Proline cis-trans isomerase

In the protein s native conformation, every X-Pro bond has to have the correct conformation (cis or trans). As the uncatalyzed transition between the two forms is very slow, there is a proline cis-trans isomerase [2] in the ER that accelerates the conversion. 

WW domains are signaling modules of ca. 40 amino acids that bind short proline-rich sequences such as PPLP or PPR motifs (review Macias et al., 2002). A subset of WW domains, found, e.g., in the proline cis-trans isomerase Pinl, however, specifically binds to phosphoserine-Pro and phosphothreonine-Pro motifs. The Pin 1 protein has an essential role in mitosis. It is thought that Pinl binding to phosphorylated mitotic proteins facilitates proline cis/trans isomerizations and subsequent conformational changes. For the Pinl substrate Cdc25 phosphatase it has been shown that proline isomerization facilitates the subsequent dephosphorylation of phosphorylated Cdc25 protein by the protein phosphatase PP2A. 

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 peptidyl-prolyl cis/trans isomerase Pin 1 isomerizes the peptide bond of a phos-phorylated serine or phosphorylated threonine followed by a proline. Through isomerization of pSer-Pro or PThr-Pro, Pinl regulates a number of proteins. Together with its ability to regulate phosphorylation and conformation of tau proteins. Pin 1 is considered a potential neuroprotective function against AD. 

Thrl67 bond in the cis Prol67Thr TEM-1 /J-lactamase variant is also characterized by a rate constant between 1 x 10-3 s-1 and 4 x 10 3 s-1 for the trans to cis interconversion [26]. Therefore the trans to cis isomerization can be rate-limiting in protein folding under native-like conditions, as was shown for a proline-free variant of the a-amylase inhibitor tendamistat [27]. This seems to be proven in the discovery of a novel protein dass, the secondary amide peptide cis/trans isomerases (APIases), which can accelerate interconversion of these peptide bonds conformers [28],... 

There is no common nomenclature for prolyl isomerases. The systematic name peptidylproline cis, trans-isomerase (E.C. 5.2.1.8) is shortened to prolyl isomerase, proline isomerase, PPI, or PPIase. Sometimes the expression rotamase is used. 

Chaperonins - In addition to the enzymes mentioned previously that assist with proper folding (e.g., cis-trans isomerase for proline and disulfide bond making enzymes), cells have a class of proteins called chaperonins, which "chaperone" a protein to help keep it properly folded and non-aggregated. 

A cis-trans isomerase and a disulfide isomerase also participate in folding. The cis-trans isomerase converts a trans peptide bond preceding a proline into the cis conformation, which is well suited for making hairpin turns. The disulfide isomerase breaks and reforms disulfide bonds between the -SH groups of two cysteine residues in transient structures formed during the folding process. After the protein has folded, cysteine-SH groups in close contact in the tertiary structure can react to form the final disulfide bonds. 

The search for an enzymatic activity that would catalyze prolyl peptide bond isomerization began soon after the proposal of the proline hypothesis. The success came in 1984, when Fischer and co-workers discovered a peptidylprolyl m—tram-isomerase activity in porcine kidney and other tissues by an assay that is based on the conformational specificity of chymotrypsin. This protease cleaves the 4-nitroanilide moiety from the peptide glutaryl-Ala-Ala-Pro-Phe-4-nitroanilide only when the Ala-Pro peptide bond is in the trans conformation. In aqueous solution 90% of the molecules are trans in the assay peptide and only 10% are cis. Therefore, in the presence of a high concentration of chymotrypsin, 90% of the hydrolysis reaction occurs within the dead time of manual mixing. Hydrolysis of the remaining 10% is slow, limited in rate by the cis — ... 

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