In support of the trap hypothesis. Chymotrypsin is not rigidly held in its complex with human alpha 2-macroglobulin

The structure and function of thioester-containing proteins in arthropods

Thioester-containing proteins (TEPs) form an ancient and diverse family of secreted proteins that play central roles in the innate immune response. Two families of TEPs, complement factors and α₂-macroglobulins, have been known and studied in vertebrates for many years, but only in the last decade have crystal structures become available. In the same period, the presence of two additional classes of TEPs has been revealed in arthropods. In this review, we discuss the common structural features TEPs and how this knowledge can be applied to the many arthropod TEPs of unknown function. TEPs perform a wide variety of functions that are driven by different quaternary structures and protein-protein interactions between a common set of folded domains. A common theme is regulated conformational change triggered by proteolysis. Structure-function analysis of the diverse arthropod TEPs may identify not just new mechanisms in innate immunity but also interfaces between immunity, development and cell death.

Natural and synthetic inhibitors of kallikrein-related peptidases (KLKs)

Including the true tissue kallikrein KLK1, kallikrein-related peptidases (KLKs) represent a family of fifteen mammalian serine proteases. While the physiological roles of several KLKs have been at least partially elucidated, their activation and regulation remain largely unclear. This obscurity may be related to the fact that a given KLK fulfills many different tasks in diverse fetal and adult tissues, and consequently, the timescale of some of their physiological actions varies significantly. To date, a variety of endogenous inhibitors that target distinct KLKs have been identified. Among them are the attenuating Zn(2+) ions, active site-directed proteinaceous inhibitors, such as serpins and the Kazal-type inhibitors, or the huge, unspecific compartment forming α(2)-macroglobulin. Failure of these inhibitory systems can lead to certain pathophysiological conditions. One of the most prominent examples is the Netherton syndrome, which is caused by dysfunctional domains of the Kazal-type inhibitor LEKTI-1 which fail to appropriately regulate KLKs in the skin. Small synthetic inhibitory compounds and natural polypeptidic exogenous inhibitors have been widely employed to characterize the activity and substrate specificity of KLKs and to further investigate their structures and biophysical properties. Overall, this knowledge leads not only to a better understanding of the physiological tasks of KLKs, but is also a strong fundament for the synthesis of small compound drugs and engineered biomolecules for pharmaceutical approaches. In several types of cancer, KLKs have been found to be overexpressed, which makes them clinically relevant biomarkers for prognosis and monitoring. Thus, down regulation of excessive KLK activity in cancer and in skin diseases by small inhibitor compounds may represent attractive therapeutical approaches.

The three-dimensional structure of the human alpha 2-macroglobulin dimer reveals its structural organization in the tetrameric native and chymotrypsin alpha 2-macroglobulin complexes

Three-dimensional electron microscopy reconstructions of the human alpha(2)-macroglobulin (alpha(2)M) dimer and chymotrypsin-transformed alpha(2)M reveal the structural arrangement of the two dimers that comprise native and proteinase-transformed molecules. They consist of two side-by-side extended strands that have a clockwise and counterclockwise twist about their major axes in the native and transformed structures, respectively. This and other studies show that there are major contacts between the two strands at both ends of the molecule that evidently sequester the receptor binding domains. Upon proteinase cleavage of the bait domains and subsequent thiol ester cleavages, which occur near the central region of the molecule, the two strands separate by 40 A at both ends of the structure to expose the receptor binding domains and form the arm-like extensions of the transformed alpha(2)M. During the transformation of the structure, the strands untwist to expose the alpha(2)M central cavity to the proteinase. This extraordinary change in the architecture of alpha(2)M functions to completely engulf two molecules of chymotrypsin within its central cavity and to irreversibly encapsulate them.

Methanethiolation of the liberated cysteine residues of human alpha 2-macroglobulin treated with methylamine generates a derivative with similar functional characteristics as native alpha 2-macroglobulin

The thiol-modifying reagent methyl methanethiosulfonate reacts with the cysteine residues of thiol esters released upon treatment of human alpha 2-macroglobulin with methylamine. This methanethiolation generates a derivative of alpha 2-macroglobulin, with an 'open trap' and slow mobility in non-denaturing PAGE, similar to native alpha 2-macroglobulin. This similarity is further substantiated by surface hydrophobicity determinations and by the fact that neither the derivative nor native alpha 2-macroglobulin are cleared from the circulation in mice. Cleavages of bait regions in the derivative and native alpha 2-macroglobulin, however, result in electrophoretically fast forms which are cleared from the circulation in mice. In contrast to native alpha 2-macroglobulin, which can bind 2 mol chymotrypsin/mol, alpha 2-macroglobulin treated with methylamine and methylmethanethiosulfonate binds only 0.8 mol chymotrypsin/mol. Protection of trypsin against inhibition by soybean trypsin inhibitor is significantly better when alpha 2-macroglobulin is modified by methylamine and methylmethanethiosulfonate than when it is modified by dinitrophenyl thiocyanate, which cyanylates the exposed thiol group. The methanethiolated derivative is also more stable than the corresponding cyanylated derivative in that it is transformed to an electrophoretically fast form with a half-life of 9 h as compared to a half-life of 7 h for the latter. The transformation to the fast form is not due to instability of the thiol modification.

Human alpha 2-macroglobulin structure. Location of Cys-949 residues within a half-molecule measured by fluorescence energy transfer

To localize the pair of thiol-ester-derived cysteine-949 residues within a half-molecule of alpha 2M and estimate the internal diameter of this alpha 2-macroglobulin trap, we have measured the separation between these cysteines. We unexpectedly found that the two cysteines of intermediate form alpha 2M had different reactivity, which permitted selective modification of one cysteine with dansyl, and the other with fluorescein fluorophores. From fluorescence energy transfer measurements, we calculated a separation of 41 +/- 10 A between these fluorophores. This indicates that the Cys-949 residues are probably located at the perimeter of the trap with an internal diameter at least as large as this separation.

Antibody binding to alpha 2-macroglobulin facilitates direct quantitation of elastase-alpha 2-macroglobulin complexes by a simple ELISA technique

In order to investigate the effect which the binding of anti-alpha 2-macroglobulin (anti-A2M) antibody has on subsequent antigen-antibody reaction between the complexed elastase and the solid phase anti-elastase antibody, elastase alpha 2-macroglobulin complex (EMC) was incubated with anti-A2M antibody and then extracted by a solid-phase bound rabbit anti-elastase antibody. A doubling dilution series of EMC generated dose related absorbance values. The critical factor permitting the immunological detection of A2M-bound elastase is the pre-incubation of anti-A2M antibody with EMC in solution. The assay exhibited a lower detection limit of 0.5 ng bound elastase per ml and EMC levels in serum samples from ten volunteers were significantly higher than in plasma (28 vs. 21 ng/ml, p < 0.05, Student's t test for paired samples). The EMC levels measured with this assay were essentially identical to those obtained when phenyl methyl sulfonyl fluoride (PMSF) was added to the assay buffer solution.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.

Separation and localization of the four cysteine-949 residues in human alpha 2-macroglobulin using fluorescence energy transfer

By use of the intermediate form (I-form) [Gettins, Crews, & Cunningham (1989) Biochemistry 28, 5613-5618], alpha 2-macroglobulin can be specifically labeled with fluorescent probes in a manner that allows the determination of the topology of the four thiol ester derived Cys949 residues within this large tetrameric protease inhibitor. Freshly prepared I-form alpha 2-macroglobulin was reacted with 5-[[2-[(iodoacetyl)-amino]ethyl]amino]naphthalene-1-sulfonate (1,5-I-AEDANS) to produce alpha 2-macroglobulin specifically and stoichiometrically labeled with 1,5-AEDANS (donor) at the two Cys949 SH groups in the first protease interaction site. Upon subsequent reaction of this labeled species with chymotrypsin, the remaining two bait regions and thiol ester linkages were opened, generating two free SH groups on the two Cys949 residues in the second protease interaction site. These SH groups were specifically and stiochiometrically labeled with 5-(iodoacetamido)fluorescein (acceptor). Fluorescence energy transfer from donor to acceptor results in 82% loss of AEDANS fluorescence intensity. By use of an R0(2/3) value of 43.5 A, calculated from the spectral parameters of this system, an R(2/3) separation between donor and acceptor of 33.9 A was calculated. From fluorescence anisotropy measurements of both donor and acceptor attached to alpha 2-macroglobulin, upper and lower limits on the separation of 43.4 and 26.1 A, respectively, were calculated. These separations, small in the context of the alpha 2-macroglobulin tetramer, which has approximate dimensions of 190 x 90 x 90 A, severely restrict the possible locations of the four Cys949 residues.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular distribution of elastase between its two main inhibitors: direct quantitation of elastase-alpha 2-macroglobulin complex with a novel ELISA technique

A novel enzyme-linked immunosorbent assay for the quantitation of elastase linked to alpha 2-macroglobulin (alpha 2M) (elastase-alpha 2M complex, EMC) in body fluids is presented. The assay has a lower detection limit of 1.5 ng bound elastase per ml. The critical factor allowing immunological detection of alpha 2M-bound elastase is the addition of phenyl methyl sulphonyl fluoride (PMSF) to the assay buffer. The assay has been used to quantitate EMC in plasma and serum of healthy volunteers, and assess the distribution of elastase between its two main inhibitors, alpha 2M and alpha 1-proteinase inhibitor (API). EMC levels in serum samples from volunteers were significantly higher than in plasma (26.9 vs 21.9 ng/ml, p = 0.05, Student's t-test for paired samples). The API-bound elastase levels were 70.2 and 170.1 ng/ml in plasma and serum respectively (statistically significantly higher in serum, p less than 0.0001). The ratio of macroglobulin to alpha 1-proteinase-bound elastase levels was 0.333 in plasma and 0.168 in serum (p less than 0.0001). As alpha 2M bound elastase retains some proteolytic activity, direct measurement of circulating levels in inflammatory conditions should be of interest.

Effect of methylamine on the reaction of alpha 2-macroglobulin with enzymes

The kinetics of reaction of alpha 2-macroglobulin (alpha 2M) with thrombin and with trypsin were studied in the presence and absence of methylamine. The rate of enzyme-induced thiol release was found to be the same whether or not amine was present. The result suggests that covalent bond formation and enzyme-catalyzed amine incorporation proceed via a common (enzyme-dependent) rate-determining step. The reaction of lysyl-modified enzymes (which show poor covalent binding with alpha 2M) was similarly unaffected by amine, indicating that enzyme-catalyzed steps were also rate determining for hydrolysis of the thiol ester. The products of the reactions were analyzed by native and denaturing gel electrophoresis. Methylamine did not affect the total binding of enzyme to alpha 2M but did cause a substantial decrease in covalent binding. Surprisingly, not all covalent complexes were affected by the presence of amine: complexes in which enzyme was covalently bound to one half-molecule increased compared to the reaction with no amine; complexes in which two half-molecules are cross-linked by two bonds to a single enzyme were substantially reduced, however. The results are consistent with a mechanism of reaction in which an enzyme-dependent step is rate determining. This step is accompanied by activation of two thiol esters. One of these reacts immediately with the bound enzyme (or may be hydrolyzed if the enzyme amine groups are blocked). The other activated center is capable of reaction with external nucleophiles such as methylamine.

Conformational change at the active center of serine-proteases upon their binding to alpha 2-macroglobulin

The inhibition rates and spectral characteristics of 2 probes specific for the active-site serine residue of proteases were examined for evidence of conformational change of the proteases upon their binding to alpha 2-macroglobulin (alpha 2M). Elastase, chymotrypsin, trypsin, and plasmin were reacted with (7-nitrobenz-2-oxa-1,3-diazole) aminoethyl- and aminopentyl methylphosphonofluoridate. The inhibition rate constants depend on the chain length of the aminoalkyl moiety of the probe and range from 10(5) to 10(4) M-1 min-1 for elastase and chymotrypsin. They are significantly modified when the proteases are stoichiometrically bound to alpha 2M. The absorption maximum of the chromophore appears in the range of 460-470 nm and 475-480 nm for the aminoethyl- and aminopentyl- conjugates, respectively. The fluorescence emission is maximal around 530 nm with a low quantum yield of about 3%. These spectral characteristics are altered in different ways by the covalent or non-covalent binding mode of the protease to alpha 2M. Finally, the CD spectrum of the NBD aminoethyl and aminopentyl elastase and chymotrypsin conjugates exhibits intense optical activity in the absorbing band of the NBD-moiety. These chiral properties are greatly altered upon binding of the protease to alpha 2M. All these results strongly suggest a conformational change in the protease at its active center upon its binding to alpha 2M; this conformational change could be taken into account to explain the alteration of the catalytic properties of the alpha 2M-bound proteases.