Isolation and characterization of a protease inhibitor from Acacia karroo with a common combining loop and overlapping binding sites for chymotrypsin and trypsin.

By using affinity and reversed-phase HPLC (RP-HPLC) chromatographies two chymotrypsin-trypsin inhibitors were isolated from seeds of Acacia karroo, a legume of the subfamily Mimosoideae. The primary structure of one of these inhibitors, named AkCI/1, was determined. The inhibitor consists of two polypeptide chains, 139 and 44 residues respectively, which are linked by a single disulfide bridge. The amino acid sequence of AkCI/1 is homologous to and showed more than 60% sequence similarity with other protease inhibitors isolated earlier from the group of Mimosoideae. AkCI/1 inhibits both chymotrypsin (EC 3.4.21.1) and trypsin (EC 3.4.21.4) in a 1:1M ratio with Ki values of 2.8 × 10(-12)M and 1.87 × 10(-12)M, respectively. The P1-P1' residues for trypsin were identified as Arg68-Ile69 by selective hydrolysis of the inhibitor at this site, with bovine trypsin and human trypsin IV. The cleavage did not affect the inhibition of trypsin, but fully abolished the chymotrypsin inhibitory activity of AkCI/1. This finding together with our studies on competition of the two enzymes for the same combining loop suggests that the same loop has to contain the binding sites for both proteases. The most likely P1 residue of AkCI/1 for chymotrypsin is Tyr67.

The catalytic aspartate is protonated in the Michaelis complex formed between trypsin and an in vitro evolved substrate-like inhibitor: a refined mechanism of serine protease action.

The mechanism of serine proteases prominently illustrates how charged amino acid residues and proton transfer events facilitate enzyme catalysis. Here we present an ultrahigh resolution (0.93 Å) x-ray structure of a complex formed between trypsin and a canonical inhibitor acting through a substrate-like mechanism. The electron density indicates the protonation state of all catalytic residues where the catalytic histidine is, as expected, in its neutral state prior to the acylation step by the catalytic serine. The carboxyl group of the catalytic aspartate displays an asymmetric electron density so that the O(δ2)-C(γ) bond appears to be a double bond, with O(δ2) involved in a hydrogen bond to His-57 and Ser-214. Only when Asp-102 is protonated on O(δ1) atom could a density functional theory simulation reproduce the observed electron density. The presence of a putative hydrogen atom is also confirmed by a residual mF(obs) - DF(calc) density above 2.5 σ next to O(δ1). As a possible functional role for the neutral aspartate in the active site, we propose that in the substrate-bound form, the neutral aspartate residue helps to keep the pK(a) of the histidine sufficiently low, in the active neutral form. When the histidine receives a proton during the catalytic cycle, the aspartate becomes simultaneously negatively charged, providing additional stabilization for the protonated histidine and indirectly to the tetrahedral intermediate. This novel proposal unifies the seemingly conflicting experimental observations, which were previously seen as either supporting the charge relay mechanism or the neutral pK(a) histidine theory.

Probing conformational plasticity of the activation domain of trypsin: the role of glycine hinges.

Trypsin-like serine proteases play essential roles in diverse physiological processes such as hemostasis, apoptosis, signal transduction, reproduction, immune response, matrix remodeling, development, and differentiation. All of these proteases share an intriguing activation mechanism that involves the transition of an unfolded domain (activation domain) of the zymogen to a folded one in the active enzyme. During this conformational change, activation domain segments move around highly conserved glycine hinges. In the present study, hinge glycines were replaced by alanine residues via site directed mutagenesis. The effects of these mutations on the interconversion of the zymogen-like and active conformations as well as on catalytic activity were studied. Mutant trypsins showed zymogen-like structures to varying extents characterized by increased flexibility of some activation domain segments, a more accessible N-terminus and a deformed substrate binding site. Our results suggest that the trypsinogen to trypsin transition is hindered by the mutations, which results in a shift of the equilibrium between the inactive zymogen-like and active enzyme conformations toward the inactive state. Our data also showed, however, that the inactive conformations of the various mutants differ from each other. Binding of substrate analogues shifted the conformational equilibrium toward the active enzyme since inhibited forms of the trypsin mutants showed similar structural features as the wild-type enzyme. The catalytic activity of the mutants correlated with the proper conformation of the active site, which could be supported by varying conformations of the N-terminus and the autolysis loop. Transient kinetic measurements confirmed the existence of an inactive to active conformational transition occurring prior to substrate binding.

When the surface tells what lies beneath: combinatorial phage-display mutagenesis reveals complex networks of surface-core interactions in the pacifastin protease inhibitor family.

Pacifastin protease inhibitors are small cysteine-rich motifs of approximately 35 residues that were discovered in arthropods. The family is divided into two related groups on the basis of the composition of their minimalist inner core. In group I, the core is governed by a Lys10-Trp26 interaction, while in group II it is organized around Phe10. Group I inhibitors exhibit intriguing taxon specificity: potent arthropod-trypsin inhibitors from this group are almost inactive against vertebrate enzymes. The group I member SGPI-1 and the group II member SGPI-2 are extensively studied inhibitors. SGPI-1 is taxon-selective, while SGPI-2 is not. Individual mutations failed to explain the causes underlying this difference. We deciphered this phenomenon using comprehensive combinatorial mutagenesis and phage display. We produced a complete chimeric SGPI-1 / SGPI-2 inhibitor-phage library, in which the two sequences were shuffled at the highest possible resolution of individual residues. The library was selected for binding to bovine trypsin and crayfish trypsin. Sequence analysis of the selectants revealed that taxon specificity is due to an intra-molecular functional coupling between a surface loop and the Lys10-Trp26 core. Five SGPI-2 surface residues transplanted into SGPI-1 resulted in a variant that retained the "taxon-specific" core, but potently inhibited both vertebrate and arthropod enzymes. An additional rational point mutation resulted in a picomolar inhibitor of both trypsins. Our results challenge the generally accepted view that surface residues are the exclusive source of selectivity for canonical inhibitors. Moreover, we provide important insights into general principles underlying the structure-function properties of small disulfide-rich polypeptides, molecules that exist at the borderline between peptides and proteins.

Unconventional translation initiation of human trypsinogen 4 at a CUG codon with an N-terminal leucine. A possible means to regulate gene expression.

Chromosomal rearrangements apparently account for the presence of a primate-specific gene (protease serine 3) in chromosome 9. This gene encodes, as the result of alternative splicing, both mesotrypsinogen and trypsinogen 4. Whereas mesotrypsinogen is known to be a pancreatic protease, neither the chemical nature nor biological function of trypsinogen 4 has been explored previously. The trypsinogen 4 sequence contains two predicted translation initiation sites: an AUG site that codes for a 72-residue leader peptide on Isoform A, and a CUG site that codes for a 28-residue leader peptide on Isoform B. We report studies that provide evidence for the N-terminal amino acid sequence of trypsinogen 4 and the possible mechanism of expression of this protein in human brain and transiently transfected cells. We raised mAbs against a 28-amino acid synthetic peptide representing the leader sequence of Isoform B and against recombinant trypsin 4. By using these antibodies, we isolated and chemically identified trypsinogen 4 from extracts of both post mortem human brain and transiently transfected HeLa cells. Our results show that Isoform B, with a leucine N terminus, is the predominant (if not exclusive) form of the enzyme in post mortem human brain, but that both isoforms are expressed in transiently transfected cells. On the basis of our studies on the expression of a series of trypsinogen 4 constructs in two different cell lines, we propose that unconventional translation initiation at a CUG with a leucine, rather than a methionine, N terminus may serve as a means to regulate protein expression.

Local binding with globally distributed changes in a small protease inhibitor upon enzyme binding.

Complexation of the small serine protease inhibitor Schistocerca gregaria chymotrypsin inhibitor (SGCI), a member of the pacifastin inhibitor family, with bovine chymotrypsin was followed by NMR spectroscopy. (1)H-(15)N correlation (HSQC) spectra of the inhibitor with increasing amounts of the enzyme reveal tight and specific binding in agreement with biochemical data. Unexpectedly, and unparalleled among canonical serine protease inhibitors, not only residues in the protease-binding loop of the inhibitor, but also some segments of it located spatially far from the substrate-binding cleft of the enzyme were affected by complexation. However, besides changes, some of the dynamical features of the free inhibitor are retained in the complex. Comparison of the free and complexed inhibitor structures revealed that most, but not all, of the observed chemical shift changes can be attributed to minor structural transitions. We suggest that the classical 'scaffold + binding loop' model of canonical inhibitors might not be fully valid for the inhibitor family studied. In our view, this feature allows for the emergence of both taxon-specific and nontaxon-specific inhibitors in this group of small proteins.

Myelin basic protein, an autoantigen in multiple sclerosis, is selectively processed by human trypsin 4.

Demyelination, the proteolytic degradation of the major membrane protein in central nervous system, myelin, is involved in many neurodegenerative diseases. In the present in vitro study the proteolytic actions of calpain, human trypsin 1 and human trypsin 4 were compared on lipid bound and free human myelin basic proteins as substrates. The fragments formed were identified by using N-terminal amino acid sequencing and mass spectrometry. The analysis of the degradation products showed that of these three proteases human trypsin 4 cleaved myelin basic protein most specifically. It selectively cleaves the Arg79-Thr80 and Arg97-Thr98 peptide bonds in the lipid bound form of human myelin basic protein. Based on this information we synthesized peptide IVTPRTPPPSQ that corresponds to sequence region 93-103 of myelin basic protein and contains one of its two trypsin 4 cleavage sites, Arg97-Thr98. Studies on the hydrolysis of this synthetic peptide by trypsin 4 have confirmed that the Arg97-Thr98 peptide bond is highly susceptible to trypsin 4. What may lend biological interest to this finding is that the major autoantibodies found in patients with multiple sclerosis recognize sequence 85-96 of the protein. Our results suggest that human trypsin 4 may be one of the candidate proteases involved in the pathomechanism of multiple sclerosis.

Inhibition of Colorado potato beetle larvae by a locust proteinase inhibitor peptide expressed in potato.

The cDNA for a 73-mer peptide containing two locust serine proteinase inhibitors was cloned, fused to the constitutive CaMV35S promoter and introduced into potato by Agrobacterium-mediated transformation. From 23 independent transgenic lines, three with high mRNA level and proteinase inhibitory activity were propagated in vitro and transferred to pots. The peptide from the leaves was identified by its N-terminal sequence and by K(i) values against chymotrypsin and trypsin. Colorado potato beetle larvae reared on transgenic plants grew slightly but significantly more slowly than those on control plants. This supports the notion that expression of multifunctional proteinase inhibitors of insect origin might be a good strategy to improve insect resistance in plants.

Extended intermolecular interactions in a serine protease-canonical inhibitor complex account for strong and highly specific inhibition.

We have previously shown that a trypsin inhibitor from desert locust Schistocerca gregaria (SGTI) is a taxon-specific inhibitor that inhibits arthropod trypsins, such as crayfish trypsin, five orders of magnitude more effectively than mammalian trypsins. Thermal denaturation experiments, presented here, confirm the inhibition kinetics studies; upon addition of SGTI the melting temperatures of crayfish and bovine trypsins increased 27 degrees C and 4.5 degrees C, respectively. To explore the structural features responsible for this taxon specificity we crystallized natural crayfish trypsin in complex with chemically synthesized SGTI. This is the first X-ray structure of an arthropod trypsin and also the highest resolution (1.2A) structure of a trypsin-protein inhibitor complex reported so far. Structural data show that in addition to the primary binding loop, residues P3-P3' of SGTI, the interactions between SGTI and the crayfish enzyme are also extended over the P12-P4 and P4'-P5' regions. This is partly due to a structural change of region P10-P4 in the SGTI structure induced by binding of the inhibitor to crayfish trypsin. The comparison of SGTI-crayfish trypsin and SGTI-bovine trypsin complexes by structure-based calculations revealed a significant interaction energy surplus for the SGTI-crayfish trypsin complex distributed over the entire binding region. The new regions that account for stronger and more specific binding of SGTI to crayfish than to bovine trypsin offer new inhibitor sites to engineer in order to develop efficient and specific protease inhibitors for practical use.

Cloning and expression of ostrich trypsinogen: an avian trypsin with a highly sensitive autolysis site.

One of ostrich (Struthio camelus) trypsinogen genes was cloned from pancreatic cDNA. Its amino acid sequence compared to known trypsin sequences from other species shows high identity and suggests that it is a member of the phylogenetically anionic trypsinogen I subfamily. After cytoplasmic over expression in Escherichia coli and renaturation, the activation properties of ostrich trypsinogen were studied and compared to those of human trypsinogen 1 (also called as human cationic trypsinogen). Ostrich trypsinogen undergoes bovine enterokinase activation and autoactivation much faster than human trypsinogen 1 and exhibits on a synthetic substrate a somewhat higher enzymatic activity than the latter one. The most interesting property of ostrich trypsin is its relatively fast autolysis that can be explained via a mechanism different from the common mechanism for rat and human 1 trypsins. The latter proteases have a site, Arg117-Val118, where the autolysis starts and then goes on in a zipper-like fashion. This is absent from ostrich trypsin. Instead it has a couple of cleavage sites within regions 67-98, including two unusual ones, Arg76-Glu77 and Arg83-Ser84. These appear to be hydrolysed fast in a non-consecutive manner. Such an autolysis mechanism could not be inhibited by a single-site mutation which in humans is proposed to lead to pancreatitis.

Comparison of proteolytic activities produced by entomopathogenic Photorhabdus bacteria: strain- and phase-dependent heterogeneity in composition and activity of four enzymes.

Twenty strains (including eight phase variant pairs) of nematode-symbiotic and insect-pathogenic Photorhabdus bacteria were examined for the production of proteolytic enzymes by using a combination of several methods, including gelatin liquefaction, zymography coupled to native and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and activity measurement with two chromogen substrate types. Four protease activities (approximately 74, approximately 55, approximately 54, and approximately 37 kDa) could be separated. The N-terminal sequences of three of the proteases were determined, and a comparison with sequences in databases allowed identification of these proteases as HEXXH metallopeptidases. Thus, the 74-kDa protease (described formerly as Php-B [J. Marokhazi, G. Koczan, F. Hudecz, L. Graf, A. Fodor, and I. Venekei, Biochem. J. 379:633-640, 2004) is an ortholog of OpdA, a member the thimet oligopeptidase family, and the 55-kDa protease is an ortholog of PrtA, a HEXXH+H peptidase in clan MB (metzincins), while the 37-kDa protease (Php-C) belongs to the HEXXH+E peptidases in clan MA. The 54-kDa protease (Php-D) is a nonmetalloenzyme. PrtA and Php-C were zymographically detected, and they occurred in several smaller forms as well. OpdA could not be detected by zymography. PrtA, Php-C, and Php-D were secreted proteases; OpdA, in contrast, was an intracellular enzyme. OpdA activity was found in every strain tested, while Php-D was detected only in the Brecon/1 strain. There was significant strain variation in the secretion of PrtA and Php-C activities, but reduced activity or a lack of activity was not specific to secondary-phase variants. The presence of PrtA, OpdA, and Php-C activities could be detected in the hemolymph of Galleria melonella larvae 20 to 40 h postinfection. These proteases appear not to be directly involved in the pathogenicity of Photorhabdus, since strains or phase variants lacking any of these proteases do not show reduced virulence when they are injected into G. melonella larvae.

Mutant rat trypsin selectively cleaves tyrosyl peptide bonds.

A double mutant of rat trypsinogen (Asp189Ser, DeltaAsp223) was constructed by site-directed mutagenesis. The recombinant protein was produced in Escherichia coli under the control of a periplasmic expression vector. The purified and enterokinase-activated enzyme was characterized by synthetic fluorogenic tetrapeptide and natural polypeptide substrates and by a recently developed method. In case of this latter method the specificity profile of the enzyme was examined by simultaneous digestion of a mixture of oligopeptide substrates each differing only at the P(1) site residue, and the results were analyzed by high-performance liquid chromatography. All these assays unanimously demonstrated that the recombinant proteinase lacks trypsin-like activity but acquired a rather unique selectivity: it preferentially hydrolyses peptide bonds C-terminal to tyrosyl residues. This narrow specificity should be useful in peptide-analytical applications such as sequence-specific fragmentation of large proteins prior to sequencing.

Isolation and functional characterization of a cyst(e)amine-sensitive protein protecting methyl-dithio groups.

We have previously shown that S-methylthio cysteine mixed disulfide and cystamine potently stimulate thiol production and glutathione synthesis of a human T-cell line in SH-poor medium. Here, we describe a simple photometric method for the determination of methylthio-mixed disulfides (MT-groups) and show that liver contains relatively large amount of MT-groups mainly associated with the globulin fractions. At least a part of methylthio (MT-) globulins is in a complex with a heat-stable protein protecting methylthio-groups against reduction and was designated as anti-methylthio factor (AMTF). Similar complexes are present in some animal sera. AMTF isolated from mouse liver was shown to specifically inhibit redox interaction of methyldithio-groups of various origin with thiols but loses this ability in the presence of some agents such as cyst(e)amine, 2-mercaptoethanol and dimethyl sulfoxide abrogating the MT-binding activity of AMTF (= cystamine-sensitive protein-CSP). AMTF purified by heat treatment and isopropanol fractionation was chromatographed on Superose-12 column. Preliminary results showed that the molecular mass of the active component is about 34 kDa consisting of two identical subunits. The possible biological role of MT-AMT complexes was discussed.

Structural characterization of the second TSP1-module of human thrombospondin.

The TSP1-module has been first identified as the type 1 repeat of thrombospondin-1. Members of this extracellular module-family have since been shown to be present in several hundred metazoan proteins as well as in proteins of some protists. Despite the widespread occurrence and biological importance of this module-type, relatively little is known about their three-dimensional structure. To define the structural features of this important module-family, we have expressed the second TSP1-domain of human thrombospondin 1 in Escherichia coli. Amino acid sequencing of proteolytic fragments of the recombinant protein have shown that its disulfide bonds connect the six conserved cysteines in a 1-5, 2-6, 3-4 pattern. Circular dichroism studies on the recombinant protein indicate that the disulfide-bonded TSP1-module consists primarily of distorted beta-strands.

Binding sites of different geometries for the 16-3 phage repressor.

Prokaryotic repressor-operator systems provide exemplars for the sequence-specific interactions between DNA and protein. The crucial atomic contacts of the two macromolecules are attained in a compact, geometrically defined structure of the DNA-protein complex. The pitch of the DNA interface seems an especially sensitive part of this architecture because changes in its length introduce new spacing and rotational relations in one step. We discovered a natural system that may serve as a model for investigating this problem: the repressor of the 16-3 phage of Rhizobium meliloti (helix-turn-helix class protein) possesses inherent ability to accommodate to various DNA twistings. It binds the cognate operators, which are 5'-ACAA-4 bp-TTGT-3' (O(L)) and 5'-ACAA-6 bp-TTGT-3' (O(R)) and thus differ 2 bp in length, and consequently the two half-sites will be rotated with respect to each other by 72 degrees in the idealized B-DNA (64 degrees by dinucleotide steps calculations). Furthermore, a synthetic intermediate (DNA sequence) 5'-ACAA-5 bp-TTGT-3' (O5) also binds specifically the repressor. The natural operators and bound repressors can form higher order DNA-protein complexes and perform efficient repression, whereas the synthetic operator-repressor complex cannot do either. The natural operators are bent when complexed with the repressor, whereas the O5 operator does not show bending in electrophoretic mobility assay. Possible structures of the complexes are discussed.

Comparative structure analysis of proteinase inhibitors from the desert locust, Schistocerca gregaria.

The solution structure of three small serine proteinase inhibitors, two natural and one engineered protein, SGCI (Schistocerca gregaria chymotrypsin inhibitor), SGCI[L30R, K31M] and SGTI (Schistocerca gregaria trypsin inhibitor), were determined by homonuclear NMR-spectroscopy. The molecules exhibit different specificities towards target proteinases, where SGCI is a good chymotrypsin inhibitor, its mutant is a potent trypsin inhibitor, and SGTI inhibits both proteinases weakly. Interestingly, SGTI is a much better inhibitor of insect proteinases than of the mammalian ones used in common assays. All three molecules have a similar fold composed from three antiparallel beta-pleated sheets with three disulfide bridges. The proteinase binding loop has a somewhat distinct geometry in all three peptides. Moreover, the stabilization of the structure is different in SGCI and SGTI. Proton-deuterium exchange experiments are indicative of a highly rigid core in SGTI but not in SGCI. We suggest that the observed structural properties play a significant role in the specificity of these inhibitors.

Remarkable phylum selectivity of a Schistocerca gregaria trypsin inhibitor: the possible role of enzyme-inhibitor flexibility.

A 35-mer polypeptide isolated from the hemolymph of desert locust Schistocerca gregaria (SG) proved to be a canonical inhibitor of bovine trypsin (K(i) = 0.2 microM). Despite having a trypsin-specific arginine at the primary specificity P(1) site, it inhibits bovine chymotrypsin almost as well (K(i) = 2 microM). Furthermore, while the latter reactivity improves 10(4)-fold by the single replacement of P(1) Arg by Leu, changing P(1)' from Lys to Met only moderately improves trypsin affinity (K(i) = 30 nM). The apparent low compatibility to trypsin, however, is not observed vs two arthropodal trypsins: SG peptides with P(1) Arg inhibit crayfish and shrimp trypsins with K(i) values in the picomolar range. This unprecedented high discrimination between orthologous enzymes is postulated to derive from flexibility differences in the protein-protein interaction. The more than four orders of magnitude phylum selectivity makes these peptides prospective candidates for agricultural use.