Limited proteolysis of thermolysin by subtilisin: isolation and characterization of a partially active enzyme derivative

Purification and characterization of Rhizoctonia solani AG-4 strain ZB-34 α-amylase produced by solid-state fermentation using corn bran

Background:

Aim of the study was to produce α-amylase cheaply from Rhizoctonia solani AG-4 strain ZB-34 by solid-state fermentation and investigate its suitability for some industries.

Methods:

Rhizoctonia solani AG-4 strain ZB-34 α-amylase was purified with starch affinity method.

Results:

The best production of enzyme was achieved by using corn bran. Optimum pH and temperature were 5.5 and 50°C, respectively. Vmax and K0.5 were determined as 238.8 U/mg protein and 0.03% from substrate-activity curve. Activity was maintained about 100% in the tested pHs after 1 day incubation. The enzyme conserved full of its activity at 4 and 28°C for 120 min. Mn2+, Ca2+, Tween 20, Triton X-100 and Triton X-114 activated the enzyme. The enzyme was highly active in the presence of some commercial detergents. The chocolate stains on the cotton fabrics were more effectively cleaned with the combination of a commercial detergent and purified enzyme. The purified enzyme also clarified the raw apple juice and desized the cotton fabrics.

Conclusion:

The results showed that Rhizoctonia solani AG-4 strain ZB-34 α-amylase might have potential application as a detergent additive. In addition, its activity between pH 4.0 and 7.0 may facilitate its use in the food and fruit processing industries.

Identifying disordered regions in proteins by limited proteolysis

Limited proteolysis experiments can be successfully used to detect sites of disorder in otherwise folded globular proteins. The approach relies on the fact that the proteolysis of a polypeptide substrate requires its binding in an extended conformation at the protease's active site and thus an enhanced backbone flexibility or local unfolding of the site of proteolytic attack. A striking correlation was found between sites of limited proteolysis and sites of enhanced chain flexibility of the polypeptide chain, this last evaluated by the crystallographically determined B-factor. In numerous cases, it has been shown that limited proteolysis occurs at chain regions characterized by missing electron density and thus being disordered. Therefore, limited proteolysis is a simple and reliable experimental technique that can detect sites of disorder in proteins, thus complementing the results that can be obtained by the use of other physicochemical and computational approaches.

Exploiting topological constraints to reveal buried sequence motifs in the membrane-bound N-linked oligosaccharyl transferases

The central enzyme in N-linked glycosylation is the oligosaccharyl transferase (OTase), which catalyzes glycan transfer from a polyprenyldiphosphate-linked carrier to select asparagines within acceptor proteins. PglB from Campylobacter jejuni is a single-subunit OTase with homology to the Stt3 subunit of the complex multimeric yeast OTase. Sequence identity between PglB and Stt3 is low (17.9%); however, both have a similar predicted architecture and contain the conserved WWDxG motif. To investigate the relationship between PglB and other Stt3 proteins, sequence analysis was performed using 28 homologues from evolutionarily distant organisms. Since detection of small conserved motifs within large membrane-associated proteins is complicated by divergent sequences surrounding the motifs, we developed a program to parse sequences according to predicted topology and then analyze topologically related regions. This approach identified three conserved motifs that served as the basis for subsequent mutagenesis and functional studies. This work reveals that several inter-transmembrane loop regions of PglB/Stt3 contain strictly conserved motifs that are essential for PglB function. The recent publication of a 3.4 Å resolution structure of full-length C. lari OTase provides clear structural evidence that these loops play a fundamental role in catalysis [ Lizak , C. ; ( 2011 ) Nature 474 , 350 - 355 ]. The current study provides biochemical support for the role of the inter-transmembrane domain loops in OTase catalysis and demonstrates the utility of combining topology prediction and sequence analysis for exposing buried pockets of homology in large membrane proteins. The described approach allowed detection of the catalytic motifs prior to availability of structural data and reveals additional catalytically relevant residues that are not predicted by structural data alone.

Random gene dissection: a tool for the investigation of protein structural organization

To investigate the domain structure of proteins and the function of individual domains, proteins are usually subjected to limited proteolysis, followed by isolation of protein fragments and determination of their functions. We have developed an approach we call random gene dissection (RGD) for the identification of functional protein domains and their interdomain regions as well as their in vivo complementing fragments. The approach was tested on a two-domain protein, the type IIS restriction endonuclease BfiI. The collection of BfiI insertional mutants was screened for those that are endonucleolytically active and thus induce the SOS DNA repair response. Sixteen isolated mutants of the wild-type specificity contained insertions that were dispersed in a relatively large region of the target recognition domain. They split the gene into two complementing parts that separately were unable to induce the SOS DNA repair response. In contrast, all 19 mutants of relaxed specificity contained the cassette inserted into a very narrow interdomain region that connects BfiI domains responsible for DNA recognition and for cleavage. As expected, only the N-terminal fragment of BfiI was required to induce SOS response. Our results demonstrate that RGD can be used as a general method to identify complementing fragments and functional domains in enzymes.

Comparison of protein fragments identified by limited proteolysis and by computational cutting of proteins

Here we present a comparison between protein fragments produced by limited proteolysis and those identified by computational cutting based on the building block folding model. The principles upon which the two methods are based are different. Limited proteolysis of natively folded proteins occurs at flexible sites and never at the level of chain segments of regular secondary structure such as alpha-helices. Therefore, the targets for limited proteolysis are locally unfolded regions. In contrast, the computational cutting algorithm considers the compactness of the fragments, their nonpolar buried surface area, and their isolatedness, that is, the surface area which was buried prior to the cutting and becomes exposed subsequently. Despite the different criteria, there is an overall correspondence between sites or regions of limited proteolysis with those identified by computational cutting. The computational cutting method has been applied to several model proteins for which detailed limited proteolysis data are available, namely apomyoglobin, cytochrome c, ribonuclease A, alpha-lactalbumin, and thermolysin. As expected, more cuts are obtained computationally than experimentally and the agreement is better when a number of proteolytic enzymes are used. For example, cytochrome c is cleaved by thermolysin at 56-57, 45-46, and at 80-81, and by proteinase K at 48-49 and 50-51. Incubation of the noncovalent and native-like complex of cytochrome c fragments 1-56 and 57-104 with proteinase K yielded the gapped protein species 1-48/57-104 and finally 1-40/57-104. Computational cutting of cytochrome c reproduced the major experimental observations, with cuts at 47, 64-65 or 65-66 and 80-81 and an unstable 32-47 region not assigned to any building block. The next step, not addressed in this work, is to probe the ability of the generated fragments to fold independently. Since both the computational algorithm and limited proteolysis attempt to dissect the protein folding problem, the general agreement between the two procedures is gratifying. This consistency allows us to propose the use of limited proteolysis to produce protein fragments that can adopt an independent folding and, therefore, to study folding intermediates. The results of the present study appear to validate the building block folding model and are in line with the proposal that protein folding is a hierarchical process, where parts constituting local minima of energy fold first, with their subsequent association and mutual stabilization to finally yield the global fold.

Chemistry of the "molecular trap" of protease-catalyzed splicing reaction of complementary segments of alpha-subunit of hemoglobin A

The complementary fragments of human Hb alpha, alpha1-30, and alpha31-141 are spliced together by V8 protease in the presence of 30% n-propanol to generate the full-length molecule (Hb alpha-semisynthetic reaction). Unlike the other protease-catalyzed protein/peptide splicing reactions of fragment complementing systems, the enzymic condensation of nonassociating segments of Hb alpha is facilitated by the organic cosolvent induced alpha-helical conformation of product acting as the "molecular trap" of the splicing reaction. The segments alpha24-30 and alpha31-40 are the shortest complementary segments that can be spliced by V8 protease. In the present study, the chemistry of the contiguous segment (product) alpha24-40 has been manipulated by engineering the amino acid replacements to the positions alpha27 and alpha31 to delineate the structural basis of the molecular trap. The location of Glu27 and Arg31 residues in the contiguous segment alpha24-40 (as well as in other larger segments) is ideal to generate (i, i + 4) side-chain carboxylate-guanidino interaction in its alpha-helical conformation. The amino acid residue replacement studies have confirmed that the side chains at alpha27 and alpha31 facilitate the semisynthetic reaction. The relative influence of the substitute at these sites on the splicing reaction depends on the chemical nature of the side chain and the location. The gamma-carboxylate guanidino side-chain interaction appears to contribute up to a maximum of 85% of the thermodynamic stability of the molecular trap. The studies also demonstrate that the thermodynamic stability of the molecular trap is determined by two interdependent conformational aspects of the peptide. One is an amino acid-sequence-specific event that facilitates the induction of an alpha-helical conformation to the contiguous segment in the presence of organic cosolvent that imparts some amount of protease resistance to Glu30-Arg31 peptide bond. The second structural aspect is a site-specific event, an i, i + 4 side-chain interaction in the alpha-helical conformation of the peptide which imparts an additional thermodynamic stability to the molecular trap. The results suggest that conformationally driven "molecular traps" of protease-mediated ligation reactions of peptides could be designed into products to facilitate the modular assembly of peptides/proteins.

Limited proteolysis of ribonuclease A with thermolysin in trifluoroethanol

We have examined the proteolysis of bovine pancreatic ribonuclease A (RNase) by thermolysin when dissolved in aqueous buffer, pH 7.0, in the presence of 50% (v/v) trifluoroethanol (TFE). Under these solvent conditions, RNase acquires a conformational state characterized by an enhanced content of secondary structure (helix) and reduced tertiary structure, as given by CD measurements. It was found that the TFE-resistant thermolysin, despite its broad substrate specificity, selectively cleaves the 124-residue chain of RNase in its TFE state (20-42 degrees C, 6-24 h) at peptide bond Asn 34-Leu 35, followed by a slower cleavage at peptide bond Thr 45-Phe 46. In the absence of TFE, native RNase is resistant to proteolysis by thermolysin. Two nicked RNase species, resulting from cleavages at one or two peptide bonds and thus constituted by two (1-34 and 35-124) (RNase Th1) or three (1-34, 35-45 and 46-124) (RNase Th2) fragments linked covalently by the four disulfide bonds of the protein, were isolated to homogeneity by chromatography and characterized. CD measurements provided evidence that RNase Th1 maintains the overall conformational features of the native protein, but shows a reduced thermal stability with respect to that of the intact species (-delta Tm 16 degrees C); RNase Th2 instead is fully unfolded at room temperature. That the structure of RNase Th1 is closely similar to that of the intact protein was confirmed unambiguously by two-dimensional NMR measurements. Structural differences between the two protein species are located only at the level of the chain segment 30-41, i.e., at residues nearby the cleaved Asn 34-Leu 35 peptide bond. RNase Th1 retained about 20% of the catalytic activity of the native enzyme, whereas RNase Th2 was inactive. The 31-39 segment of the polypeptide chain in native RNase forms an exposed and highly flexible loop, whereas the 41-48 region forms a beta-strand secondary structure containing active site residues. Thus, the conformational, stability, and functional properties of nicked RNase Th1 and Th2 are in line with the concept that proteins appear to tolerate extensive structural variations only at their flexible or loose parts exposed to solvent. We discuss the conformational features of RNase in its TFE-state that likely dictate the selective proteolysis phenomenon by thermolysin.

Probing the partly folded states of proteins by limited proteolysis

The folding of a polypeptide chain of a relatively large globular protein into its unique three-dimensional and functionally active structure occurs via folding intermediates. These partly folded states of proteins are difficult to characterize, because they are usually short lived or exist as a distribution of possible conformers. A variety of experimental techniques and approaches have been utilized in recent years in numerous laboratories for characterizing folding intermediates that occur at equilibrium, including spectroscopic techniques, solution X-ray scattering, calorimetry and gel filtration chromatography, as well as genetic methods and theoretical calculations. In this review, we focus on the use of proteolytic enzymes as probes of the structure and dynamics of folding intermediates and we show that this simple biochemical technique can provide useful information, complementing that obtained by other commonly used techniques and approaches. The key result of the proteolysis experiments is that partly folded states (molten globules) of proteins can be sufficiently rigid to prevent extensive proteolysis and appear to maintain significant native-like structure.

Structural features of neutral protease from Bacillus subtilis deduced from model-building and limited proteolysis experiments

The overall folding of neutral protease from Bacillus subtilis has been predicted by computer-aided modelling, taking as a basis the known three-dimensional structure of thermolysin. As expected from the 50% similarity of sequence between the two proteins, the structure of B. subtilis protease is similar to that of thermolysin, including the two-domain topology and location of elements of regular secondary structure (helices and strands), whereas specific differences were predicted in loop regions. A protruding and loose loop predicted in B. subtilis has been detected also experimentally by a limited proteolysis approach. Incubation of B. subtilis protease at pH 9.0 for 24 h at room temperature with trypsin at 20:1 ratio (by mass) leads to a specific and almost quantitative fission of the Arg214-Asn215 peptide bond located in a highly exposed, and thus probably flexible, loop of the protease. On the other hand, thermolysin was completely resistant to tryptic hydrolysis when reacted under identical conditions. The 'nicked' B. subtilis protease can be isolated by gel filtration chromatography at neutral pH, whereas the two constituting fragments 1-214 and 215-300 are separated under protein-denaturing conditions. Overall, these results indicate that the limited proteolysis approach can pinpoint a peculiar difference in surface structure between the two similar protein molecules of B. subtilis neutral protease and thermolysin and emphasize the potential use of proteolytic enzymes as structural probes of globular proteins.

N-terminal domain of pepsin as a model for retroviral dimeric aspartyl protease

Autolysis of porcine pepsin at pH 4 affords a derivative possessing intrinsic proteolytic activity. This derivative was isolated by alumina pseudo-affinity chromatography and gel-filtration and was found to result from the tight association of two identical molecules, 135 amino acids long, emerging from the N-terminal domain of pepsin. This finding emphasizes a similarity with the only aspartyl-proteases known to act as dimers, the retroviral proteases.

Folding of thermolysin fragments. Hydrodynamic properties of isolated domains and subdomains

Sedimentation analysis in the analytical ultracentrifuge has been used to characterize the size and shape of thermolysin and a number of its fragments obtained by chemical or enzymatic cleavage of the protein. Four fragments (121-316, 206-316, 225/226-316 and 255-316) originate from the C-terminal domain, and two (1-155 and 1-205) from the N-terminal domain of the intact molecule. In aqueous solution at neutral pH the hydrodynamic properties of the C-terminal fragments, except 255-316, are consistent with compact homogeneous monomers. Fragment 255-316 is a monomeric species below 0.08 mg/ml concentration and forms a dimer above this concentration. Dimerization does not lead to changes in fragment conformation, as determined by far-ultraviolet circular dichroic measurements, but to an increase of 5.6 degrees C (to 68.2 degrees C at 1.0 mg/ml) in the temperature for thermal unfolding and a corresponding increase of 4.6 kJ/mol in the free energy of unfolding. Fragments derived from the N-terminal domain show a strong tendency to form high-molecular-mass aggregates. Previous experiments utilizing circular dichroic measurements and antibody binding data suggested that the C-terminal fragments listed above are able to refold in aqueous solution at neutral pH into a stable conformation of native-like characteristics [Dalzoppo, D., Vita, C. & Fontana, A. (1985) J. Mol. Biol. 182, 331-340] (and references cited therein). Present data establish that all these C-terminal fragments are globular monomeric species in solution (at concentrations approximately 0.1 mg/ml) and thus represent 'isolated' domains (or subdomains) with intrinsic conformational stability typical of small globular proteins.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides: ligand-induced conformational changes

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is inactivated by trypsin, chymotrypsin, pronase E, thermolysin, 4.0 M urea, and by heating to 49 degrees C. It is protected, to varying degrees, against all these forms of inactivation by glucose 6-phosphate, NAD+, and NADP+. When these ligands are present at 10 times their respective KD concentrations, protection by NAD+ or glucose 6-phosphate is substantially greater than protection by NADP+. A detailed analysis was undertaken of the protective effects of these ligands, at varying concentrations, on proteolysis of glucose-6-phosphate dehydrogenase by thermolysin. This study confirmed the above conclusion and permitted calculation of KD values for NAD+, NADP+, and glucose 6-phosphate that agree with such values determined by independent means. For NADP+, two KD values, 6.1 microM and 8.0 mM, can be derived, associated with protection against thermolysin by low and high NADP+ concentrations, respectively. The former value is in agreement with other determinations of KD and the latter value appears to represent binding of NADP+ to a second site which causes inhibition of catalysis. A Ki value of 10.5 mM for NADP+ was derived from inhibition studies. The principal conclusion from these studies is that NAD+ binding to L. mesenteroides glucose-6-phosphate dehydrogenase results in a larger global conformational change of the enzyme than does NADP+ binding. Presumably, a substantially larger proportion of the free energy of binding of NAD+, compared to NADP+, is used to alter the enzyme's conformation, as reflected in a much higher KD value. This may play an important role in enabling this dual nucleotide-specific dehydrogenase to accommodate either NAD+ or NADP+ at the same binding site.

Structure and stability of thermophilic enzymes. Studies on thermolysin

The molecular mechanisms responsible for the unusual stability of enzymes isolated from thermophilic microorganisms are much more complex and subtle than was originally thought. In particular, a general mechanism cannot be proposed, since individual enzymes can be stabilized by specific molecular interactions and forces. The results of studies on thermophilic enzymes obtained in recent years in our laboratory will be summarized, with particular emphasis being placed on those obtained with thermolysin, a stable metalloendopeptidase isolated from Bacillus thermoproteolyticus. Fragmentation of thermolysin by limited proteolysis by added protease (subtilisin) or autolysis mediated by heat or the ion-chelating agent EDTA leads to quite selective peptide bond fissions, allowing isolation of 'nicked' thermolysin species. Correlation of the sites of proteolytic cleavage with the known three-dimensional structure of thermolysin allowed us to infer some of the key characteristics of the structure, folding, dynamics and stability of the thermolysin molecule. The potential utility of these and other studies on thermophilic enzymes in devising strategies for enhancing the stability of mesophilic enzymes using genetic engineering techniques is discussed.

The specific activities of mesophilic and thermophilic proteinases

1. Most enzymes from extreme thermophiles do not possess higher specific activities than similar enzymes from mesophiles (measured at their respective growth temperatures). 2. However, using protein substrates, the specific activities of thermophilic proteinases are considerably higher than those of most microbial and eukaryotic proteinases. 3. This property could be attributed to purely kinetic influences on the enzyme, to some specific "design" feature of the proteinase, or to the effects of temperature on the substrate. 4. Comparisons of the rates of hydrolysis of large and small substrates by both mesophilic and thermophilic proteinases suggest that temperature-induced changes in substrate susceptibility are a major factor.

Independent folding of autolytic fragments of thermolysin and their domain-like properties

High molecular weight autolysis products from thermolysin have been isolated and identified. The primary fragments correspond to residues 1 to 187-204 (21kD) and residues 187-204 to 316 (12kD), respectively. The fragments are both capable of independent refolding upon removal of denaturant. On the basis of these results, we suggest that the first step in the unfolding pathway of thermolysin involves unfolding of an interdomain region and domain separation. Bound calcium ions at sites 1, 2 and 4 play a major role in protecting the protein against both autolysis and unfolding, probably by stabilizing the interdomain region and enhancing domain-domain interactions.

Permissible discontinuity region of the alpha-chain of hemoglobin: noncovalent interaction of heme and the complementary fragments alpha 1-30 and alpha 31-141

Generation of a fragment-complementing system of the alpha-chain on limited proteolysis with Staphylococcus aureus V8 protease has been investigated. Digestion of the alpha-chain (0.4 mM) of hemoglobin with V8 protease in phosphate buffer at pH 6.0 and 37 degrees C is limited to the peptide bonds of Glu-23, Glu-27, Glu-30, and Asp-47. Gel filtration of a V8 protease digest of the alpha-chain on a Sephadex G-50 column did not release any heme to the low molecular weight region, though some peptides were released from the protein. The filtration studies revealed the presence of two heme-containing components in the digest, the major one eluting at the alpha-chain position and the minor one eluting slightly ahead of the alpha-chain position. Reversed-phase high-performance liquid chromatography and amino-terminal sequence analysis demonstrated that the component eluting at the alpha-chain position contains species generated by the noncovalent interactions of heme and the complementary fragments alpha 1-30 and alpha 31-141. In dilute solutions (0.04 mM) the V8 protease digestion occurred exclusively on the carboxyl side of Glu-30(alpha). This high selectivity was also observed at pH 4.0 and pH 7.8. The visible spectra and the ultraviolet circular dichroic spectra of the digest reflect the native-like structure of the noncovalent fragment system. The dissociation constant of alpha 1-30 appears to be in the range of 10(-8) M. In tetrameric hemoglobin A the peptide bond of Glu-30-Arg-31 of the alpha-chain is not accessible to V8 protease digestion.(ABSTRACT TRUNCATED AT 250 WORDS)

Catabolism of intracellular protein: molecular aspects

All living cells regulate the content and composition of their resident proteins, but the mechanisms by which this is accomplished are not understood. The process of protein degradation has an important role in determining steady state and fluctuations of protein concentrations in mammalian cells. This process may be regulated by innate properties of the protein substrates, by factors that interact or "brand" proteins for degradation or by the degradative machinery of the cell. For a specific protein, there appears to be a committed step, an irreversible event that leads to rapid and extensive degradation. That initial event may or may not involve 1) proteolysis, 2) a nonproteolytic covalent modification or branding event (e.g., oxidation, ubiquitin conjugation), 3) denaturation or unfolding of the protein, or 4) sequestration. The degradative machinery of cells may either recognize proteins committed to degradation or initiate degradation, but the process must be selective because there is great heterogeneity in the rates of degradation for different proteins of one cell. The degradative process certainly requires proteases, and it is probable that lysosomal and extralysosomal proteases are involved in the catabolism of cellular proteins. We review here briefly what is currently known about the factors that may determine the half-life of a protein in a mammalian cell, the role of the protein substrate and sequestration in the process, the proteolytic and nonproteolytic enzymes that may initiate the degradative process, and the regulation of extensive degradation of proteins in cells.

Autolysis of thermolysin. Isolation and characterization of a folded three-fragment complex

Incubation of the neutral metalloendopeptidase thermolysin at pH 7.2 in the presence of EDTA and/or low concentrations of calcium ions produces fast enzyme inactivation as a result of autolysis. The 'nicked' protein is a folded species composed of three tightly associated protein fragments. Dissociation of this complex can be achieved under denaturing conditions, such as gel filtration on a column equilibrated with 5 M guanidine hydrochloride or reverse-phase high-performance liquid chromatography (HPLC) at acidic pH. The positions of the peptide bond cleavages were defined by isolation of the individual fragments by HPLC and their characterization by amino acid analysis after acid hydrolysis, end-group determination and partial amino acid sequencing. The results of these analyses indicated that the nicked protein is composed of fragments 1-196, 197-204 and 205-316 and thus that the corresponding sites of limited proteolysis occur at the polypeptide chain loop involved in the binding of Ca(4) in native thermolysin [Matthews, B. W., Weaver, L. H. and Kester, W. R. (1974) J. Biol. Chem. 249, 8030-8044]. The overall conformational properties of nicked thermolysin are quite similar to those of the intact protein, as judged by spectroscopic measurements and by the fact that rabbit antibodies against native thermolysin recognize and precipitate the nicked protein in immunodiffusion assays. The nicked protein was much less stable to heat and unfolding agents than intact thermolysin. These results contribute to a better knowledge of the molecular mechanism of stabilization of native thermolysin by the four bound calcium ions and demonstrate that the function of Ca(4) is to stabilize the loop 190-205 on the surface of the molecule against autolysis.

The importance of disulfide bridges in human endopeptidase (enkephalinase) after proteolytic cleavage

The neutral endopeptidase (NEP) is a membrane-bound enzyme, which is solubilized by treatment with the protease, papain. Papain did not affect the apparent catalytic activity or the molecular mass of the purified human enzyme in SDS-PAGE. When NEP was treated with a reducing agent after papain digestion, it dissociated into smaller, lower molecular mass fragments. Amino acid analysis and s-carboxymethylation of the half cystine residues indicated that NEP contains four S-S bridges. We concluded that, although covalent bonds appear to be cleaved in NEP by papain, its activity and structure are sustained by S-S bridges.

Folding of thermolysin fragments. Correlation between conformational stability and antigenicity of carboxyl-terminal fragments

Circular dichroism (CD) and immunochemical measurements have been used to examine conformational properties of COOH-terminal fragments 121-316, 206-316 and 225(226)-316 of thermolysin, and to compare these properties to those of native thermolysin and thermolysin S, the stable partially active two-fragment complex composed of fragments 5-224(225) and 225(226)-316. In aqueous solution at neutral pH, all the COOH-terminal fragments attain a native-like conformation, as judged both by the content of secondary structure deduced from far-ultraviolet CD spectra and by the recognition of rabbit polyclonal antibodies specific for the COOH-terminal region in native thermolysin. The three fragments showed reversible cooperative unfolding transitions mediated by both heat and guanidine hydrochloride (Gdn X HCl). The phase transition curves were analyzed for Tm (temperature of half-denaturation) and Gibbs free energies (delta GD) of unfolding from native to denatured state. The observed order of thermal stability is 225(226)-316 less than or equal to 206-316 less than 121-316 less than thermolysin S less than thermolysin. The ranking of delta GD values for the three fragments correlates with the size of each fragment. Competitive binding studies by radioimmunoassay using 14C-labeled thermolysin and affinity purified antibodies specific for native antigenic determinants in segment 206-316 of native thermolysin indicate that the COOH-terminal fragments adopt native-like conformations which are in equilibrium with non-native conformations. These equilibria are shifted towards the native state as the fragment size increases from 225(226)-316, to 206-316, to 121-316. Fragment 225(226)-316, when combined with fragment 5-224(225) in the thermolysin S complex, adopts a more stable native-like conformation and becomes much more antigenic. It has been shown that the degree of antigenicity of COOH-terminal fragments towards thermolysin antibodies correlates directly with their conformational stability. The results of this study are discussed in relation to the recently proposed correlation between antigenicity and segmental mobility of globular proteins.