--- title: "Disulfide bonds: cysteine and cystine Template | MolDraw" url: https://www.moldraw.com/templates/disulfide-bond-cystine/ description: "Disulfide bonds: cysteine and cystine template: A cysteine thiolate attacks an oxidised disulfide (in cells PDI or GSSG) and a second thiolate closes…" lang: en publisher: Scidart Academy generated: 2026-11-13 --- [MolDraw](https://www.moldraw.com/) › [Templates](https://www.moldraw.com/templates/) › [Biochemistry](https://www.moldraw.com/templates/biochemistry/) › Disulfide bonds: cysteine and cystine [Biochemistry](https://www.moldraw.com/templates/biochemistry/)Premium # Disulfide bonds: cysteine and cystine template ![Disulfide bonds: cysteine and cystine template: A cysteine thiolate attacks an oxidised disulfide (in cells PDI or GSSG) and a second thiolate closes cystine. DTT reverses it: one thiolate cuts the S–S bond, the other closes DTT’s own six-membered ring.](https://www.moldraw.com/templates/disulfide-bond-cystine.webp) [Edit in MolDraw](https://www.moldraw.com/)Download with Pro Premium template. Preview and edit it free in MolDraw; downloading and exporting it is part of [MolDraw Pro](https://www.moldraw.com/pricing). A cysteine thiolate attacks an oxidised disulfide (in cells PDI or GSSG) and a second thiolate closes cystine. DTT reverses it: one thiolate cuts the S–S bond, the other closes DTT’s own six-membered ring. ## Mechanism step by step Two cysteine thiols are oxidised to the S–S bond of cystine; thiols such as DTT reduce it again by thiol–disulfide exchange 1. **Thiol–disulfide exchange** – Oxidation: a Cys thiolate attacks the oxidant’s S–S (R′ = glutathione or PDI) 2. **Second exchange** – Another Cys thiolate releases R′S⁻ and closes cystine 3. **Reduction by DTT** – A DTT thiolate cuts the S–S bond: a mixed disulfide 4. **Ring closure** – DTT’s second thiolate attacks: oxidised DTT (a 1,2-dithiane) and free cysteine ## Key points - Overall 2 R–SH ⇌ R–S–S–R + 2 H⁺ + 2 e⁻. The thiolate (Cys SH pKa ≈ 8.3) is the nucleophile, attacking S in an SN2-like way. - Disulfides form in the oxidising ER (protein disulfide isomerase) and outside cells; the cytosol, rich in glutathione (GSH), keeps cysteines reduced. - DTT (Cleland’s reagent) reduces completely at low concentration because its second step is intramolecular; β-mercaptoethanol needs a large excess. - S–S bridges link insulin’s A and B chains and stabilise secreted proteins. They are reduced (and the thiols blocked) before SDS-PAGE or sequencing. Drawn with 149 atoms, 13 arrows, 8 shapes – every one editable in MolDraw. ## Sources - [Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 3](https://openlibrary.org/isbn/9781319228002) Textbook - [Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019](https://openlibrary.org/isbn/9781319114657) Textbook - [Cleland, Biochemistry 1964, 3, 480](https://doi.org/10.1021/bi00892a002) Original paper ## How to use this template 1. Press **Edit in MolDraw**. The template opens on the canvas of the free MolDraw editor. 2. Change anything: move structures, edit atoms and labels, recolour highlights or delete what you do not need. 3. Add your own structures, arrows and text next to it, or combine several templates on one page. 4. Export the drawing as PNG, SVG or PDF for slides, worksheets, theses and papers. ## Frequently asked questions ### Is the Disulfide bonds: cysteine and cystine template free? You can preview the Disulfide bonds: cysteine and cystine template here and open and edit it in MolDraw for free. Downloading or exporting it is part of MolDraw Pro, together with the other premium templates. ### Can I edit the Disulfide bonds: cysteine and cystine template? Yes. It opens as an ordinary MolDraw drawing, so every structure, arrow, label and colour can be changed, and you can add your own content around it. ### Is the chemistry checked? Every structure is checked with RDKit and every curly arrow passes MolDraw’s electron-flow audit. The sources used are listed on this page. ### Are the biochemical pathways drawn with structures? Yes. 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