Regions within a protein chain or in separate chains sometimes are cross-linked through disulfide bonds. Disulfide bonds are commonly found in extracellular proteins, where they help stabilize the folded structure. The smallest amino acid, glycine, has a single hydrogen atom as its R group. Its small size allows it to fit into tight spaces. Unlike the other common amino acids, the side chain of proline bends around to form a ring by covalently bonding to the nitrogen atom (amino group) attached to the Ca. As a result, proline is very rigid and creates a fixed kink in a protein chain, limiting how a protein can fold in the region of proline residues.
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