Atlas One · Atomic · Chalcogen / Redox Active / Structural
Period 3, Group 16 — thiol chemistry enables redox sensing, protein folding, and energy transfer
| Functional Group | pKa / ΔG | Examples | Biological Role |
|---|---|---|---|
| Thiol (–SH) | pKa 8.3 (Cys); ~5–6 (active-site Cys near Arg) | Cysteine residues; GSH; Trx active site | Nucleophile in catalysis; redox sensor; metal ligand |
| Disulfide (–S–S–) | −125 kJ/mol per bond (protein stabilisation) | IgG inter/intra-chain; insulin A-B chain; keratin | Protein folding/stability in ER oxidising environment; PDI catalysed |
| Thioether (–S–) | Non-ionisable | Methionine; SAM; biotin | SAM transmethylation; Met oxidation (biomarker) |
| Thioester (–C(O)–S–) | ~31 kJ/mol hydrolysis | Acetyl-CoA; succinyl-CoA; malonyl-CoA | High-energy acyl carrier; fatty acid/TCA/ketone metabolism |
| Sulfate ester | Stable at pH 7 | Heparan sulfate; chondroitin sulfate; cholesterol sulfate | ECM structure; anticoagulation; lipid bilayer |
Biological Roles
Glutathione antioxidant cycle, CoA thioester metabolism, SAM methylation, Fe-S cluster relay
Glutathione Antioxidant System
γ-Glu–Cys–Gly (GSH, tripeptide; thiol from Cys)
│
├─ GPx (selenoprotein): 2 GSH + H₂O₂ → GSSG + 2H₂O
│ 2 GSH + PLOOH → GSSG + PLOH + H₂O (GPx4/ferroptosis)
│
├─ GST (phase II): GSH + electrophile → GSH-conjugate (hepatic detox)
│ Exported by MRP1/2; mercapturate pathway → urinary excretion
│
└─ GSSG ──Glutathione reductase (GR) + NADPH──► 2 GSH
(NADPH from Pentose Phosphate Pathway, G6PD)
G6PD deficiency → ↓ NADPH → ↓ GSH → haemolytic anaemia under oxidative stress
Coenzyme A — Thioester Energy Carrier
CoA (pantothenate-derived) forms thioesters with acyl groups (~31 kJ/mol). Acetyl-CoA enters TCA cycle (citrate synthase); is substrate for fatty acid synthesis (ACC → malonyl-CoA); donates acetyl group to histones (HATs → chromatin remodelling). Succinyl-CoA is a TCA intermediate and haem synthesis precursor (ALA synthase). Malonyl-CoA inhibits CPT1 (fatty acid import into mitochondria) — key in fed/fasted switching.
SAM and One-Carbon Transmethylation
S-adenosylmethionine (SAM): Met + ATP → SAM (by MAT). SAM donates methyl groups to: DNA (DNMT → cytosine methylation / gene silencing), histones (HMTs → H3K4me3/H3K27me3), norepinephrine (PNMT → epinephrine), phosphatidylethanolamine (PE → PC), and many other substrates. SAM → SAH (S-adenosylhomocysteine) → Hcy (homocysteine) → remethylated (B12/folate) or transsulfurated (B6/CBS → cystathionine → Cys → GSH).
Iron-Sulfur Clusters
[2Fe-2S] and [4Fe-4S] clusters: assembled by ISC (iron-sulfur cluster) machinery in mitochondria. Key sites: Complex I (8 Fe-S clusters), Complex II (3 Fe-S clusters), Complex III (Rieske [2Fe-2S]), aconitase ([4Fe-4S]), ferredoxin. Cys residues ligate iron atoms. Reduced [4Fe-4S]²⁺ / [4Fe-4S]¹⁺ transitions relay single electrons. Loss of Fe-S in aconitase = citrate accumulation, IRP1 activation (iron sensor).
Absorption & Metabolism
Methionine cycle, transsulfuration, and sulfate conjugation
Dietary sulfur arrives as Met (~50% essential), Cys (conditionally essential), and inorganic sulfate. Met is absorbed by enterocytes via ASCT1/2 and B0AT1. The transsulfuration pathway (CBS + CSE, B6-dependent) converts Hcy → Cys → GSH. Cys/Met oxidation produces sulfite (oxidised by SUOX to sulfate) and H₂S (gasotransmitter; made by CBS/CSE in vascular smooth muscle — vasodilation).
| Pathway | Key Enzymes | Products |
|---|---|---|
| Met cycle | MAT, SAHH, MTHFR, MS (B12) | SAM (methylation), SAH, Hcy |
| Transsulfuration | CBS (B6), CSE (B6) | Cystathionine, Cys, H₂S |
| GSH synthesis | GCL (γ-GCS, rate-limiting), GSS | GSH (γ-Glu-Cys-Gly) |
| Phase II (liver) | GSTs (α,µ,π,θ classes) | GSH-conjugates → mercapturates → urine |
| Sulfate conjugation | SULTs (sulfotransferases) | PAPS + R-OH → R-O-SO₃⁻ (bile acids, steroids, drugs) |
Deficiency & Toxicity
| Condition | Mechanism | Signs | Treatment |
|---|---|---|---|
| Homocystinuria | CBS deficiency (B6-responsive ~50% or B6-non-responsive); Hcy accumulates | Lens dislocation (ectopia lentis), Marfanoid habitus, intellectual disability, thromboembolic events | Pyridoxine (B6); betaine (Hcy remethylation); Met restriction + Cys supplementation |
| Cystinuria | SLC3A1/SLC7A9 mutations; defective cystine reabsorption in PCT and intestine | Recurrent cystine nephrolithiasis (hexagonal crystals); renal insufficiency | High fluid intake; urinary alkalinisation; D-penicillamine (cystine solubilising) |
| Friedreich's ataxia | FXN (frataxin) deficiency → mitochondrial Fe-S assembly failure → Fe accumulation in mitochondria → ROS | Progressive spinocerebellar ataxia; hypertrophic cardiomyopathy; DM | Antioxidants (idebenone); omaveloxolone (Nrf2 activator, FDA 2023); supportive |
| G6PD deficiency | ↓ NADPH → ↓ GSH → susceptible erythrocytes to oxidative haemolysis | Haemolytic anaemia triggered by fava beans, primaquine, dapsone, viral illness | Avoid oxidant drugs; folic acid; supportive transfusion; gene therapy trials |
| Hydrogen sulfide toxicity | Endogenous H₂S excess or environmental H₂S exposure; inhibits CcO (Complex IV) | Smell of rotten eggs; at high concentrations: olfactory fatigue, pulmonary oedema, death | Nitrites (methb formation sequesters H₂S); hydroxocobalamin |
Clinical Use
| Application | Details |
|---|---|
| N-acetylcysteine (NAC) | GSH precursor: reduces Cys-Gly → replenishes hepatic GSH; acetaminophen overdose antidote (4-hour nomogram); mucolytic (breaks disulfide bonds in mucus) |
| Dimercaprol (BAL) | Dithiol chelator; chelates As, Hg, Pb; competes with enzyme SH groups |
| D-penicillamine | Thiol compound; cystinuria (complexes cystine), Wilson disease (Cu chelation), RA (immunomodulation) |
| Mesna (MESNA) | –SH group reacts with acrolein (toxic cyclophosphamide metabolite) in urine → prevents haemorrhagic cystitis |
| Heparin / LMWH | Sulfated polysaccharide (heparan sulfate-like); potentiates ATIII to inhibit thrombin and Xa; O-sulfate groups are critical for activity |
Connections
References
- Lu SC. "Glutathione synthesis." Biochim Biophys Acta 2013;1830:3143–3153.
- Lill R. "Function and biogenesis of iron-sulphur proteins." Nature 2009;460:831–838.
- Stipanuk MH. "Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine." Annu Rev Nutr 2004;24:539–577.
- Prigione A, et al. "SAM in health and disease." Nat Rev Mol Cell Biol 2019.
- Wang R. "Two's company, three's a crowd: can H₂S be the third endogenous gaseous transmitter?" FASEB J 2002;16:1792–1798.