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3.3: Cysteine Chemistry

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    65987

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    Two cysteine side chains can covalently interact in a protein to produce a disulfide. Just as HOOH (hydrogen peroxide) is more oxidized than HOH, RSSR is the oxidized form and RSH is the reduced form of thiols.

    disulfide.gif

    Figure: DISULFIDE - CYSTINE - REACTIONS

    When a protein folds, two Cys side chains might approach each other, and form an intrachain disulfide bond. Likewise, two Cys side chains on separate proteins might approach each other and form an interchain disulfide. Such disulfides must be cleaved, and the chains separated before analyzing the sequence of the protein. The disulfide in protein can be cleaved by reducing agents such as beta-mercaptoethanol or dithiothreitol (DTT).

    The inside of cells are maintained in a reduced environment by the presence of many "reducing" agents, such as the tripeptide g-glu-cys-gly (glutathione). Hence intracellular proteins usually do not contain disulfides, which are abundant in extracellular proteins (such as those found in blood) or in certain organelles such as the endoplasmic reticulum and mitochondrial intermembrane space where disulfides can be introduced.

    disulfidebreak.gif

    Figure: Cleaving Disulfide Bonds in Proteins

    Cysteine Redox Chemistry

    The sulfur in cysteine is redox-active and hence can exist in a wide variety of states, depending on the local redox environment and the presence of oxidizing and reducing agents. A potent oxidizing agent that can be made in cells is hydrogen peroxide, which can lead to more drastic and irreversible chemical modifications to the Cys side chains. If a reactive Cys is important to protein function, then the function of the protein can be modulated (sometimes reversibly, sometimes irreversibly) with various oxidizing agents, as shown in the figure below.

    CysChem.gif

    Figure: Redox state of Cysteine

    This page titled 3.3: Cysteine Chemistry is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Henry Jakubowski.