Kinetics of collagen fold formation in human type I procollagen and the effect of disulfide bonds

The interaction of sodium dodecyl sulfate and urea with cat-fish collagen solutions in acetate buffer: hydrodynamic and thermodynamic studies

Cat-fish collagen was extracted and characterized. Shrinkage temperature of cat-fish collagen is 54.5 degrees C. SDS-PAGE pattern indicated that the cat-fish collagen is Type I in nature. The ratio of proline and hydroxyproline is 1:2 and it suggests cat-fish collagen is vertebrate. The molecular weight of cat-fish collagen was determined by using molecular sieve chromatography and it was found to be 3 20,000 Da. The mutual interaction of cat-fish collagen with SDS and urea was studied at various temperatures. The results suggest that the aggregation of collagen is facilitated by the presence of SDS, whereas hindered by urea. The various thermodynamic parameters were estimated from viscosity measurements and the transfer of collagen into SDS micelles, urea and the reverse phenomenon was analysed. These transfer properties are temperature-dependent. Our thermodynamic results are also able to predict the exact denaturation temperature as well as the structural order of water in the collagen in various environments. The hydrated volumes, Vh of collagen in buffer, SDS, and urea environments using Simha-Einstein equation and intrinsic viscosity were also calculated. The low intrinsic viscosity [eta] and high Vh value of collagen in an SDS environment compared to buffer and other environments suggested a more workable system in cosmetic and dermatological preparations. The one and two-hydrogen-bonded models of this collagen in various environments have been analysed. The calculated thermodynamic parameters varied with the concentration of collagen as well as concentration of additives. The change of thermodyanamic parameters from coiled-coil to random-coil conformation upon denaturation of collagen were calculated from the amount of proline and hydroxyproline residues and compared with viscometric results. Denaturation enthalpy of the catfish collagen in buffer, SDS and urea environments has also been determined by differential scanning calorimetric (DSC) measurements, and the results are in good agreement with the viscosity-derived values. The assymmetry and molecular geometry of this collagen in buffer, SDS and urea environments are also computed. Overall, our hydrodynamic and thermodynamic results suggest that the stability of the collagen in the additive environments is in the following order: SDS > buffer > urea.

Thermal stability and folding of the collagen triple helix and the effects of mutations in osteogenesis imperfecta on the triple helix of type I collagen

Osteogenesis imperfecta (OI) is an inherited disease in which 90% of the cases result from mutations in the 2 genes, pro alpha 1 and pro alpha 2, coding for type I collagen. Type I collagen is a trimeric molecule, (alpha 1)2 alpha 2, which is dominated both structurally and functionally by the 300 nm triple-helical domain. Most OI mutations occur in this domain and almost all point mutations result in the substitution of other amino acids for the obligate glycine which occurs at every third residue. The phenotypic effects of these mutations are frequently attributed in part to alterations in the stability and rate of folding of the triple helix. In order to better understand the relationship between glycine substitutions and stability we review current concepts of the forces governing triple helical stability, denaturational and predenaturational unfolding, and the techniques of measuring stability. From observations on the stability of several collagen types as well as synthetic tripeptides, we present a model for stability based on the contribution of individual and neighboring tripeptide units to the local stability. Although in preliminary form, this empirical model can account for the observed shifts in the Tm of many of the point mutations described. The folding of the triple helix is reviewed. The involvement of peptidyl prolyl cis-trans isomerase in this process in vivo is demonstrated by the inhibition of collagen folding in fibroblasts by cyclosporin A. An hypothesis based on the relationship between the thermal stability at the site of mutation and the propensity for renucleation of folding is proposed.

Presence of a thermostable domain in the helical part of the type I collagen molecule and its role in the mechanism of triple helix folding

A study has been done of the effect of neutral salts (NaCl and CaCl2) on the mechanism of type I collagen triple helix folding and unfolding in concentrated acetic acid solutions (2-8.8 M). It is shown that in these conditions, thermoabsorption and secondary structure change in heated solutions proceed in two consecutive stages. Salts exert a different destabilizing effect on different sites of the macromolecule, promoting the detection of a thermostable domain. The presence of a thermostable domain permits one to carry out reversible denaturation of collagen and to study the mechanism of the triple helix folding. Proceeding from the mechanism of the triple helix folding, an assumption has been made on the localization of the thermostable domain and its biological role.

Underhydroxylated minor cartilage collagen precursors cannot form stable triple helices

Matrix-free cells from chick-embryo sterna were incubated with various concentrations of 2,2'-bipyridyl, an iron chelator that inhibits prolyl hydroxylase and lysyl hydroxylase. At concentrations in the region of 0.1 mM, significant effects on cartilage collagen hydroxylation and secretion were observed. When the underhydroxylated collagens were subsequently digested with chymotrypsin or chymotrypsin plus trypsin at 4 degrees C for 15 min, the minor cartilage collagen precursors (namely types IX and XI) were extensively degraded; type II procollagen was only partially susceptible and was converted into underhydroxylated collagen. The results demonstrate that there were significant differences in triple-helix stability among cartilage collagens such that the underhydroxylated minor collagen precursors were unable to attain a native structure under conditions where type II procollagen was successful.

Production of monoclonal antibodies recognising N-ethylmaleimide during attempted generation of monoclonal antibodies to human type I procollagen

Mice were immunised for the production of monoclonal antibodies to human procollagen (I) using antigen purified from fibroblast conditioned medium. The procedure for procollagen (I) preparation included the addition of proteinase inhibitors N-ethylmaleimide, phenylmethylsulphonyl fluoride and ethylenediaminetetraacetic acid to prevent damage by proteolytic cleavage. Four of the five monoclonal antibodies subsequently produced were found to recognise the thiol proteinase inhibitor N-ethylmaleimide but not type I procollagen prepared in the absence of N-ethylmaleimide. One of these monoclonal antibodies was examined further and shown to recognise beta-galactosidase after it had been reacted with N-ethylmaleimide. As far as we are aware this is the first time that monoclonal antibodies have been produced which recognise N-ethylmaleimide. Our findings indicate an unexpected reaction between procollagen and N-ethylmaleimide and prompt the suggestion that the use of N-ethylmaleimide in the purification of procollagen and other proteins should be reexamined.

Biosynthesis of collagen

During the biosynthesis and assembly of collagen structures, disulfide links can serve several functions. During biosynthesis they successively stabilize intrapeptide folding and associations of three chains into one molecule. Studies on the refolding and reassociation of reduced and denatured carboxyl propeptides of procollagen I showed that successive interactions of folding and assembly are successively weaker. Disulfide bridges were reestablished within correctly refolded carboxyl propeptides. Rearrangements of disulfide bridges may occur during the processing of type V procollagen molecules as these collagens become incorporated into extracellular matrix. The basement membrane procollagen IV molecules become disulfide linked at each end into networks, and there are indications that further rearrangements of disulfide links may allow additional modulation.

Formation of the triple helix of type I procollagen in cellulo. Temperature-dependent kinetics support a model based on cis in equilibrium trans isomerization of peptide bonds

The kinetics of triple-helix formation in type I procollagen at 37 degrees C in cellulo have been found to agree with predictions from the following model: triple-helix formation is initiated after completion of the synthesis of the procollagen polypeptide chains and after the chains associate to form interchain disulfide bonds within the C-propeptide; triple-helix formation propagates from this single nucleation site toward the N terminus of the molecule, interrupted by the random occurrence of peptide bonds in the cis configuration; cis-trans isomerization controls the rate of triple-helix formation. This model predicts that the activation energy of the rate-limiting process should be strongly positive. However, studies of triple-helix formation in vitro using thermally denatured material have shown only a low, or even negative, dependence of the rate on temperature in the physiological range. Here we report the temperature dependence of the rate of triple-helix formation in cellulo and a novel procedure for analyzing the resulting data to give an estimate of the Arrhenius activation energy of the rate-controlling process. It was found that this rate showed a strong, positive dependence on temperature, as expected, and that the activation energy was in satisfactory agreement with independent direct determinations of this parameter for cis-trans isomerizations. These findings lend further support to the model of triple-helix formation described above.

p-HMW-collagen, a minor collagen obtained from chick embryo cartilage without proteolytic treatment of the tissue

The fragments of minor collagens of cartilages, called HMW and LMW, were isolated after pepsin treatment of sternal cartilages of young chickens and were shown to be entirely triple-helical molecules as judged by their circular dichroic spectra. Studies on renaturation kinetics of HMW suggested that the interchain disulfide bonds in HMW reside at one of the ends of the so-called long arm. Polyclonal antibodies against HMW were raised and affinity purified. These antibodies did not cross-react with type II collagen nor with other minor collagens such as LMW and 1 alpha, 2 alpha, 3 alpha collagen in native or denatured structure. The antibodies were used to identify HMW-related molecules which were synthesized by embryonic chick cartilages in vitro. Some of these molecules were secreted into the organ culture medium and could be recovered from it by ammonium sulfate precipitation. Polyacrylamide gel electrophoresis of this precipitate gave one band of high molecular weight which could be reduced to two bands migrating slightly faster than the alpha 1(II) chain when identified by immunoblotting. These bands could also be identified among about six radiolabelled polypeptides present in the ammonium sulfate precipitate of medium proteins when analysed by polyacrylamide gel electrophoresis followed by fluorography. The same polypeptides could be recovered from the medium by immunoprecipitation with anti-HMW antibodies. Their presence in cartilage tissue was shown by immunoblotting of material extracted from cartilage tissue and separated on polyacrylamide gels. We suggest that the protein containing these polypeptide chains represents the parent molecule of the peptic fragment HMW as it is synthesized in vivo and have designated it p-HMW-collagen.

Protein disulphide-isomerase activity in various cells synthesizing collagen

A high correlation was found between the activities of protein disulphide isomerase and prolyl 4-hydroxylase when assayed in cells synthesizing various collagen types or the same type at markedly different rates. The highest activities of both enzymes were found in freshly isolated chick-embryo tendon and cartilage cells, intermediate activities in confluent cultures of human skin and lung fibroblasts and mouse 3T6 fibroblasts, and the lowest values in three human sarcoma cell lines, the difference in protein disulphide isomerase activity between the freshly isolated tendon cells and confluent simian-virus-40-transformed human lung fibroblasts being about 25-fold. All these differences are in good agreement with differences reported between the various cells in their rates of collagen synthesis. A great similarity was also found between the changes in the two enzyme activities measured per cell during the growth of 3T6 fibroblast cultures from the early logarithmic phase to the stationary phase. No correlation was found between protein disulphide isomerase activity and the type of collagen synthesized. The data suggest that protein disulphide isomerase may be involved in the formation of intra-chain and inter-chain disulphide bonds in procollagens, but there is no collagen type-related variation in this enzyme activity of a magnitude that would explain the marked differences in the rates of formation of inter-chain disulphide bonds between the various collagen types.

Assembly of procollagen mRNA translation products into pepsin-resistant structures

Proteolytic digestion was used to probe the conformation of the prepro alpha chains synthesized by a mRNA-dependent reticulocyte lysate from chicken calvarial RNA. Pepsin-resistant alpha 1- and alpha 2-like chains were recovered even from translation reactions that were not preincubated below the reported Tm of the unhydroxylated triple helix. The pepsin-resistant structures were stable to thermal denaturation at 45 degrees C and a fraction remained resistant to peptic digestion at 30 degrees C. Interchain disulfide bonds did not appear to be required for the formation or thermal stability of these structures. Pepsin resistance is normally interpreted as evidence for a triple-helical conformation. Therefore, these results suggest that the in vitro synthesized prepro alpha chains contain the requisite information to associate in register for correct helix folding. The unusual thermal stability of these structures is not understood, but this may indicate assembly into higher orders of structure.

Kinetics of reduction of the intersubunit disulfides of the carboxyl propeptide of type I procollagen

The carboxyl propeptide produced by proteolysis during maturation of type I procollagen to collagen was purified to homogeneity from the medium of cultured chick embryo calvaria by a new method. The propeptide was identified as such by its amino acid composition and migration pattern through sodium dodecyl sulfate-polyacrylamide gels in the absence and presence of dithiothreitol. Reduction of the intersubunit disulfides, which covalently join the two C1 and one C2 polypeptides of the carboxyl propeptide, was studied by incubating the propeptide in the presence of dithiothreitol for various times under nondenaturing conditions at pH 8.2. The reduction process was characterized by the appearance of disulfide-linked dimers. The appearance of dimers correlated with the disappearance of the carboxyl propeptide. Monomers, retaining intrasubunit disulfides, appeared concomitant with dimer formation. Reduction of the intersubunit disulfides of the dimers followed; intrasubunit disulfides were retained. The rate of the first process, trimer to dimer plus monomer, was an order of magnitude larger than the rate for the second process, dimer to monomers. The dimeric intermediates were composed of approximately equivalent amounts of (C1)2 and (C1, C2). The kinetics of formation and reduction of (C1)2 and (C1, C2) could not be differentiated by the techniques used. The relative amounts of intermediates found were not those expected if quasi-equivalent intersubunit disulfides were reduced in a random fashion. A possible model for reduction of the intersubunit disulfides of the propeptide has been proposed, and implications for the intersubunit polypeptide surface contacts have been discussed.