential scanning fluorimetry across a broad pH range revealed only a modest, pH-dependent decrease in thermal stability associated with the exchange, most pronounced at alkaline pH. These data suggest that the disease-protective effect of M187K is unlikely to originate from gross structural rearrangement or active-site perturbation.

The human mitochondrial amidoxime reducing component 1 (mARC1) is a molybdenum-dependent enzyme whose protein-coding variants confer protection against common metabolic liver diseases. Whereas the frequent A165T variant acts largely through accelerated cellular degradation, the basis of protection by the rarer M187K variant remains obscure, as it has been reported that there are no differences between the "wild-type" and M187K variant protein in terms of cellular protein levels and localisation. Here, the crystal structure of the human mARC1 M187K variant, crystallised as a T4 lysozyme fusion after iterative micro-seeding, was determined at 1.63 A resolution. The variant structure is essentially superimposable with the previously reported "wild-type" and A165T structures, with pairwise root-mean-square deviations of 0.3 - 0.4 A, and the pentacoordinated molybdenum cofactor is fully intact. Differential scanning fluorimetry across a broad pH range revealed only a modest, pH-dependent decrease in thermal stability associated with the exchange, most pronounced at alkaline pH. These data suggest that the disease-protective effect of M187K is unlikely to originate from gross structural rearrangement or active-site perturbation.

Lysozymes are an essential part of immunity and nutrition in metazoans, degrading bacterial cell walls via the hydrolysis of peptidoglycan. Although various lysozymes have been reported for higher animals, the origin of animal lysozymes remains elusive as they seem to be lacking in all early branching phyla. In this study, we investigated a putative goose-type lysozyme (PLys, glycoside hydrolase family 23, GH23) of the placozoan Trichoplax sp. H2. We show that PLys is highly active and primarily produced by cells of the placozoan ventral epithelium. PLys contains a non-conserved cysteine-rich domain N-terminal of the GH23 lysozyme domain, which stabilizes the protein and is truncated during maturation. Using a pH-sensitive fluorescence reporter, we show that Trichoplax sp. H2 acidifies its temporary feeding grooves pulsatively during digestive events close to the optimum pH for PLys activity. To elucidate the evolutionary origin of the metazoan GH23 lysozyme family, we applied structure-based phylogenetics to show that the metazoan g-type GH23 lysozymes originated from a horizontal gene transfer event from bacteria to an early pre-bilaterian ancestor. GH23 lysozymes have then been retained and expanded in many phyla acting as first animal lysozyme and a key component in the antibacterial arsenal since early animal evolution.

N-Hydroxyurea has been known since the 1960s as an antiproliferative drug and is used both in oncology and for treatment of hematological disorders such as sickle cell anemia where very high daily doses are administered. It is assumed that the cellular effect of N-hydroxyurea is caused by inhibition of ribonucleotide reductase, while alternative mechanisms, e.g., generation of nitric oxide, have also been proposed. Despite its many therapeutic applications, the metabolism of hydroxyurea is largely unexplored. The major elimination pathway of N-hydroxyurea is the reduction to urea. Since the mitochondrial amidoxime reducing component (mARC) is known for its N-reductive activity, we investigated the reduction of NHU by this enzyme system. This study presents in vitro and in vivo evidence that this reductive biotransformation is specifically mediated by the mARC1. Inactivation by mARC1 is a possible explanation for the high doses of NHU required for treatment.

A combination of X-ray absorption and low-temperature electronic absorption spectroscopies has been used to probe the geometric and electronic structures of the human mitochondrial amidoxime reducing component enzyme (hmARC1) in the oxidized Mo(VI) and reduced Mo(IV) forms. Extended X-ray absorption fine structure analysis revealed that oxidized enzyme possesses a 5-coordinate [MoO2(S-Cys)(PDT)](-) (PDT = pyranopterin dithiolene) active site with a cysteine coordinated to Mo. A 5-coordinate geometry is retained in the reduced state, with the equatorial oxo being protonated. Low-temperature electronic absorption spectroscopy of hmARC1 reveals a spectrum for the oxidized enzyme that is significantly different from what has been reported for sulfite oxidase family enzymes. Time-dependent density functional theory computations on oxidized and reduced hmARC1, and a small molecule analogue for hmARC1(ox), have been used to assist us in making detailed band assignments and developing a greater understanding of enzyme electronic structure contributions to reactivity. Our understanding of the hmARC(red) HOMO and the LUMO of the benzamidoxime substrate reveal a potential pi-bonding interaction between these redox orbitals, with two-electron occupation of the substrate LUMO along the reaction coordinate activating the O-N bond for cleavage and promoting oxygen atom transfer to the Mo site.

Background: Mutations in the gene MTARC1 (mitochondrial amidoxime–reducing component 1) protect carriers from metabolic dysfunction–associated steatohepatitis (MASH) and cirrhosis. MTARC1 encodes the mARC1 enzyme, which is localized to the mitochondria and has no known MASH-relevant molecular function. Our studies aimed to expand on the published human genetic mARC1 data and to observe the molecular effects of mARC1 modulation in preclinical MASH models. Methods and Results: We identified a novel human structural variant deletion in MTARC1, which is associated with various biomarkers of liver health, including alanine aminotransferase levels. Phenome-wide Mendelian Randomization analyses additionally identified novel putatively causal associations between MTARC1 expression, and esophageal varices and cardiorespiratory traits. We observed that protective MTARC1 variants decreased protein accumulation in in vitro overexpression systems and used genetic tools to study mARC1 depletion in relevant human and mouse systems. Hepatocyte mARC1 knockdown in murine MASH models reduced body weight, liver steatosis, oxidative stress, cell death, and fibrogenesis markers. mARC1 siRNA treatment and overexpression modulated lipid accumulation and cell death consistently in primary human hepatocytes, hepatocyte cell lines, and primary human adipocytes. mARC1 depletion affected the accumulation of distinct lipid species and the expression of inflammatory and mitochondrial pathway genes/proteins in both in vitro and in vivo models. Conclusions: Depleting hepatocyte mARC1 improved metabolic dysfunction–associated steatotic liver disease–related outcomes. Given the functional role of mARC1 in human adipocyte lipid accumulation, systemic targeting of mARC1 should be considered when designing mARC1 therapies. Our data point to plasma lipid biomarkers predictive of mARC1 abundance, such as Ceramide 22:1. We propose future areas of study to describe the precise molecular function of mARC1, including lipid trafficking and subcellular location within or around the mitochondria and endoplasmic reticulum. Export

The mitochondrial amidoxime reducing component (mARC) is a human molybdoenzyme known to catalyze the reduction of various N-oxygenated substrates. The physiological function of mARC enzymes, however, remains unknown. In this study, we examine the reduction of hydrogen peroxide (H₂O₂) by the human mARC1 and mARC2 enzymes. Furthermore, we demonstrate an increased sensitivity toward H₂O₂ for HEK-293T cells with an MTARC1 knockout, which implies a role of mARC enzymes in the cellular response to oxidative stress. H₂O₂ is a reactive oxygen species (ROS) formed in all living cells involved in many physiological processes. Furthermore, H₂O₂ constitutes the first mARC substrate without a nitrogen–oxygen bond, implying that mARC enzymes may have a substrate spectrum going beyond the previously examined N-oxygenated compounds.

The mitochondrial amidoxime-reducing component (mARC) is one of the simplest molybdenum-containing enzymes. mARC is among a few known reducing enzymes playing an important role in drug metabolism in mammals. Here, an assay based on the fluorescence of NADH is reported for the rapid detection of substrates and potential inhibitors of mARC. So far unknown inhibitors might be useful for the development of drugs assigned to nonalcoholic fatty liver disease (NAFLD) and similar diseases. Kinetics of reactions catalyzed by mARC can be recorded with high sensitivity and precision. On a microtiter plate scale, the assay presented could be applied for high-throughput screening of substance libraries and detection of novel mARC substrate candidates. For instance, molnupiravir was also identified as a new substrate by this assay. For better comparison for such substances, the inhibitor or substrate-to-BAO ratio was introduced. After normalization of enzyme activities to the standard benzamidoxime, substrates can reproducibly be classified.

A study recently published in hepatology communications provided insights into a variant of MTARC1 protein, which conveys protection against liver disease. Here, we report a crystal structure of the variant protein at near‐atomic resolution and compare it to the structure of the wildtype protein.