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Substitutions:
Cysteine shows no preference generally for substituting
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with
any other amino acid, though it can tolerate substitutions with
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other
small amino acids. Largely the above preferences can be
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accounted
for by the extremely varied roles that Cysteines play in
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proteins
(see below). The substitutions preferences shown above are
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derived
by analysis of all Cysteines, in all contexts, meaning that what
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are
really quite varied preferences are averaged and blurred; the result
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being quite
meaningless.
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Role
in structure: The role of Cysteines in structure is very dependent
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on
the cellular location of the protein in which they are contained.
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Within
extracellular proteins, cysteines are frequently involved in
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disulphide
bonds, where pairs of cysteines are oxidised to form a
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covalent
bond. These bonds serve mostly to
stabilise the protein
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structure,
and the structure of many extracellular proteins is almost
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entirely
determined by the topology of multiple disulphide bonds
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