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Prolyl isomerase
Prolyl isomerase (also known as peptidylprolyl isomerase or PPIase) is an enzyme () found in both prokaryotes and eukaryotes that interconverts the ''cis'' and ''trans'' isomers of peptide bonds with the amino acid proline. Proline has an unusually conformationally restrained peptide bond due to its cyclic structure with its side chain bonded to its secondary amine nitrogen. Most amino acids have a strong energetic preference for the ''trans'' peptide bond conformation due to steric hindrance, but proline's unusual structure stabilizes the ''cis'' form so that both isomers are populated under biologically relevant conditions. Proteins with prolyl isomerase activity include cyclophilin, FKBPs, and parvulin, although larger proteins can also contain prolyl isomerase domains.
== Protein folding ==
Proline is unique among the natural amino acids in having a relatively small difference in free energy between the ''cis'' configuration of its peptide bond and the more common ''trans'' form. The activation energy required to catalyse the isomerisation between ''cis'' and ''trans'' is relatively high: ~20kcal/mol (c.f. ~0kcal/mol for regular peptide bonds). Unlike regular peptide bonds, the X-prolyl peptide bond will not adopt the intended conformation spontaneously, thus, the process of ''cis-trans'' isomerization can be the rate-limiting step in the process of protein folding. Prolyl isomerases therefore function as protein folding chaperones. ''Cis'' peptide bonds N-terminal to proline residues are often located at the first residue of certain types of tight turns in the protein backbone. Proteins that contain structural ''cis''-prolines in the native state include ribonuclease A, ribonuclease T1, beta lactamase, cyclophilin, and some interleukins.
Prolyl isomerase folding can be autocatalytic and therefore the speed of folding depends on reactant concentration. Parvulin and human cytosolic FKBP are thought to catalyze their own folding processes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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