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Circular proteins are a recently discovered phenomenon. They
presumably evolved to confer advantages over ancestral linear
proteins while maintaining the intrinsic biological functions of
those proteins. In general, these advantages include a reduced
sensitivity to proteolytic cleavage and enhanced stability. In
one remarkable family of circular proteins, the cyclotides, the
cyclic backbone is additionally braced by a knotted arrangement of
disulfide bonds that confers additional stability and topological
complexity upon the family. This article describes the discovery,
structure, function and biosynthesis of the currently known
circular proteins. The discovery of naturally occurring circular
proteins in the past few years has been complemented by new
chemical and biochemical methods to make synthetic circular
proteins; these are also briefly described.
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