Tissue transglutaminase acylation: Proposed role of conserved active site Tyr and Trp residues revealed by molecular modeling of peptide substrate binding

Peptidic Inhibitors and a Fluorescent Probe for the Selective Inhibition and Labelling of Factor XIIIa Transglutaminase

Factor XIIIa (FXIIIa) is a transglutaminase of major therapeutic interest for the development of anticoagulants due to its essential role in the blood coagulation cascade. While numerous FXIIIa inhibitors have been reported, they failed to reach clinical evaluation due to their lack of metabolic stability and low selectivity over transglutaminase 2 (TG2). Furthermore, the chemical tools available for the study of FXIIIa activity and localization are extremely limited. To combat these shortcomings, we designed, synthesised, and evaluated a library of 21 novel FXIIIa inhibitors. Electrophilic warheads, linker lengths, and hydrophobic units were varied on small molecule and peptidic scaffolds to optimize isozyme selectivity and potency. A previously reported FXIIIa inhibitor was then adapted for the design of a probe bearing a rhodamine B moiety, producing the innovative KM93 as the first known fluorescent probe designed to selectively label active FXIIIa with high efficiency (k_inact/K_I = 127,300 M^-1 min^-1) and 6.5-fold selectivity over TG2. The probe KM93 facilitated fluorescent microscopy studies within bone marrow macrophages, labelling FXIIIa with high efficiency and selectivity in cell culture. The structure-activity trends with these novel inhibitors and probes will help in the future study of the activity, inhibition, and localization of FXIIIa.

Structure-Activity Relationships of Potent, Targeted Covalent Inhibitors That Abolish Both the Transamidation and GTP Binding Activities of Human Tissue Transglutaminase

Human tissue transglutaminase (hTG2) is a multifunctional enzyme. It is primarily known for its calcium-dependent transamidation activity that leads to formation of an isopeptide bond between glutamine and lysine residues found on the surface of proteins, but it is also a GTP binding protein. Overexpression and unregulated hTG2 activity have been associated with numerous human diseases, including cancer stem cell survival and metastatic phenotype. Herein, we present a series of targeted covalent inhibitors (TCIs) based on our previously reported Cbz-Lys scaffold. From this structure-activity relationship (SAR) study, novel irreversible inhibitors were identified that block the transamidation activity of hTG2 and allosterically abolish its GTP binding ability with a high degree of selectivity and efficiency (k_inact/K_I > 10⁵ M^-1 min^-1). One optimized inhibitor (VA4) was also shown to inhibit epidermal cancer stem cell invasion with an EC₅₀ of 3.9 μM, representing a significant improvement over our previously reported "hit" NC9.

Separation of transglutaminase by thermo-responsive affinity precipitation using l-thyroxin as ligand

Transglutaminase (TGase) is widely used in the food industry. In this study, TGase was purified by affinity precipitation using l-thyroxin, coupled to a thermo-responsive polymer (PNBN), as an affinity ligand. The lower critical solution temperature and recovery of the affinity polymer were 31.0 °C and 99.6 %, respectively. The optimal adsorption condition was 0.02 mol/L phosphate buffer (pH 5.0). The recoveries 99.01 % (protein) and 98.85 % (activity) were obtained by 0.2 mol/L Gly-NaOH buffer (pH 10.0) as the elution agent. Circular dichroism spectroscopy and FortéBio Octet system were used to explore the interactions between l-thyroxin and TGase. The results show that l-thyroxin is suitable for affinity precipitation of TGase. The purity of the final product was verified using sodium dodecyl sulfate polyacrylamide gel electrophoresis.

Isopeptidase activity of human transglutaminase 2: disconnection from transamidation and characterization by kinetic parameters

Transglutaminase 2 (TG2) is a multifunctional protein with diverse catalytic activities and biological roles. Its best studied function is the Ca(2+)-dependent transamidase activity leading to formation of γ-glutamyl-ε-lysine isopeptide crosslinks between proteins and γ-glutamyl-amine derivatives. TG2 has a poorly studied isopeptidase activity cleaving these bonds. We have developed and characterised TG2 mutants which are significantly deficient in transamidase activity while have normal or increased isopeptidase activity (W332F) and vice versa (W278F). The W332F mutation led to significant changes of both the K m and the V max kinetic parameters of the isopeptidase reaction of TG2 while its calcium and GTP sensitivity was similar to the wild-type enzyme. The W278F mutation resulted in six times elevated amine incorporating transamidase activity demonstrating the regulatory significance of W278 and W332 in TG2 and that mutations can change opposed activities located at the same active site. The further application of our results in cellular systems may help to understand TG2-driven physiological and pathological processes better and lead to novel therapeutic approaches where an increased amount of crosslinked proteins correlates with the manifestation of degenerative disorders.

Novel 3-arylethynyl-substituted thieno[3,4-b]pyrazine derivatives as human transglutaminase 2 inhibitors

In the process of optimization, we developed a novel core skeleton of thieno[3,4-b]pyrazine via GK-13. The derivatives synthesized were shown to inhibit TGase 2 activity in cancer cells. Some of the hit compounds such as the arylethynyl group-coupled thieno[3,4-b]pyrazine derivatives were shown to exhibit promising activity for use as potential therapeutic small-molecules in renal cancer by inhibiting TGase 2 activity.

Inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) catalyzes the cross-linking of proteins by the formation of isopeptide bonds between glutamine (Gln) and lysine (Lys) side chains. Although TG2 is essential for the stabilization of the extracellular matrix, its unregulated activity has been implicated in celiac disease, fibrosis, and cancer metastasis, among other disorders. Given the importance and range of TG2-related pathologies, recent work has focused on the development of potent and selective inhibitors against TG2. In this review, we present the latest and most noteworthy irreversible and reversible inhibitors of TG2, and offer perspectives for the design of future inhibitors, in the hope that lead compounds with therapeutic potential may soon be discovered.

Acyl transfer mechanisms of tissue transglutaminase

Tissue transglutaminase (TG2) is a calcium-dependent enzyme that catalyses several acyl transfer reactions. The most biologically relevant of these involve protein-bound Gln residues as an acyl-donor substrate, and either water or a primary amine as an acyl-acceptor substrate. The former leads to deamidation of Gln to Glu, whereas the latter leads to transamidation, typically resulting in protein cross-linking when the amine substrate is a protein-bound Lys residue. In this review, we present an overview of over fifty years of mechanistic studies that have led to our current understanding of TG2-mediated hydrolysis and transamidation.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Expression, purification and kinetic characterisation of human tissue transglutaminase

The expression of soluble recombinant transglutaminase (TGase) has proven to be a challenge for many research groups. Herein, we report a complementary method for the expression, in BL21(DE3) Escherichia coli, of recombinant human tissue transglutaminase (hTG2) whose solubility is enhanced through N-terminal fusion to glutathione S-transferase (GST). Moreover, we report the cleavage of the GST tag using PreScission™ Protease (PSP) and purification of hTG2 in its untagged form, distinctively suitable for subsequent studies of its remarkable conformational equilibrium. The effects of co-solvents and storage conditions on stability of purified hTG2 are also reported. Furthermore, we demonstrate for the first time the use of a convenient chromogenic assay to measure the activity of the human enzyme. The utility of this assay was demonstrated in the measurement of the kinetic parameters of a wide variety of substrates and inhibitors of both hTG2 and the extensively studied guinea pig liver TGase. Finally, comparison of these results provides further evidence for the functional similarity of the two enzymes.

"Activation of tissue tranglutsaminase by removal of carboxyl-terminal peptides"

Tissue transglutaminase (TGC or TG2) functions as transglutaminase (cross-linking), deamidase, kinase, and disulfide isomerase and its activities are implicated in the pathogenesis of several human diseases. Proteolytic activation of zymogens in the transglutaminase family is not unusual. Plasma transglutaminase (FXIIIa), epidermal transglutaminase (TG 3), transglutaminase-5, and microbial transglutaminase (MTG) can be subjected to proteolysis from specific proteases to generate the active functional enzyme. In the present study, calcium or GTP was essential for activation of TGC cross-linking activity by trypsin in membrane fractions from human RBC and was accompanied by the conversion of TGC (80 kDa) to a smaller TG form (55 kDa). While bacterially expressed TGC showed no activity, bacterial expression of C-terminal domain deletion constructs with carboxy-terminal ends ranging from lysine 464 (TG464) to glycine 480 (TG480) produced enzymes that were highly active in cross-linking activity. The product of a construct with a coding region ended at proline 446 (TG446), which interrupted the calcium-binding domain, exhibited weak cross-linking activity. TG480 and TG512 were characterized by about 80% and 10%, respectively, of the cross-linking activities of TG464. This may indicate that the longer the peptide after the calcium binding domain, the less the enzymatic activity expressed, possibly because the folding of such peptide which interfere with the calcium binding site or the catalytic site. Western analysis of MCF7 and T47D human breast cancer cells transfected with TGC showed TGC as a major protein and TG as a minor fragment. Incubation of lysate from transfected cells with serum resulted in the conversion of the TGC to TG, a condition that may be comparable to injury or wounds that lead to rapid enzymatic transamidation activation.

Probing the NADH- and Methyl Red-binding site of a FMN-dependent azoreductase (AzoA) from Enterococcus faecalis

AzoA from Enterococcus faecalis is a member of the polymeric flavin-dependent NADH-preferred azoreductase group. Little is known about the binding and interaction of NADH and azo dye in the azoreductase group. A synergetic strategy based on computational prediction, reverse genetics validation coupled with site-directed mutagenesis, and reconstruction of mutation network was used to investigate the binding and interaction of NADH and a model azo dye, Methyl Red, with AzoA. Methyl Red and NADH interacted in a unique binding mode in which the benzoic acid moiety of Methyl Red and the nicotinamide ring of NADH were not parallel to the flavin isoalloxazine ring, but lay against it at angles of ∼45° and ∼35°, respectively. The adenine ribose moiety of NADH was surrounded by loop ℓ2 on chain B and α3 on chain A in a typical Rossmann fold. There were 12 and 19 amino acid residues that could participate in the binding of Methyl Red and NADH, respectively, especially the residues Tyr-129 and Asp-184. The functional perturbation effects of 13 residues, including Tyr-129 and Asp-184, were mapped to reconstruct the mutation network, which confirmed the proposed binding modes and also provided insights into the interaction among NADH, FMN and Methyl Red.

Recent advances in the development of tissue transglutaminase (TG2) inhibitors

Tissue transglutaminase (TG2) is a Ca(2+)-dependent enzyme and probably the most ubiquitously expressed member of the mammalian transglutaminase family. TG2 plays a number of important roles in a variety of biological processes. Via its transamidating function, it is responsible for the cross-linking of proteins by forming isopeptide bonds between glutamine and lysine residues. Intracellularly, Ca(2+) activation of the enzyme is normally tightly regulated by the binding of GTP. However, upregulated levels of TG2 are associated with many disease states like celiac sprue, certain types of cancer, fibrosis, cystic fibrosis, multiple sclerosis, Alzheimer's, Huntington's and Parkinson's disease. Selective inhibitors for TG2 both cell penetrating and non-cell penetrating would therefore serve as novel therapeutic tools for the treatment of these disease states. Moreover, they would provide useful tools to fully elucidate the cellular mechanisms TG2 is involved in and help comprehend how the enzyme is regulated at the cellular level. The current paper is intended to give an update on the recently discovered classes of TG2 inhibitors along with their structure-activity relationships. The biological properties of these derivatives, in terms of both activity and selectivity, will also be reported in order to translate their potential for future therapeutic developments.

Structural and phylogenetic analyses of the GP42 transglutaminase from Phytophthora sojae reveal an evolutionary relationship between oomycetes and marine Vibrio bacteria

Transglutaminases (TGases) are ubiquitous enzymes that catalyze selective cross-linking between protein-bound glutamine and lysine residues; the resulting isopeptide bond confers high resistance to proteolysis. Phytophthora sojae, a pathogen of soybean, secretes a Ca(2+)-dependent TGase (GP42) that is activating defense responses in both host and non-host plants. A GP42 fragment of 13 amino acids, termed Pep-13, was shown to be absolutely indispensable for both TGase and elicitor activity. GP42 does not share significant primary sequence similarity with known TGases from mammals or bacteria. This suggests that GP42 has evolved novel structural and catalytic features to support enzymatic activity. We have solved the crystal structure of the catalytically inactive point mutant GP42 (C290S) at 2.95 Å resolution and identified residues involved in catalysis by mutational analysis. The protein comprises three domains that assemble into an elongated structure. Although GP42 has no structural homolog, its core region displays significant similarity to the catalytic core of the Mac-1 cysteine protease from Group A Streptococcus, a member of the papain-like superfamily of cysteine proteases. Proteins that are taxonomically related to GP42 are only present in plant pathogenic oomycetes belonging to the order of the Peronosporales (e.g. Phytophthora, Hyaloperonospora, and Pythium spp.) and in marine Vibrio bacteria. This suggests that a lateral gene transfer event may have occurred between bacteria and oomycetes. Our results offer a basis to design and use highly specific inhibitors of the GP42-like TGase family that may impair the growth of important oomycete and bacterial pathogens.

Transglutaminase-mediated methods for site-selective modification of human growth hormone

Two methods for the site-selective modification of native human growth hormone (hGH) using microbial transglutaminase were developed. In the first method, 1,3-bisaminoxypropane was attached to hGH, providing a direct incorporation of reactive aminoxy groups for further modification. The reaction was shown to be selective for Gln(141), with minor modification at Gln(40). In the second method, modified glutamine substrates were developed for attachment to Lys(145) in hGH. A series of glutamine-substrates were screened, and it was shown that microbial transglutaminase was selective towards substitutions on the glutamine core structure. Products from both methods could be transformed to site selectively mono-PEGylated hGH-derivatives in good isolated yield.

Synthesis, enzymatic evaluation, and docking studies of fluorogenic caspase 8 tetrapeptide substrates

The synthesis, enzymatic evaluation, and molecular modeling studies of new fluorogenic tetrapeptide-based substrates selective for caspase 8, having the general structure Ac-IETD-AXX, are described. Various fluorescent reporter groups (AXX), i.e., 3- and 4-substituted coumarins and quinolin-2(1H)-ones were synthesized by von Pechmann condensation. They were subsequently coupled with the caspase-8-selective tetrapeptide Ac-IETD-OH under newly developed synthetic conditions to give the desired substrates in good yields and in high enantiomeric purity. Based on K(M) and V(max) values, the new compounds proved to be excellent substrates for recombinant human caspase 8. In contrast, the K(M) values for the same compounds as substrates for human caspase 3 were approximately 10-20-fold higher. Molecular modeling studies based on the X-ray crystal structures of both human caspases 3 and 8 revealed that there is sufficient room within both active sites to accommodate substrates with moderately bulky substituents in the 3- and 4-positions of the fluorogenic coumarins and quinolin-2(1H)-ones. Automated docking of the substrates into the active sites of both human caspases 3 and 8 with the program AutoDock 3 gave structures similar to the published crystallographic structures for the same tetrapeptide bound to caspase 8 in the form of an irreversible inhibitor. The calculated binding energies for the new substrates to either caspase 3 or 8 showed little difference between the substrates, consistent with the K(M) data. In addition, the calculated binding energies (DeltaG) to caspase 8 were considerably more negative than those to caspase 3, also consistent with the K(M) data. A possible molecular interaction that might explain the selectivity of the IETD tetrapeptide motif for caspase 8 over caspase 3 is discussed.

Photolabeling of tissue transglutaminase reveals the binding mode of potent cinnamoyl inhibitors

We have recently developed a new class of cinnamoyl derivatives as potent tissue transglutaminase (TG2) inhibitors. Herein, we report the synthesis of a diazirine derivative of these inhibitors and its application to the photolabeling of its binding site on guinea pig liver transglutaminase. Two novel homology models were generated for this commonly studied TG2, which differ in the conformational state they represent. Tryptic digest and mass spectrometric analysis of the photolabeling experiment showed that only residue Cys230 was labeled, and our homology models were used to visualize these results. This visualization suggested that Cys230 is somewhat more solvent-exposed in the "closed" conformation of TG2, compared to the "open" conformation. Docking experiments suggested binding modes consistent with the labeling pattern that would block access to the tunnel leading to the active site, consistent with the observed mode of inhibition. However, while these modeling simulations favored the closed conformation as the target of our cinnamoyl inhibitors, native PAGE experiments indicated the open conformation of the enzyme in fact predominates in the presence of our photolabeling derivative. These results are important for understanding the binding modes of TG2 inhibitors in general and will be critical for the structure-based design of future inhibitors.

Synthesis of potent water-soluble tissue transglutaminase inhibitors

Dipeptide-based sulfonium peptidylmethylketones derived from 6-diazo-5-oxo-L-norleucine (DON) have been investigated as potential water-soluble inhibitors of extracellular transglutaminase. The lead compounds were prepared in four steps and exhibited potent activity against tissue transglutaminase.

Reversible and competitive cinnamoyl triazole inhibitors of tissue transglutaminase

A series of 15 cinnamoyl triazole derivatives was prepared by Cu(I)-catalyzed azide/alkyne [3+2]-cycloaddition reactions and examined as inhibitors of guinea-pig liver transglutaminase. Several compounds exhibited activity as reversible inhibitors that were competitive with acyl donor transglutaminase substrates. For example, triazole 4d has a K(i) value of 174 nM and represents one of the most potent reversible transglutaminase inhibitors reported to date.

The bioorganic chemistry of transglutaminase — from mechanism to inhibition and engineering

Through a multidisciplinary approach comprising organic synthesis, molecular biology, and physical organic kinetic studies, we have studied the mechanism of transglutaminase-mediated transamidation. More recently, we have applied our understanding of the mechanism to the design of reversible inhibitors and affinity labels for biological application. We have also undertaken the engineering of transglutaminase for its application as a “peptide synthase”. Herein, we present a brief overview of previously published work as well as recent results presented at the 2007 Merck–Frosst Centre for Therapeutic Research Award Lecture.Key words: transglutaminase, enzyme mechanism, enzyme inhibition, protein engineering

Gamma-glutamyl hydrolase: kinetic characterization of isopeptide hydrolysis using fluorogenic substrates

Gamma-glutamyl hydrolase, a cysteine peptidase, catalyzes the hydrolysis of poly-gamma-glutamate derivatives of folate cofactors and many antifolate drugs. We have used internally quenched fluorogenic derivatives of glutamyl-gamma-glutamate and (4,4-difluoro)glutamyl-gamma-glutamate to examine the effect of fluorine substitution adjacent to the scissile isopeptide bond. Using a newly developed continuous fluorescence assay, the hydrolysis of both substrates could be described by Michaelis-Menten kinetics. Fluorine substitution resulted in a significant decrease in observed rates of hydrolysis under steady-state conditions due primarily to a approximately 15-fold increase in Km. Using stopped-flow techniques, hydrolysis of the non-fluorinated isopeptide was characterized by a burst phase followed by a steady-state rate, indicating that formation of the acyl enzyme is not rate-limiting for hydrolysis of this isopeptide. This conclusion was confirmed by analysis of the progress curves over a wide range of substrate concentration, which demonstrated that the acylation rate (k2) is approximately 10-fold higher than the deacylation rate (k3). The increased value of Km associated with the difluoro derivative limited the ability to obtain comparable pre-steady-state kinetics data at saturating concentration of substrate due to inner filter effects. However, even under nonsaturating conditions, a modest burst was observed for the difluoro derivative. These data indicate that either deacylation or rearrangement of the enzyme-product complex is rate-limiting in this isopeptide hydrolysis reaction.

A highly sensitive fluorometric assay for determination of human coagulation factor XIII in plasma

Based on the iso-peptidase activity of human plasma FXIII, a novel fluorometric assay that determines FXIII concentrations in human plasma below 0.05 IU/ml is introduced. We considered a peptide sequence derived from alpha(2)-antiplasmin (n =12) to yield high sensitivity. Peptide Abz-NE(Cad-Dnp)EQVSPLTLLK exhibits a K(m) value of 19.8+/-2.8 microM and is used in a concentration of 50 microM. The assay design is suitable for measurements in cuvettes (1 ml volume) as well as for the microtiter plate (MTP) format (0.2 ml volume). It provides linear dose-response curves over a wide range of FXIII concentrations (0.05-8.8 IU/ml). The assay was validated with respect to precision, detection and quantitation limits, accuracy/specificity, linearity, and range. A comparison of the fluorometric assay with the photometric assay for FXIII determinations in plasma pools as well as single donor plasma revealed suitability of the fluorometric assay for FXIII determination in plasma of healthy individuals. FXIII concentrations in plasma samples of patients with severe FXIII deficiency are discussed in the context of FXIII antigen levels. These assays correlate well in the critical range below 0.1 IU/ml, whereas the photometric assay may overestimate residual FXIII activity in severe FXIII-deficient patients.

Acyl transfer from carboxylate, carbonate, and thiocarbonate esters to enzymatic and nonenzymatic thiolates

Transglutaminases (EC 2.3.2.13) (TGases) catalyze calcium-dependent acyl transfer reactions between peptide-bound glutamine residues as acyl donors and peptide-bound lysine residues as acyl acceptors, resulting in the formation of intermolecular ε-(γ-glutamyl)lysine crosslinks. The mechanistic details of its "ping-pong" transamidation reaction remain unknown. In particular, few studies have been published probing the nucleophilicity of TGase using acyl-donor substrates of varied electrophilicity. Herein we report the synthesis of activated esters of carbonates, carbamates, and thiocarbonates and their reactions with simple thiols, as a nonenzymatic point of reference, and with the catalytic cysteine residue of guinea pig liver TGase. Our kinetic results show that the simple substitution of a side chain methylene unit by oxygen or sulphur had a surprising effect on both substrate affinity and acylation reactivity. Furthermore, they provide unexpected insight into the importance of a side chain heteroatom for conferring affinity for tissue TGase as well as revealing an interesting class of irreversible inhibitors.Key words: enzyme kinetics, enzyme inhibition, transglutaminase, acyl-transfer reactions, carbamate, thiocarbonate, carbonate.

Synthesis and evaluation of peptidic maleimides as transglutaminase inhibitors

A series of novel transglutaminase inhibitors was prepared, based on the scaffold of a commonly used peptide substrate and bearing an electrophilic maleimide group. These compounds were evaluated in vitro and shown to lead to irreversible inactivation of tissue transglutaminase. Comparison with inhibitors studied previously provides insight into the steric environment of the enzyme active site.

Mechanism of allosteric regulation of transglutaminase 2 by GTP

Allosteric regulation is a fundamental mechanism of biological control. Here, we investigated the allosteric mechanism by which GTP inhibits cross-linking activity of transglutaminase 2 (TG2), a multifunctional protein, with postulated roles in receptor signaling, extracellular matrix assembly, and apoptosis. Our findings indicate that at least two components are involved in functionally coupling the allosteric site and active center of TG2, namely (i) GTP binding to mask a conformationally destabilizing switch residue, Arg-579, and to facilitate interdomain interactions that promote adoption of a compact, catalytically inactive conformation and (ii) stabilization of the inactive conformation by an uncommon H bond between a cysteine (Cys-277, an active center residue) and a tyrosine (Tyr-516, a residue located on a loop of the beta-barrel 1 domain that harbors the GTP-binding site). Although not essential for GTP-mediated inhibition of cross-linking, this H bond enhances the rate of formation of the inactive conformer.

Synthesis and evaluation of peptidic irreversible inhibitors of tissue transglutaminase

Herein we report the synthesis and the evaluation of eight novel compounds as irreversible inhibitors of transglutaminase (TGase). These compounds are based on a minimal peptidic scaffold shown previously [Chem. Biol.2005, 12, 469-475] to confer affinity for the TGase active site and bear electrophilic groups such as alpha,beta-unsaturated amide, chloroacetamide or maleimide; their general structure being Cbz-Phe-spacer-electrophile. The affinity conferred by the Cbz-Phe scaffold was determined by comparison to N-propylacrylamide and the length of the spacer was also varied to evaluate its importance. The inhibitory efficiencies (k(inact)/K(I)) of these compounds vary up to 10(5)M(-1)min(-1), among the highest reported for derivatives based on this simple Cbz-Phe peptidic scaffold.

Tissue transglutaminase inhibition

Khosla and coworkers report the synthesis of peptidic dihydroisoxazole derivatives, the in vitro evaluation of these novel compounds as inhibitors of recombinant human tissue transglutaminase (TG2), and their oral bioavailability and efficacy for the synergistic treatment of glioblastoma tumors.

The C-terminal Residues in the Alpha-interacting Domain (AID) Helix Anchor CaVβ Subunit Interaction and Modulation of CaV2.3 Channels

The alpha-interacting domain (AID) in the I-II linker of high voltage-activated (HVA) Ca(2+) channel alpha1 subunits binds with high affinity to Ca(V)beta auxiliary subunits. The recently solved crystal structures of the AID-Ca(V)beta complex in Ca(V)1.1/1.2 have revealed that this interaction occurs through a set of six mostly invariant residues Glu/Asp(6), Leu(7), Gly(9), Tyr(10), Trp(13), and Ile(14) (where the superscript refers to the position of the residue starting with the QQ signature doublet) distributed among three alpha-helical turns in the proximal section of the I-II linker. We show herein that alanine mutations of N-terminal AID residues Gln(1), Gln(2), Ile(3), Glu(4), Glu(6), Leu(7), and Gly(9) in Ca(V)2.3 did not abolish [(35)S]Ca(V)beta 1b or [(35)S]Ca(V)beta 3 subunit overlay binding to fusion proteins nor did they prevent the typical modulation of whole cell currents by Ca(V)beta 3. Mutations of the invariant Tyr(10) with either hydrophobic (Ala), aromatic (Phe), or positively charged (Arg, Lys) residues yielded Ca(V)beta 3-responsive whole cell currents, whereas mutations with negatively charged residues (Asp, Glu) disrupted Ca(V)beta 3 binding and modulation. In contrast, modulation and binding by Ca(V)beta 3 was significantly weakened in I14A (neutral and hydrophobic) and I14S (neutral and polar) mutants and eradicated in negatively charged I14D and I14E or positively charged I14R and I14K mutants. Ca(V)beta 3-induced modulation was only preserved with the conserved I14L mutation. Molecular replacement analyses carried out using a three-dimensional homology model of the AID helix from Ca(V)2.3 suggests that a high degree of hydrophobicity and a restrained binding pocket could account for the strict structural specificity of the interaction site found at position Ile(14). Altogether these results indicate that the C-terminal residues Trp(13) (1) and Ile(14) anchor Ca(V)beta subunit functional modulation of HVA Ca(2+) channels.