Isethionate sulfite-lyase

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Degradation of Isethionate catalyzed by IslA.

Isethionate sulfite-lyase (IslA, IseA or IseG) is a glycyl radical enzyme that catalyzes the degradation of isethionate into acetaldehyde and sulfite through the cleavage of a carbon-sulfur bond.[1] This conversion is a necessary step for taurine catabolism in anaerobic bacteria like Bilophila wadsworthia. IslA is activated by the enzyme IslB which uses S-adenoslymethionine (SAM) as the initial radical donor.

Crystallographic structure of isethionate sulfite-lyase (IslA) with substrate isethionate bound.

Structure[edit]

IslA, like all other characterized glycyl radical enzymes, is a dimeric protein. The IslA monomer contains a barrel made of alpha helices that envelop two five-stranded half-beta barrels positioned antiparallel to each other.[2] Hidden within this barrel is the active site of the enzyme. It is believed that the positioning of the active site within the barrel protects the radical species (formed during the activation of the enzyme) from solvent quenching.[3]

Function[edit]

Enzyme activation[edit]

Activation of IslA depends on binding of glycyl radical enyzme-activating enzyme IslB, which catalyzes the initial formation of the radical S-adenosylmethionine (rSAM) species. rSAM is formed by the one-electron reduction of an iron-sulfur cluster, and the resulting radical is stabilized by amino acid residues within the enzyme.[4] The formation of the stable complex between the two enzymes and the binding of glycine in the active site of IslB are prerequisites for successful activation of IslA.[4]

Migration mechanism of IseG suggested by Deng et al. after quantum mechanical and molecular mechanical calculations.

Mechanism of action[edit]

The radical-based cleavage of IslA is thought to occur through a direct elimination reaction.[2] However, recent research indicates that a 1,2-SO3-radical migration may occur after a catalytically active cysteine residue radical grabs a hydrogen atom from isethionate, followed by hydrogen atom transfer from cysteine to a 1-hydroxylethane-1-sulfonate radical intermediate.[5] The elimination of sulfite from 1-hydroxylethane-1-sulfonate to result in the final product is likely to occur outside the enzyme.[5] This mechanism is similar to the reported fragmentation-recombination mechanism of B12-dependent glutamate mutase.[5]

Evolution of structure[edit]

Radicals are very chemically unstable species and must be carefully controlled in biological systems. Research supports the theory that GREs converged on glycyl radical formation due to the better conformational accessibility of the glycine radical loop, rather than the highest radical stability of the formed peptide radicals.[6]

Physiological role[edit]

Disease[edit]

Isethionate sulfite-lyase active site hydrogen bond interaction with surrounding amino acid residues.

Isla produced by Bilophila wadsworthia is known to convert organosulfides including taurine and isethionate into acetaldehyde and sulfite. Sulfite is converted into hydrogen sulfide, which can degrade the mucous lining of the colon and cause pathological conditions including colorectal cancer, inflammatory bowel diseases, and colitis.[3] Moreover, hydrogen sulfide has been known to induce antibiotic resistance suggesting that the production of this molecule could prompt blooms of opportunistic bacteria during antibiotic treatment.[7] Conversely, hydrogen sulfide may also act as a signaling molecule within the homeostasis of a host's circulatory system such as regulating blood pressure control.[8] Ultimately, although the role of hydrogen sulfide within disease may be unclear, efforts to find inhibitors for IslA may help mitigate the excess production of hydrogen sulfide.

Bacterial microcompartments[edit]

Within the same gene cluster that encodes IslA and IslB enzymes are several genes that encode shell proteins of bacterial microcompartments (BMCs). It has been found that the IslA and IslB enzymes are likely contained within BMCs which isolate the products of IslA (acetaldehyde and sulfite) from the cytosol and limits their harmful effects.[9] Flavin molecules, which are also present in the BMCs, may be used to shuttle electrons to the IslB enzyme which is necessary to install the glycyl radical on the IslA enzyme upon activation.

Industrial relevance[edit]

Anaerobic radical enzymes such as IslA have the potential to functionally modify a substrate without oxygen incorporation, requiring less expensive adaptation of downstream synthetic methodologies than from oxygen-rich biomass-derived feedstocks.[10] The ability to catabolize amino acids to generate a broad range of branched and unbranched hydrocarbon chains could be useful in production of biofuels. In addition, radical catalysis enables a range of specialist reactions of industrial interest, including carbon-skeleton rearrangements, aminomutases, and eliminases.[10]

References[edit]

  1. ^ Peck SC, Denger K, Burrichter A, Irwin SM, Balskus EP, Schleheck D (February 2019). "A glycyl radical enzyme enables hydrogen sulfide production by the human intestinal bacterium Bilophila wadsworthia". Proceedings of the National Academy of Sciences of the United States of America. 116 (8): 3171–3176. Bibcode:2019PNAS..116.3171P. doi:10.1073/pnas.1815661116. PMC 6386719. PMID 30718429.
  2. ^ a b Dawson CD, Irwin SM, Backman LR, Le C, Wang JX, Vennelakanti V, et al. (September 2021). "Molecular basis of C-S bond cleavage in the glycyl radical enzyme isethionate sulfite-lyase". Cell Chemical Biology. 28 (9): 1333–1346.e7. doi:10.1016/j.chembiol.2021.03.001. PMC 8473560. PMID 33773110.
  3. ^ a b Waqas M, Halim SA, Ullah A, Ali AA, Khalid A, Abdalla AN, et al. (January 2023). "Multi-Fold Computational Analysis to Discover Novel Putative Inhibitors of Isethionate Sulfite-Lyase (Isla) from Bilophila wadsworthia: Combating Colorectal Cancer and Inflammatory Bowel Diseases". Cancers. 15 (3): 901. doi:10.3390/cancers15030901. PMC 9913583. PMID 36765864.
  4. ^ a b Nicolet Y (13 April 2020). "Structure–function relationships of radical SAM enzymes". Nature Catalysis. 3 (4): 337–350. doi:10.1038/s41929-020-0448-7. ISSN 2520-1158. S2CID 215747856.
  5. ^ a b c Deng WH, Lu Y, Liao RZ (December 2021). "Revealing the Mechanism of Isethionate Sulfite-Lyase by QM/MM Calculations". Journal of Chemical Information and Modeling. 61 (12): 5871–5882. doi:10.1021/acs.jcim.1c00978. PMID 34806370. S2CID 244518952.
  6. ^ Hanževački M, Croft AK, Jäger CM (July 2022). "Activation of Glycyl Radical Enzymes─Multiscale Modeling Insights into Catalysis and Radical Control in a Pyruvate Formate-Lyase-Activating Enzyme". Journal of Chemical Information and Modeling. 62 (14): 3401–3414. doi:10.1021/acs.jcim.2c00362. PMC 9326890. PMID 35771966.
  7. ^ Shatalin K, Shatalina E, Mironov A, Nudler E (November 2011). "H2S: a universal defense against antibiotics in bacteria". Science. 334 (6058): 986–990. Bibcode:2011Sci...334..986S. doi:10.1126/science.1209855. PMID 22096201. S2CID 24829077.
  8. ^ Tomasova L, Konopelski P, Ufnal M (November 2016). "Gut Bacteria and Hydrogen Sulfide: The New Old Players in Circulatory System Homeostasis". Molecules. 21 (11): 1558. doi:10.3390/molecules21111558. PMC 6273628. PMID 27869680.
  9. ^ Burrichter AG, Dörr S, Bergmann P, Haiß S, Keller A, Fournier C, et al. (December 2021). "Bacterial microcompartments for isethionate desulfonation in the taurine-degrading human-gut bacterium Bilophila wadsworthia". BMC Microbiology. 21 (1): 340. doi:10.1186/s12866-021-02386-w. PMC 8667426. PMID 34903181.
  10. ^ a b Jäger CM, Croft AK (Jun 30, 2018). "Anaerobic Radical Enzymes for Biotechnology". ChemBioEng Reviews. 5 (3): 143–162. doi:10.1002/cben.201800003.
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