PPIases FAQ
Questions most frequently asked about folding helper enzymes: |
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Which proteins belong to the group of folding helper enzymes? The group of folding helper enzymes encompasses the enzyme classes of peptidyl prolyl cis/trans isomerases (EC 5.2.1.8) and the protein disulphide isomerases (EC 5.3.4.1) Is there a principal difference between the catalytic functions of peptidyl prolyl cis/trans isomerases and protein disulphide isomerases? Yes, because the unique peptidyl prolyl cis/trans isomerases are catalyzing a conformational change in a polypeptide chain, whereas protein disulphide isomerases are catalyzing the formation of covalent bonds. What are the first papers about peptidyl prolyl cis/trans isomerases? In a paper, entitled "Determination of enzymatic catalysis for the cis/trans-isomerization of peptide bonds in proline-containing peptide" by G. Fischer, H. Bang and C. Mech published in Biomed. Biochim. Acta 43, 1101-1111 (1984) the discovery of peptidyl prolyl cis/trans isomerases was described. Originally, pig kidney cyclophilin was identified as the enzyme. The FKBP family of peptidyl prolyl cis/trans isomerases was discovered by two groups in 1989. The references are: J. J. Siekierka, S. H. Hung, M. Poe, C. S. Lin, and N. H. Sigal: Nature 341, 7655 (1989) and M. W. Harding, A. Galat, D. E. Uehling and S. L. Schreiber: Nature, 341, 758 (1989). The first member of the parvulin family was identified in E. coli in 1994 (J. U. Rahfeld; A. Schierhorn, K. Mann and G. Fischer: FEBS-Lett. 343, 65-69 (1994)). What about the substrates for peptidyl prolyl cis/trans isomerases? The enzymes are directed toward the peptide bond preceding proline in oligopeptides and proteins. Secondary amide peptide bonds cannot substitute for the prolyl peptide bond. The active cleft of the enzymes may adopt as much as 20 amino acid residues. Secondary binding sites are utilized to express catalytic activity and substrate specificity. Catalysis is obtained for the reversible cis/trans isomerization of the prolyl peptide bond. The ratio of isomers cannot be shifted in the presence of catalytic amounts of the enzymes. How many protein families exist in the enzyme class of peptidyl prolyl cis/trans isomerases? Currently, three families are existing which do not have similarity in the amino acid sequences, the cyclophilins, the FK506-binding proteins and the parvulins. Which alternative names can be found for peptidyl prolyl cis/trans isomerases in the literature? According to the recommendations of the Nomenclature Committee of the IU/BMB the discoverer termed the enzymes peptidyl prolyl cis/trans isomerases (Abbreviated: PPIase) in 1984. The short terms "prolyl isomerase" and "proline isomerase" are also in use. The term "rotamase" conveys an imprecise picture of the enzyme function and should be avoided. The biomedical literature the term "immunophilin" is frequently used in an attempt to replace the term peptidyl prolyl cis/trans isomerase. It relates to the putative involvement in the cyclosporin A (FK506)-mediated immunosuppression of a few mammalian members of the cyclophilin and FKBP family (Cyp18 and FKBP12). Many proteins not related to peptidyl prolyl cis/trans isomerases are immunophilic (ubiquitin, G-actin, interleukin 8, and many others), whereas many peptidyl prolyl cis/trans isomerases do not exhibit immunophilic properties. Thus, the meaning of both expressions does not coincide. How many different peptidyl prolyl cis/trans isomerases exist in human cells? At least 11 cyclophilins, 18 FKBPs and 2 parvulins are encoded in the human genome. How large is the degree of evolutionary conservation of the peptidyl prolyl cis/trans isomerase function? All genomes analysed so far contain at least a single member of these enzymes but often many of them are encoded. Do peptidyl prolyl cis/trans isomerases belong to the chaperones? Generally, they do not because the prototypic peptidyl prolyl cis/trans isomerases, human Cyp18, human FKBP12 and E.coli Par10 cannot functionally substitute chaperones in protein refolding assays designed to monitor chaperone properties. However, larger peptidyl prolyl cis/trans isomerases with domains complementing the catalytic core may exhibit chaperone properties in addition to the isomerase function. The chaperone function is critically linked to the presence of the additional domains. Are peptidyl prolyl cis/trans isomerases involved in the de novo protein folding? Yes, because the prokariotic trigger factor, a peptidyl prolyl cis/trans isomerases of the FKBP type, is a peripheral ribosomal protein with affinity to a subset of newly synthesized proteins. Synthetic lethality was shown for a double-deletion of the chaperone DnaK and the trigger factor in E. coli. Do we already know the biological function of peptidyl prolyl cis/trans isomerases? Unfortunately, knowledge is still limited about the molecular mechanism of enzyme action in vivo. May be that there is a wide variety of functions but not even one, like catalysis of de novo protein folding. For example, the enzyme activity is involved in regulation of the eukaryotic cell cycle, the function of the ryanodine receptor, the maturation of photoreceptors, the degradation of peptide hormones, the function of receptor protein kinases, the activation of steroid receptors, the HIV-1 replication cycle, the replication of facultative intracellular parasites causing multiple human diseases, the development of familial amyotrophic lateral sclerosis. |