The serine protease inhibitor canonical loop conformation: examples found in extracellular hydrolases, toxins, cytokines and viral proteins

Identification of Kazal Inhibitor Scaffolds with Identical Canonical Binding Loops and Their Effects on Binding Properties

Kazal inhibitors hold high potential as scaffolds for therapeutic molecules, taking advantage of the easily exchangeable canonical binding loop. Different Kazal inhibitor backbones have been suggested to be therapeutically useful, but the impact of different Kazal-like scaffolds on binding properties is still largely unknown. Here, we identified trypsin-targeting human serine protease inhibitor Kazal type 1 (SPINK1) homologues in different mammalian species that cluster in two P2-P1 combinations, implying the coevolution of these residues. We generated loop exchange variants of human SPINK1 for comparison with Kazal inhibitors from related species. Using comprehensive biophysical characterization of the inhibitor-enzyme interactions, we found not only affinity but also pH resistance to be highly backbone-dependent. Differences are mostly observed in complex stability, which varies by over one order of magnitude. We provide clear evidence for high backbone dependency within the Kazal family. Hence, when designing Kazal inhibitor-based therapeutic molecules, testing different backbones after optimizing the canonical binding loop can be beneficial and may result in increased affinity, complex stability, specificity, and pH resistance.

A protein scaffold, engineered SPINK2, for generation of inhibitors with high affinity and specificity against target proteases

Proteases are one of attractive therapeutic targets to play key roles in pharmacological action. There are many protease inhibitors in nature, and most of them structurally have cystine knot motifs. Their structures are favorable for recognition of active pockets of proteases, leading to the potent inhibition. However, they also have drawbacks, such as broad cross-reactivity, on the therapeutic application. To create therapeutic proteins derived from a disulfide-rich scaffold, we selected human serine protease inhibitor Kazal type 2 (SPINK2) through a scaffold screening, as a protein scaffold with requirements for therapeutic proteins. We then constructed a diverse library of the engineered SPINK2 by introducing random mutations into its flexible loop region with the designed method. By phage panning against four serine proteases, we isolated potent inhibitors against each target with picomolar K_D and sub-nanomolar K_i values. Also, they exhibited the desired specificities against target proteases without inhibiting non-target proteases. The crystal structure of kallikrein related peptidase 4 (KLK4)-engineered SPINK2 complex revealed the interface with extensive conformational complementarity. Our study demonstrates that engineered SPINK2 can serve as a scaffold to generate therapeutic molecules against target proteins with groove structures.

Microbial serine protease inhibitors and their therapeutic applications

Serine protease inhibitors, inhibit serine proteases either partially or completely after forming complexes with their respective proteases. Protease actions are significant for many physiological pathways found in living forms and any anomalies may lead to numerous physiological complications. Each cell or organism has its own mechanism for controlling these protease actions. It is often regulated by the action of inhibitors or activators. Among the proteases, serine proteases are the most common that are involved in many life and death processes. Selective inhibitors of physiologically relevant proteases can be used as a lead compound for the drug development. Therefore, it is imperative to identify small peptides and proteins that selectively inhibit serine proteases from various sources. Microbes can be considered as a major source of diverse serine protease inhibitors since they have the prominent and diverse domain in nature. Most of the microbial serine protease inhibitors are intracellular and few are extracellular. Microbes produce protease inhibitors for protection against its own proteases or against other environmental factors. The status and future prospects of microbial serine protease inhibitors and their therapeutic benefits in treating cancer, blood coagulation disorders and viral infections, are reviewed here.

Sequence and conformational specificity in substrate recognition: several human Kunitz protease inhibitor domains are specific substrates of mesotrypsin

Mesotrypsin is an isoform of trypsin that is uniquely resistant to polypeptide trypsin inhibitors and can cleave some inhibitors rapidly. Previous studies have shown that the amyloid precursor protein Kunitz protease inhibitor domain (APPI) is a specific substrate of mesotrypsin and that stabilization of the APPI cleavage site in a canonical conformation contributes to recognition by mesotrypsin. We hypothesized that other proteins possessing potential cleavage sites stabilized in a similar conformation might also be mesotrypsin substrates. Here we evaluated a series of candidate substrates, including human Kunitz protease inhibitor domains from amyloid precursor-like protein 2 (APLP2), bikunin, hepatocyte growth factor activator inhibitor type 2 (HAI2), tissue factor pathway inhibitor-1 (TFPI1), and tissue factor pathway inhibitor-2 (TFPI2), as well as E-selectin, an unrelated protein possessing a potential cleavage site displaying canonical conformation. We find that Kunitz domains within APLP2, bikunin, and HAI2 are cleaved by mesotrypsin with kinetic profiles of specific substrates. TFPI1 and TFPI2 Kunitz domains are cleaved less efficiently by mesotrypsin, and E-selectin is not cleaved at the anticipated site. Cocrystal structures of mesotrypsin with HAI2 and bikunin Kunitz domains reveal the mode of mesotrypsin interaction with its canonical substrates. Our data suggest that major determinants of mesotrypsin substrate specificity include sequence preferences at the P1 and P'2 positions along with conformational stabilization of the cleavage site in the canonical conformation. Mesotrypsin up-regulation has been implicated previously in cancer progression, and proteolytic clearance of Kunitz protease inhibitors offers potential mechanisms by which mesotrypsin may mediate pathological effects in cancer.

Interactions outside the proteinase-binding loop contribute significantly to the inhibition of activated coagulation factor XII by its canonical inhibitor from corn

Activated factor XII (FXIIa) is selectively inhibited by corn Hageman factor inhibitor (CHFI) among other plasma proteases. CHFI is considered a canonical serine protease inhibitor that interacts with FXIIa through its protease-binding loop. Here we examined whether the protease-binding loop alone is sufficient for the selective inhibition of serine proteases or whether other regions of a canonical inhibitor are involved. Six CHFI mutants lacking different N- and C-terminal portions were generated. CHFI-234, which lacks the first and fifth disulfide bonds and 11 and 19 amino acid residues at the N and C termini, respectively, exhibited no significant changes in FXIIa inhibition (K_i = 3.2 ± 0.4 nm). CHFI-123, which lacks 34 amino acid residues at the C terminus and the fourth and fifth disulfide bridges, inhibited FXIIa with a K_i of 116 ± 16 nm. To exclude interactions outside the FXIIa active site, a synthetic cyclic peptide was tested. The peptide contained residues 20–45 (Protein Data Bank code 1BEA), and a C29D substitution was included to avoid unwanted disulfide bond formation between unpaired cysteines. Surprisingly, the isolated protease-binding loop failed to inhibit FXIIa but retained partial inhibition of trypsin (K_i = 11.7 ± 1.2 μm) and activated factor XI (K_i = 94 ± 11 μm). Full-length CHFI inhibited trypsin with a K_i of 1.3 ± 0.2 nm and activated factor XI with a K_i of 5.4 ± 0.2 μm. Our results suggest that the protease-binding loop is not sufficient for the interaction between FXIIa and CHFI; other regions of the inhibitor also contribute to specific inhibition.

SUMOMO: A PROTEIN SURFACE MOTIF MINING MODULE

Protein surface motifs, which can be defined as commonly appearing patterns of shape and physical properties in protein molecular surfaces, can be considered "possible active sites".

We have developed a system for mining surface motifs: SUMOMO which consists of two phases: surface motif extraction and surface motif filtering. In the extraction phase, a given set of protein molecular surface data is divided into small surfaces called unit surfaces. After extracting several common unit surfaces as candidate motifs, they are repetitively merged into surface motifs. However, a large amount of surface motifs is extracted in this phase, making it difficult to distinguish whether the extracted motifs are significant to be considered active sites. Since active sites from proteins with a particular function have similar shape and physical properties, proteins can be classified based on similarity among local surfaces. Thus, in the filtering phase, local surfaces extracted from proteins of the same group are considered significant motifs, and the rest are filtered out.

The proposed method was applied to discover surface motifs from 15 proteins belonging to four function groups. Motifs corresponding to all 4 known functional sites were recognised.

A computational approach for detecting peptidases and their specific inhibitors at the genome level

Background

Peptidases are proteolytic enzymes responsible for fundamental cellular activities in all organisms. Apparently about 2–5% of the genes encode for peptidases, irrespectively of the organism source. The basic peptidase function is "protein digestion" and this can be potentially dangerous in living organisms when it is not strictly controlled by specific inhibitors. In genome annotation a basic question is to predict gene function. Here we describe a computational approach that can filter peptidases and their inhibitors out of a given proteome. Furthermore and as an added value to MEROPS, a specific database for peptidases already available in the public domain, our method can predict whether a pair of peptidase/inhibitor can interact, eventually listing all possible predicted ligands (peptidases and/or inhibitors).

Results

We show that by adopting a decision-tree approach the accuracy of PROSITE and HMMER in detecting separately the four major peptidase types (Serine, Aspartic, Cysteine and Metallo- Peptidase) and their inhibitors among a non redundant set of globular proteins can be improved by some percentage points with respect to that obtained with each method separately. More importantly, our method can then predict pairs of peptidases and interacting inhibitors, scoring a joint global accuracy of 99% with coverage for the positive cases (peptidase/inhibitor) close to 100% and a correlation coefficient of 0.91%. In this task the decision-tree approach outperforms the single methods.

Conclusion

The decision-tree can reliably classify protein sequences as peptidases or inhibitors, belonging to a certain class, and can provide a comprehensive list of possible interacting pairs of peptidase/inhibitor. This information can help the design of experiments to detect interacting peptidase/inhibitor complexes and can speed up the selection of possible interacting candidates, without searching for them separately and manually combining the obtained results. A web server specifically developed for annotating peptidases and their inhibitors (HIPPIE) is available at

Identification of function-associated loop motifs and application to protein function prediction

MOTIVATION

The detection of function-related local 3D-motifs in protein structures can provide insights towards protein function in absence of sequence or fold similarity. Protein loops are known to play important roles in protein function and several loop classifications have been described, but the automated identification of putative functional 3D-motifs in such classifications has not yet been addressed. This identification can be used on sequence annotations.

RESULTS

We evaluated three different scoring methods for their ability to identify known motifs from the PROSITE database in ArchDB. More than 500 new putative function-related motifs not reported in PROSITE were identified. Sequence patterns derived from these motifs were especially useful at predicting precise annotations. The number of reliable sequence annotations could be increased up to 100% with respect to standard BLAST.

CONTACT

boliva@imim.es

SUPPLEMENTARY INFORMATION

Supplementary Data are available at Bioinformatics online.

Predicting protein interaction sites: binding hot-spots in protein–protein and protein–ligand interfaces

MOTIVATION

Protein assemblies are currently poorly represented in structural databases and their structural elucidation is a key goal in biology. Here we analyse clefts in protein surfaces, likely to correspond to binding 'hot-spots', and rank them according to sequence conservation and simple measures of physical properties including hydrophobicity, desolvation, electrostatic and van der Waals potentials, to predict which are involved in binding in the native complex.

RESULTS

The resulting differences between predicting binding-sites at protein-protein and protein-ligand interfaces are striking. There is a high level of prediction accuracy (< or =93%) for protein-ligand interactions, based on the following attributes: van der Waals potential, electrostatic potential, desolvation and surface conservation. Generally, the prediction accuracy for protein-protein interactions is lower, with the exception of enzymes. Our results show that the ease of cleft desolvation is strongly predictive of interfaces and strongly maintained across all classes of protein-binding interface.

Single mutation at P1 of a chymotrypsin inhibitor changes it to a trypsin inhibitor: X-ray structural (2.15 A) and biochemical basis

Change in specificity, caused by the mutations at P1 site, of the serine protease inhibitors of different families is reported in the literature, but Kunitz (STI) family inhibitors are almost unexplored in this regard. In this paper, we present the crystal structure of a P1 variant of winged bean chymotrypsin inhibitor (WCI) belonging to Kunitz (STI) family, supplemented by biochemical, phylogenetic and docking studies on the mutant. A single mutation (Leu-->Arg) at P1 converted WCI to a strong inhibitor of trypsin with an association constant of 4.8x10(10) M(-1) which is comparable to other potent trypsin inhibitors of the family. The crystal structure (2.15 A) of this mutant (L65R) shows that its reactive site loop conformation deviates from that of WCI and adopts a structure similar to that of Erythrina caffra trypsin inhibitor (ETI) belonging to the same family. Mutation induced structural changes have also been propagated in a concerted manner to the neighboring conserved scaffolding residue Asn14, such that the side chain of this residue took an orientation similar to that of ETI and optimized the hydrogen bonds with the loop residues. While docking studies provide information about the accommodation of non-specific residues in the active site groove of trypsin, the basis of the directional alteration of the reactive site loop conformation has been understood through sequence analysis and related phylogenetic studies.

Potential folding-function interrelationship in proteins

The possibility is addressed that protein folding and function may be related via regions that are critical for both folding and function. This approach is based on the building blocks folding model that describes protein folding as binding events of conformationally fluctuating building blocks. Within these, we identify building block fragments that are critical for achieving the native fold. A library of such critical building blocks (CBBs) is constructed. Then, it is asked whether the functionally important residues fall in these CBB fragments. We find that for over two-thirds of the proteins in our library with available functional information, the catalytic or binding site residues lie within the CBB regions. From the evolutionary standpoint, a folding-function relationship is advantageous, since the need to guard against mutations is limited to one region. Furthermore, conformationally similar CBBs are found in globally unrelated proteins with different functions. Hence, substituting CBBs may lead to designed proteins with altered functions. We further find that the CBBs in our library are conformationally unstable.

Towards a structural classification of phosphate binding sites in protein-nucleotide complexes: an automated all-against-all structural comparison using geometric matching

A method is described for the rapid comparison of protein binding sites using geometric matching to detect similar three-dimensional structure. The geometric matching detects common atomic features through identification of the maximum common sub-graph or clique. These features are not necessarily evident from sequence or from global structural similarity giving additional insight into molecular recognition not evident from current sequence or structural classification schemes. Here we use the method to produce an all-against-all comparison of phosphate binding sites in a number of different nucleotide phosphate-binding proteins. The similarity search is combined with clustering of similar sites to allow a preliminary structural classification. Clustering by site similarity produces a classification of binding sites for the 476 representative local environments producing ten main clusters representing half of the representative environments. The similarities make sense in terms of both structural and functional classification schemes. The ten main clusters represent a very limited number of unique structural binding motifs for phosphate. These are the structural P-loop, di-nucleotide binding motif [FAD/NAD(P)-binding and Rossman-like fold] and FAD-binding motif. Similar classification schemes for nucleotide binding proteins have also been arrived at independently by others using different methods.

The relationship between sequence and interaction divergence in proteins

There is currently a gap in knowledge between complexes of known three-dimensional structure and those known from other experimental methods such as affinity purifications or the two-hybrid system. This gap can sometimes be bridged by methods that extrapolate interaction information from one complex structure to homologues of the interacting proteins. To do this, it is important to know if and when proteins of the same type (e.g. family, superfamily or fold) interact in the same way. Here, we study interactions of known structure to address this question. We found all instances within the structural classification of proteins database of the same domain pairs interacting in different complexes, and then compared them with a simple measure (interaction RMSD). When plotted against sequence similarity we find that close homologues (30-40% or higher sequence identity) almost invariably interact the same way. Conversely, similarity only in fold (i.e. without additional evidence for a common ancestor) is only rarely associated with a similarity in interaction. The results suggest that there is a twilight zone of sequence similarity where it is not possible to say whether or not domains will interact similarly. We also discuss the rare instances of fold similarities interacting the same way, and those where obviously homologous proteins interact differently.

In vitro activities of small peptides from snake venom against clinical isolates of drug-resistant Mycobacterium tuberculosis

The recent re-emergence of tuberculosis, especially the multidrug-resistant cases, has highlighted the importance of screening effective novel drugs against Mycobacterium tuberculosis. In this study, the in vitro activities of small peptides isolated from snake venom were investigated against multidrug-resistant M. tuberculosis. Minimum inhibitory concentrations (MICs) were determined by the Bactec TB-460 radiometric method. A small peptide with the amino acid sequence ECYRKSDIVTCEPWQKFCYREVTFFPNHPVYLSGCASECTETNSKWCCTTDKCNRARGG (designated as vgf-1) from Naja atra (isolated from Yunnan province of China) venom had in vitro activity against clinically isolated multidrug-resistant strains of M. tuberculosis. The MIC was 8.5 mg/l. The antimycobacterial domain of this 60aa peptide is under investigation.

A conserved cis peptide bond is necessary for the activity of Bowman-Birk inhibitor protein

The Bowman-Birk inhibitor (BBI) family of protease inhibitors has an inhibitory region comprising a disulfide-linked nine-residue loop that adopts the characteristic canonical motif found in many serine protease inhibitors. A unique feature of the BBI loop is the presence of a cis peptide bond at the edge of the inhibitory loop. BBI-related protein fragments that encapsulate this loop retain the structure and inhibitory activity of the parent protein. The most common BBI loop sequence has a proline-proline element with a cis-trans geometry at P3'-P4'. We have examined this element by analysis of the inhibitory activity and structure for a series of synthetic fragments where each of these proline residues has been systematically replaced with alanine. The results show that only when a proline is present at P3' are potent inhibition and a cis peptide bond at that position in the solution structure observed, suggesting that this conformation is required for biological activity. Though a P4' proline is not essential for activity, it effectively stabilizes the cis conformation at P3' by suppressing alternative conformations. This is most evident from the Pro-Ala variant, which comprises a 1:1 mixture of slowly exchanging and structurally different cis and trans isomers. Monitoring the action of trypsin on this mixture by NMR shows that this protease interacts selectively with the cis P3' structure, providing direct evidence for the link between activity and the nativelike structure of the cis isomer. This is, to the best of our knowledge, the first example where cis isomer selectivity can be demonstrated for a proteinase.

A clogged gutter mechanism for protease inhibitors

A classical peptide inhibitor of serine proteases that is hydrolyzed approximately 10(7) times more slowly than a good substrate is shown to form an acyl-enzyme intermediate rapidly. Despite this quick first step, further reaction is slowed dramatically because of tight and oriented binding of the cleaved peptide, preventing acyl-enzyme hydrolysis and favoring the reverse reaction. Moreover, this mechanism appears to be common to a large class of tight-binding serine protease inhibitors that mimic good substrates. The arrest of enzymatic reaction at the intermediate stage allowed us to determine that the consensus nucleophilic attack angle is close to 90 degrees in the reactive Michaelis complexes.

Predicting function from structure: examples of the serine protease inhibitor canonical loop conformation found in extracellular proteins

The prediction of protein function from structure is becoming of growing importance in the age of structural genomics. We have focused on the problem of identifying sites of potential serine protease inhibitor interactions on the surface of proteins of known structure. Given that there is no sequence conservation within canonical loops from different inhibitor families we first compare representative loops to all fragments of equal length among proteins of known structure by calculating main-chain RMS deviation. Fragments with RMS deviation below a certain threshold (hits) are removed if residues have solvent accessibilities appreciably lower than those observed in the search structure. These remaining hits are further filtered to remove those occurring largely within secondary structure elements. Likely functional significance is restricted further by considering only extracellular protein domains. Also a test is performed to see if the loop can dock into the binding site of the serine protease trypsin without unacceptable steric clashes. By comparing different canonical loop structures to the protein structure database we show that the method was able to detect previously known inhibitors. In addition, we discuss potentially new canonical loop structures found in secreted hydrolases, toxins, viral proteins, cytokines and other proteins. We discuss the possible functional significance of several of the examples found.

The role of Asn14 in the stability and conformation of the reactive-site loop of winged bean chymotrypsin inhibitor: crystal structures of two point mutants Asn14-->Lys and Asn14-->Asp

A double-headed chymotrypsin inhibitor, WCI, from winged bean seeds was cloned for structural and biochemical studies. The inhibitor was subjected to two point mutations at a conserved position, Asn14. This residue, known to have a pivotal role in stabilizing the first reactive-site loop (Gln63-Phe68) of the inhibitor, is highly conserved in the sequences of the other members of Kunitz (STI) family as well as in the sequences of Kazal family of serine protease inhibitors. The mutants, N14K and N14D, were subjected to biochemical assay and their characteristics were compared with those of the recombinant inhibitor (rWCI). Crystallographic studies of the recombinant and the mutant proteins are discussed. These studies were primarily aimed at understanding the importance of the protein scaffolding towards the conformational rigidity of the reactive-site loop. Our analysis reveals that, as the Lys14 side chain takes an unusual fold in N14K and the Asp14 side chain in N14D interacts with the loop residues by water-mediated hydrogen bonds, the canonical conformation of the loop has remained effectively intact in both the mutant structures. However, minor alterations such as a 2-fold increase in the inhibitory affinity towards the cognate enzyme were observed.

On the evolution of protein folds: are similar motifs in different protein folds the result of convergence, insertion, or relics of an ancient peptide world?

This paper presents and discusses evidence suggesting how the diversity of domain folds in existence today might have evolved from peptide ancestors. We apply a structure similarity detection method to detect instances where localized regions of different protein folds contain highly similar sequences and structures. Results of performing an all-on-all comparison of known structures are described and compared with other recently published findings. The numerous instances of local sequence and structure similarities within different protein folds, together with evidence from proteins containing sequence and structure repeats, argues in favor of the evolution of modern single polypeptide domains from ancient short peptide ancestors (antecedent domain segments (ADSs)). In this model, ancient protein structures were formed by self-assembling aggregates of short polypeptides. Subsequently, and perhaps concomitantly with the evolution of higher fidelity DNA replication and repair systems, single polypeptide domains arose from the fusion of ADSs genes. Thus modern protein domains may have a polyphyletic origin.

Review: protein secondary structure prediction continues to rise

Methods predicting protein secondary structure improved substantially in the 1990s through the use of evolutionary information taken from the divergence of proteins in the same structural family. Recently, the evolutionary information resulting from improved searches and larger databases has again boosted prediction accuracy by more than four percentage points to its current height of around 76% of all residues predicted correctly in one of the three states, helix, strand, and other. The past year also brought successful new concepts to the field. These new methods may be particularly interesting in light of the improvements achieved through simple combining of existing methods. Divergent evolutionary profiles contain enough information not only to substantially improve prediction accuracy, but also to correctly predict long stretches of identical residues observed in alternative secondary structure states depending on nonlocal conditions. An example is a method automatically identifying structural switches and thus finding a remarkable connection between predicted secondary structure and aspects of function. Secondary structure predictions are increasingly becoming the work horse for numerous methods aimed at predicting protein structure and function. Is the recent increase in accuracy significant enough to make predictions even more useful? Because the recent improvement yields a better prediction of segments, and in particular of beta strands, I believe the answer is affirmative. What is the limit of prediction accuracy? We shall see.

Sialidase-like Asp-boxes: sequence-similar structures within different protein folds

Sequence similarity is the most common measure currently used to infer homology between proteins. Typically, homologous protein domains show sequence similarity over their entire lengths. Here we identify Asp box motifs, initially found as repeats in sialidases and neuraminidases, in new structural and sequence contexts. These motifs represent significantly similar sequences, localized to beta hairpins within proteins that are otherwise different in sequence and three-dimensional structure. By performing a combined sequence- and structure-based analysis we detect Asp boxes in more than nine protein families, including bacterial ribonucleases, sulfite oxidases, reelin, netrins, some lipoprotein receptors, and a variety of glycosyl hydrolases. Although the function common to each of these proteins, if any, remains unclear, we discuss possible functions of Asp boxes on the basis of previously determined experimental results and discuss different evolutionary scenarios for the origin of Asp-box containing proteins.

Assessing the protease and protease inhibitor content of the human genome

The revealing of the entire complement of protease and protease inhibitor sequences by the Human Genome Project will be of great importance to both academic and pharmaceutical research. Although the finishing phase is not yet complete, a selection of secondary annotation sources and comparisons with completed model organism genomes already allow useful estimates to be made. Conservative extrapolation suggests a total of approximately 1.8% for human proteases. This is close to the figures for yeast (1.7%) and worm (1.8%) but lower than the fly (3.4%) which has a large trypsin-like protease content. Using estimates for the human proteome of between 40,000 and 60,000 genes would extrapolate to 700-1,100 proteases, compared with approximately 360 currently represented as GenBank mRNAs. Preliminary comparisons between domain annotations for predicted human gene products and completed proteins suggest the genomic protease family and mechanistic class distributions will broadly reflect those in the current transcript data. The protease:inhibitor ratio at the mRNA level is currently approximately 9:1, but genome annotation data indicate that inhibitory domains are more widespread than this ratio would indicate.