The Cysteine residue exists in its thiol form in intracellular proteins, yet for

Question

The Cysteine residue exists in its thiol form in intracellular proteins, yet forms a disulfide linkage with a second Cysteine residue (to form “Cystine”) in extracellular proteins because of the more oxidizing environment that extracellular proteins are exposed to (for example proteins of the human circulatory system). Cystine formation requires that the two participating Cysteine residues are closely located in three-dimensional space in the folded protein (i.e. tertiary structure) and as well as properly oriented for the formation of a disulfide bond that is free of strain. Typically, the presence of a Cystine creates a covalent link between two Cysteine residues that are located at distant positions within the primary structure, such that when the protein unfolds (i.e., denatures) the protein chain forms a looped structure. Explain why (how) the presence of this loop in the unfolded protein serves to stabilize the folded protein. (Hint-this has nothing to do with SH vs S-S bond energies!).

Explanation / Answer

Every system wants to minimize their energy, lower the energy higher the stabilization. Now, in unfolded protein every amino acid residue has several orientation around them. In these thousands of possibilities of orientation of amino acid only one possibility is there, where that protein gets its minimum energy. In this orientation disulphide bond will form according to the condition of the environment.

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