Cyclophilins isomerization

Immunophillins are abundant proteins that catalyze the cis-trans isomerization of proline residues within proteins, generally to aid in protein folding. Immunophillins are not essential proteins, are the intracellular binding proteins of several immunosuppressive drugs. Cyclosporin A exerts its action after binding to cyclophilin. Tacrolimus and sirolimus predominantly bind to the protein FKBP-12 (FK binding protein-12). 

The design of this successful system started with an enzyme with a ready-made peptide binding site, with well established specificity. The cyclophilins catalyze the cis-trans isomerization of peptide bonds to proline ... 

A cytosolic binding protein for CsA was first isolated in 1984 and named cyclophilin, later cyclophilin A (CyPA), in reference to its high affinity for CsA (18). CyPA is a basic, abundant protein with a mass of 18 kDa, and it is found in a variety of tissues. The first clue to its function came in 1989 when two independent groups isolated the enzyme that catalyzes pepti-dyl proline isomerization/peptidylprolyl cis-trans isomerase (EC 5.2.1.8 PPIase), in protein chains and (re)discovered CyPA (19, 20). CyPA is a potent PPIase, and its enzymatic activity is strongly inhibited by CsA. 

Disulfide bond formation within the individual propeptides precedes folding and trimers are then formed by association of the C-terminal propeptides." Disulfide bonds between the chains are then formed and this formation is most likely catalyzed by PDI." As triple helix formation proceeds, the rate-limiting step in this process is the cis—trans isomerization of peptidyl-Pro bonds. This process can be catalyzed by peptidyl-prolyl cis—trans isomerases (cyclophilins and FKBPs). This activity is required to convert the proline residues to the trans form required for triple helix formation." " " ... 

The smallest member of a new family of prolyl iso-merases (unrelated to the cyclophilins or the FK-506 binding proteins) that catalyzes the proline-limited folding of a variant of ribonuclease T1 with a KJK value of 30,000 M s With the tetrapeptide succinyl-Ala-Leu-Pro-Phe-4-nitroanilide as a substrate in parvulin-catalyzed prolyl isomerization, this parameter is 1.1 x 10 M s Parvulin also accelerates its own refolding in an autocatalytic fashion. 

At least two major slow processes occur in the folding of disulfide-containing proteins the cis-trans isomerizations of Xaa-Pro peptide bonds and the formation of the correct disulfide bonds. The latter is catalyzed by protein disulfide-isomerase (PDl). This enzyme occurs at high concentration in the endoplasmic reticulum (Hawkins et al., 1991) and there is good experimental evidence that PDl is required for the de novo folding of nascent secretory proteins (Bulleid and Freedman, 1988). Cyclophilins have recently also been localized in the ER (Hasel et al., 1991) and in other compartments of the secretory pathway (Caroni et al., 1991). Their biological function is not known. 

Fig. 12.5 Schematic representation of the interaction between CsA and cyclophilin hCypl 8 structure of CsA analog 1 (SDZ NIM 811). CsA bonds that isomerize during hCyp binding are indicated with rotating arrows.

In the maturation of the collagen triple helix, prolyl and hydroxyprolyl isomerizations are rate-limiting steps (Bachinger et al., 1980 Buevich et al., 2000) and accelerated by Cypl8 (Bachinger, 1987 Davis et al., 1989). For mouse dihydrofolate reductase, the overall formation of the catalytically active protein is accelerated by cyclophilin, but it is not the... 

The prolyl isomerases catalyze isomerizations only at prolyl bonds and not at nonprolyl peptide bonds. The refolding of the P39A variant of RNase Tl, which is limited in rate by the very slow trans —> cis reisomerization of the Tyr38-Ala39 bond (see Section IV.B), is not catalyzed by cyclophilins, FKBPs, or parvulins. These enzymes are also unable to catalyze amide bond isomerizations in the proline-free model peptide Ala-Ala-Tyr-Ala-Ala (Scholz etal., 1998b). 

Shift the reaction path (cyclophilin). Cyclophilin, which catalyzes the cis-trans isomerization of peptidyl-proline bonds, is converted into protease by an engineered introduction of the proteolytic triad, Asp-His-Ser (Qu6m6neur et al, 1998). 

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