bioRxiv Science⌕ Search
Home/Archive/bioRxiv

bioRxiv · 10.1101/2024.03.12.584659

Metabolism of FAD, FMN and riboflavin (vitamin B2) in the human parasitic blood fluke Schistosoma mansoni

Da'dara, A.·Nation, C.·Skelly, P.
View original source ↗Export metadata ↓

Abstract

Schistosomiasis is a parasitic disease caused by trematode worms of the genus Schistosoma. The intravascular worms acquire the nutrients necessary for their survival from host blood. Since all animals are auxotrophic for riboflavin (vitamin B2), schistosomes too must import it to survive. Riboflavin is an essential component of the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD); these support key functions of dozens of flavoenzymes. In this work we focus on the biochemistry of riboflavin and its metabolites in Schistosoma mansoni. We show that when schistosomes are incubated in murine plasma, levels of FAD decrease over time while the levels of FMN increase. We show that live schistosomes can cleave exogenous FAD to generate FMN and this ability is significantly blocked when expression of the surface ectoenzyme SmNPP5 is suppressed using RNAi. Recombinant SmNPP5 cleaves FAD with a Km of 178 {+/-} 5.9 {micro}M. The FAD-dependent enzyme IL-4I1 drives the oxidative deamination of phenylalanine to produce phenylpyruvate and H2O2 in the extracellular environment. Since schistosomes can be damaged by H2O2, we determined if SmNPP5 could impede H2O2 production by blocking IL-4I1 action in vitro. We found that this was not the case, suggesting that covalently bound FAD on IL-4I1 is inaccessible to SmNPP5. We also report here that live schistosomes can cleave exogenous FMN to generate riboflavin and this ability is significantly impeded when expression of a second surface ectoenzyme (alkaline phosphatase, SmAP) is suppressed. Recombinant SmAP cleaves FMN with a Km of 3.82 {+/-} 0.58 mM. Thus, the sequential hydrolysis of FAD by tegumental ecto-enzymes SmNPP5 and SmAP can generate free vitamin B2 around the worms from where it can be conveniently imported by, we hypothesize, the recently described schistosome riboflavin transporter SmaRT. In this work we also identified in silico schistosome homologs of enzymes that are involved in intracellular vitamin B2 metabolism. These are riboflavin kinase (SmRFK) as well as FAD synthase (SmFADS); cDNAs encoding these two enzymes were cloned and sequenced. SmRFK is predicted to convert riboflavin to FMN while SmFADS could further act on FMN to regenerate FAD in order to facilitate robust vitamin B2-dependent metabolism in schistosomes.

Explore related subjects

biochemistryPreprint

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Da'dara, A., Nation, C., Skelly, P.. 2024-03-12. Metabolism of FAD, FMN and riboflavin (vitamin B2) in the human parasitic blood fluke Schistosoma mansoni. https://doi.org/10.1101/2024.03.12.584659

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related preprints

bioRxiv2026

MgATP/MgADP-dependent conformational dynamics and intrinsically disordered regions of vascular KATP channels revealed by cryoEM

Vascular smooth muscle KATP channels, composed of the pore-forming Kir6.1 and regulatory SUR2B subunits, control vascular tone, dysfunction of which causes systemic disease. Vascular KATP is regulated by Mg-nucleotides, but the underlying structural mechanism has remained elusive. Here, we determined cryoEM structures of these channels in the presence of MgATP and MgADP. Two key structures captured, one showing the SUR2B-nucleotide binding domains (NBDs) separated and one showing the SUR2B-NBDs dimerized, reveal conformation-specific organization of intrinsically disordered regions (IDRs) found in both Kir6.1 and SUR2B. In the NBD-separated conformation, the Kir6.1-N terminal IDR (KNt) sits within the central cleft of the ABC-core of SUR2B. In the NBD-dimerized conformation, KNt is excluded from the central cleft and instead forms contacts with an ED domain comprising 15 consecutive glutamate and aspartate residues within a SUR2B IDR, the N1-T2 linker connecting NBD1 (N1) to transmembrane domain 2 (T2). Moreover, within the N1-T2 linker a regulatory helix seen between the two NBDs in the NBD-separated conformation moves to outside the dimerized NBDs, interacting with the C-terminal residues unique to SUR2B, in the NBD-dimerized conformation. MD simulations further reveal that transient but frequent interactions mediated by the IDRs may facilitate Mg-nucleotide dependent conformational switch in vascular KATP channels.

biochemistry↗
bioRxiv2026

Probing the sequence variability tolerance in a de novo α-helical barrel biocatalyst

De novo-designed enzymes have recently achieved high catalytic activity and stereoselectivity while demonstrating exceptional thermostability in entirely novel protein scaffolds. Among these, -helical barrel protein scaffolds are attractive structures for biocatalysis due to their structural simplicity, high thermostability, and rationalizable sequence patterning. However, enabling major structural reengineering of these scaffolds while maintaining the structure, stability and catalytic activity while also improving soluble protein production remain major challenges and pose the fundamental question how engineerable a de novo backbone-sequence pair is. Here, we combine deep learning based and classic computational protein design to modify and optimize de novo -helical barrel biocatalysts. Using the previously reported six-helical barrel 6H5L as a model scaffold, AlphaFold2-guided RosettaRemodel enabled the design of a truncated variant, whose crystal structure closely matches the design model. Additional sequence-redesign using ProteinMPNN generated a variant with a tenfold increase of soluble protein yield in Escherichia coli. Biochemical, biophysical, and structural analyses showed that both variants retained the overall barrel architecture, high thermal stability, and catalytic activity for both purified protein and whole-cell systems. Detailed kinetic analysis on the variants showed both variation in kcat and Km, reflecting changes in catalytic turnover and substrate binding. Together, these approaches provide new insights and possibilities for the further engineering of functional de novo -helical barrels, their ability to withstand dramatically large sequence changes and their broader application in biocatalysis and biotechnology.

biochemistry↗
bioRxiv2026

Cytokine-induced nuclear translocation of STAT1 via a non-transferable NLS

The targeting function of nuclear localization signals (NLSs) is generally considered independent of a protein's native sequence or fold and is readily transferable to heterologous cargos. Contrary to this paradigm, rapid nuclear translocation of phosphorylated STAT1 (pSTAT1) following cytokine stimulation requires importin {beta}, Ran-GTP, and the importin 5 isoform, which recognizes a non-transferable NLS. Here, we present cryo-EM structures of pSTAT1 bound to importin 5, revealing an asymmetric 2:1 complex that diverges from canonical NLS-mediated cargo recognition. Importin 5 occupies the DNA-binding groove of the pSTAT1 dimer, with a single STAT1 N-terminal domain positioning the C-terminal Armadillo repeats 9-10 (S1B domain) orthogonal to the DNA-binding interface. This interface is also targeted by the Ebola virus protein VP24, an antagonist of interferon signaling. We further show that Ran-GTP alone is insufficient to trigger nuclear release of pSTAT1, which additionally requires the exportin CAS. A cryo-EM reconstruction of the CAS-Ran-GTP-5 complex, supported by in vitro competition assays, demonstrates that CAS and pSTAT1 are mutually exclusive ligands for importin 5. Together, these findings define the molecular choreography of cytokine-induced STAT1 nuclear translocation and release, establishing a general paradigm for STAT family signaling.

biochemistry↗