The oligomeric state of the Caldivirga maquilingensis type III sulfide:Quinone Oxidoreductase is required for membrane binding

An Expanded Substrate Spectrum of Sulfide:Quinone Oxidoreductase Found in Pollutant Degrading Bacteria

Sulfide:Quinone Oxidoreductase (Sqr) Catalyzes The Initial Procedure On Sulfide Transformation, Alongside Sulfide (H2S, S2-) Oxidization Coupled With Coenzyme Q (CoQ) Reducing And Reactive Sulfur Species (RSS) Production. Here, We Assessed The Reactivity Of Propanethiol (PT) As An Alternative Substrate For Sqr To Maintain Intracellular Homeostasis In Strain S-1 Capable Of Degrading Emerging Sulfur-Containing Pollutants. We Deleted A Gene Encoding Sqr, And Serial Transcriptional Difference Induced By RSS Dynamics Was Therefore Revealed. Next, The Reaction Properties Of Two Sqr Homologs From Strains JMP134 And S-1 Were Comparatively Characterized, Respectively. As A Result, An Additional Role Of Sqr In Yielding RSS From PT Was Found In Reaction Mixture Prepared By Cell-Free Extracts Or Purified Enzymes. Interestingly, The Transformation Velocity Of PT By Sqr Was Slower Than That Of Sulfides. From This Scenario, It Was A Rate-Determining Step That PT As A Nucleophilic Compound Can Be Added Into Sqr Cysteine To Form Disulfide Bond And Likely Serve Nonoptimal Sulfur Recipient. In Addition, The Role Of Persulfidation Driven By RSS In Combating Oxidative And Sulfur Stresses Required To Be Further Clarified. Nevertheless, This Promiscuity Of Sqr-Binding Organosulfur Compounds And Its Catalytic Modulation Underscored That Expanded Substrates Might Benefit Sulfide Homeostasis In Thiol-Degrading Bacteria.

Insights into Transient Dimerisation of Carnitine/Acylcarnitine Carrier (SLC25A20) from Sarkosyl/PAGE, Cross-Linking Reagents, and Comparative Modelling Analysis

The carnitine/acylcarnitine carrier (CAC) is a crucial protein for cellular energy metabolism, facilitating the exchange of acylcarnitines and free carnitine across the mitochondrial membrane, thereby enabling fatty acid β-oxidation and oxidative phosphorylation (OXPHOS). Although CAC has not been crystallised, structural insights are derived from the mitochondrial ADP/ATP carrier (AAC) structures in both cytosolic and matrix conformations. These structures underpin a single binding centre-gated pore mechanism, a common feature among mitochondrial carrier (MC) family members. The functional implications of this mechanism are well-supported, yet the structural organization of the CAC, particularly the formation of dimeric or oligomeric assemblies, remains contentious. Recent investigations employing biochemical techniques on purified and reconstituted CAC, alongside molecular modelling based on crystallographic AAC dimeric structures, suggest that CAC can indeed form dimers. Importantly, this dimerization does not alter the transport mechanism, a phenomenon observed in various other membrane transporters across different protein families. This observation aligns with the ping-pong kinetic model, where the dimeric form potentially facilitates efficient substrate translocation without necessitating mechanistic alterations. The presented findings thus contribute to a deeper understanding of CAC's functional dynamics and its structural parallels with other MC family members.

Quinone binding site in a type VI sulfide:quinone oxidoreductase

Monotopic membrane-bound flavoproteins, sulfide:quinone oxidoreductases (SQRs), have a variety of physiological functions, including sulfide detoxification. SQR enzymes are classified into six groups. SQRs use the flavin adenine dinucleotide (FAD) cofactor to transfer electrons from sulfide to quinone. A type VI SQR of the photosynthetic purple sulfur bacterium, Thiocapsa roseopersicina (TrSqrF), has been previously characterized, and the mechanism of sulfide oxidation has been proposed. This paper reports the characterization of quinone binding site (QBS) of TrSqrF composed of conserved aromatic and apolar amino acids. Val331, Ile333, and Phe366 were identified near the benzoquinone ring of enzyme-bound decylubiquinone (dUQ) using the TrSqrF homology model. In silico analysis revealed that Val331 and Ile333 alternately connected with the quinone head group via hydrogen bonds, and Phe366 and Trp369 bound the quinones via hydrophobic interactions. TrSqrF variants containing alanine (V331A, I333A, F366A) and aromatic amino acid (V331F, I333F, F366Y), as well as a C-terminal α-helix deletion (CTD) mutant were generated. These amino acids are critical for quinone binding and, thus, catalysis. Spectroscopic analyses proved that all mutants contained FAD. I333F replacement resulted in the lack of the charge transfer complex. In summary, the interactions described above maintain the quinone molecule's head in an optimal position for direct electron transfer from FAD. Surprisingly, the CTD mutant retained a relatively high level of specific activity while remaining membrane-anchored. This is a unique study because it focuses on the QBS and the oxidative stage of a type VI sulfide-dependent quinone reduction. KEY POINTS: • V331, I333, F366, and W369 were shown to interact with decylubiquinone in T. roseopersicina SqrF • These amino acids are involved in proper positioning of quinones next to FAD • I333 is essential in formation of a charge transfer complex from FAD to quinone.