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S Pascarella

Publications and source records attributed to S Pascarella.

At least 19 recordsLinked to original sources
PubMed2001

Selective inhibition of human mast cell tryptase by gabexate mesylate, an antiproteinase drug.

Gabexate mesylate is a non-antigenic synthetic inhibitor of trypsin-like serine proteinases that is therapeutically used in the treatment of pancreatitis and disseminated intravascular coagulation and as a regional anticoagulant for hemodialysis. Considering the structural similarity between gabexate mesylate and arginine-based inhibitors of trypsin-like serine proteinases, the effect of gabexate mesylate on human and bovine mast cell tryptase action was investigated. Values of the inhibition constant (K(i)) for gabexate mesylate binding to human and bovine tryptase were 3.4 x 10(-9) M and 1.8 x 10(-7) M (at pH 7.4 and 37.0 degrees ), respectively. Furthermore, gabexate mesylate inhibited the fibrinogenolytic activity of human tryptase. On the basis of the available x-ray crystal structure of human tryptase, the possible binding mode of gabexate mesylate to human and bovine tryptase was analyzed. Human tryptase inhibition by gabexate mesylate may account for the reported prevention of inflammation, erosion, and ulceration of skin and mucosae.

Animals↗
PubMed2001

l-Threonine aldolase, serine hydroxymethyltransferase and fungal alanine racemase. A subgroup of strictly related enzymes specialized for different functions.

Serine hydroxymethyltransferase (SHMT) is a member of the fold type I family of vitamin B6-dependent enzymes, a group of evolutionarily related proteins that share the same overall fold. The reaction catalysed by SHMT, the transfer of Cbeta of serine to tetrahydropteroylglutamate (H4PteGlu), represents in the cell an important link between the breakdown of amino acids and the metabolism of folates. In the absence of H4PteGlu and when presented with appropriate substrate analogues, SHMT shows a broad range of reaction specificity, being able to catalyse at appreciable rates retroaldol cleavage, racemase, aminotransferase and decarboxylase reactions. This apparent lack of specificity is probably a consequence of the particular catalytic apparatus evolved by SHMT. An interesting question is whether other fold type I members that normally catalyse the reactions which for SHMT could be considered as 'forced errors', may be close relatives of this enzyme and have a catalytic apparatus with the same basic features. As shown in this study, l-threonine aldolase from Escherichia coli is able to catalyse the same range of reactions catalysed by SHMT, with the exception of the serine hydroxymethyltransferase reaction. This observation strongly suggests that SHMT and l-threonine aldolase are closely related enzymes specialized for different functions. An evolutionary analysis of the fold type I enzymes revealed that SHMT and l-threonine aldolase may actually belong to a subgroup of closely related proteins; fungal alanine racemase, an extremely close relative of l-threonine aldolase, also appears to be a member of the same subgroup. The construction of three-dimensional homology models of l-threonine aldolase from E. coli and alanine racemase from Cochliobolus carbonum, and their comparison with the SHMT crystal structure, indicated how the tetrahydrofolate binding site might have evolved and offered a starting point for further investigations.

Alanine Racemase↗
PubMed2001

Structural adaptation of enzymes to low temperatures.

A systematic comparative analysis of 21 psychrophilic enzymes belonging to different structural families from prokaryotic and eukaryotic organisms is reported. The sequences of these enzymes were multiply aligned to 427 homologous proteins from mesophiles and thermophiles. The net flux of amino acid exchanges from meso/thermophilic to psychrophilic enzymes was measured. To assign the observed preferred exchanges to different structural environments, such as secondary structure, solvent accessibility and subunit interfaces, homology modeling was utilized to predict the secondary structure and accessibility of amino acid residues for the psychrophilic enzymes for which no experimental three-dimensional structure is available. Our results show a clear tendency for the charged residues Arg and Glu to be replaced at exposed sites on alpha-helices by Lys and Ala, respectively, in the direction from 'hot' to 'cold' enzymes. Val is replaced by Ala at buried regions in alpha-helices. Compositional analysis of psychrophilic enzymes shows a significant increase in Ala and Asn and a decrease in Arg at exposed sites. Buried sites in beta-strands tend to be depleted of VAL: Possible implications of the observed structural variations for protein stability and engineering are discussed.

Alanine↗
PubMed2000

Structure-function relationships in sorcin, a member of the penta EF-hand family. Interaction of sorcin fragments with the ryanodine receptor and an Escherichia coli model system.

Sorcin, a 21.6 kDa cytosolic EF-hand protein which undergoes a Ca(2+)-induced translocation from cytoplasm to membranes, has been assigned to the newly defined penta EF-hand family. A molecular model of the C-terminal Ca(2+)-binding domain has been generated using as a template the X-ray coordinates of the corresponding domain in the calpain light subunit, the family prototype [Lin, G., et al. (1997) Nat. Struct. Biol. 4, 539-546]. The model indicates that in sorcin the three-dimensional structure is conserved and in particular that of EF1, the novel EF-hand motif characteristic of the family. On this basis, two stable fragments have been obtained and characterized. Just like the native protein, the sorcin Ca(2+)-binding domain (residues 33-198) is largely dimeric, interacts with the ryanodine receptor at physiological calcium concentrations, and undergoes a reversible, Ca(2+)-dependent translocation from cytosol to target proteins on Escherichia coli membranes. In contrast, the 90-198 fragment (residues 90-198), which lacks EF1 and EF2, does not bind Ca(2+) with high affinity and is unable to translocate. Binding of calcium to the EF1-EF2 pair is therefore required for the activation of sorcin which uses the C-terminal calcium-binding domain for interaction with the ryanodine receptor, a physiological target in muscle cells.

Amino Acid Sequence↗
PubMed2000

Aspartate aminotransferase from the Antarctic bacterium Pseudoalteromonas haloplanktis TAC 125. Cloning, expression, properties, and molecular modelling.

The gene encoding aspartate aminotransferase from the psychrophilic bacterium Pseudoalteromonas haloplanktis TAC 125 was cloned, sequenced and overexpressed in Escherichia coli. The recombinant protein (PhAspAT) was characterized both at the structural and functional level in comparison with the E. coli enzyme (EcAspAT), which is the most closely related (52% sequence identity) bacterial counterpart. PhAspAT is rapidly inactivated at 50 degrees C (half-life = 6.8 min), whereas at this temperature EcAspAT is stable for at least 3 h. The optimal temperature for PhAspAT activity is approximately 64 degrees C, which is some 11 degrees C below that of EcAspAT. The protein thermal stability was investigated by following changes in both tryptophan fluorescence and amide ellipticity; this clearly suggested that a first structural transition occurs at approximately 50 degrees C for PhAspAT. These results agree with the expected thermolability of a psychrophilic enzyme, although the observed stability is much higher than generally found for enzymes isolated from cold-loving organisms. Furthermore, in contrast with the higher efficiency exhibited by several extracellular psychrophilic enzymes, both kcat and kcat/Km of PhAspAT are significantly lower than those of EcAspAT over the whole temperature range. This behaviour possibly suggests that the adaptation of this class of endocellular enzymes to a cold environment may have only made them less stable and not more efficient. The affinity of PhAspAT for both amino-acid and 2-oxo-acid substrates decreases with increasing temperature. However, binding of maleate and 2-methyl-L-aspartate, which both inhibit the initial steps of catalysis, does not change over the temperature range tested. Therefore, the observed temperature effect may occur at any of the steps of the catalytic mechanism after the formation of the external aldimine. A molecular model of PhAspAT was constructed on the basis of sequence homology with other AspATs. Interestingly, it shows no insertion or extension of loops, but some cavities and a decrease in side chain packing can be observed.

Amino Acid Sequence↗
PubMed2000

Characterization of five new low-molecular-mass trypsin inhibitors from white mustard (Sinapis alba L.) seed.

Five new low-molecular-mass trypsin inhibitors belonging to the RTI/MTI-2 family were identified from white mustard (Sinapis alba L. ; MTI-2) seed. Purified MTI-2 consisted of a peptide mixture, displaying Ile or Arg at position 43, Trp or kynurenine (Kyn) at position 44, and C-terminal ragged ends. The occurrence of Ile or Arg at position 43 suggested that MTI-2 inhibitors originated from different genes. The presence of 5-oxo-proline (pyroglutamic acid; 5-oxoPro1) and Kyn44 reflected post-translational processing of the serine proteinase inhibitor. MTI-2 showed approximately 70% amino-acid identity with low-molecular-mass trypsin inhibitors isolated from oil rape (Brassica napus var. oleifera; RTI-III) seed and with serine proteinase inhibitors mapped in Arabidopsis thaliana chromosome II (ATTI). Furthermore, MTI-2 was homologous to brazzein, the sweet-tasting protein from Pentadiplandra brazzeana Baillon fruit ( approximately 30% amino-acid identity). Although snake-venom toxins showed a low amino-acid identity (< 20%) with MTI-2, RTI-III, and ATTI, some structurally relevant residues were conserved. The disulfide bridge pattern of MTI-2 (Cys5-Cys27, Cys18-Cys31, Cys42-Cys52, and Cys54-Cys57) corresponded to that of RTI-III and of snake-venom toxins, being different from that of brazzein. Therefore, protein similarity might be attributable to the three-dimensional arrangement rather than to the amino-acid sequence. Values of Ka for MTI-2 binding to bovine beta-trypsin (trypsin) and bovine alpha-chymotrypsin were 6.3 x 109 M-1 and 2.0 x 106 M-1, respectively, at pH 8.0 and 21.0 degrees C. Moreover, values of kon for MTI-2 binding to trypsin and of koff for the dissociation of the serine proteinase:inhibitor complex were 5.6 x 105 M-1.s-1 and 8.9 x 10-5 M-1.s-1, respectively, at pH 8.0 and 21.0 degrees C. Despite the heterogeneity of the purified inhibitor peptide mixture, the inhibition properties of the different MTI-2 inhibitors were indistinguishable.

Amino Acid Sequence↗
PubMed1999

Characterization of low-molecular-mass trypsin isoinhibitors from oil-rape (Brassica napus var. oleifera) seed.

A new low-molecular-mass (6767.8 Da) serine proteinase isoinhibitor has been isolated from oil-rape (Brassica napus var. oleifera) seed, designated 5-oxoPro1-Gly62-RTI-III. The 5-oxoPro1-Gly62-RTI-III isoinhibitor is longer than the Asp2-Pro61-RTI-III and the Ser3-Pro61-RTI-III forms, all the other amino acid residues being identical. In RTI-III isoinhibitors, the P1-P1' reactive site bond (where residues forming the reactive site have been identified as PnellipsisP1 and P1'ellipsisPn', where P1-P1' is the inhibitor scissile bond) has been identified at position Arg21-Ile22. The inhibitor disulphide bridges pattern has been determined as Cys5-Cys27, Cys18-Cys31, Cys42-Cys52 and Cys54-Cys57. The disulphide bridge arrangement observed in the RTI-III isoinhibitors is reminiscent of that found in a number of toxins (e.g. erabutoxin b). Moreover, the organization of the three disulphide bridges subset Cys5-Cys27, Cys18-Cys31 and Cys42-Cys52 is reminiscent of that found in epidermal growth factor domains. Preliminary 1H-NMR data indicates the presence of alphaalphaNOEs and 3JalphaNH coupling constants, typical of the beta-structure(s). These data suggest that the three-dimensional structure of the RTI-III isoinhibitors may be reminiscent of that of toxins and epidermal growth factor domains, consisting of three-finger shaped loops extending from the crossover region. Values of the apparent association equilibrium constant for RTI-III isoinhibitors binding to bovine beta-trypsin and bovine alpha-chymotrypsin are 3.3 x 109 m-1 and 2.4 x 106 m-1, respectively, at pH 8.0 and 21.0 degrees C. The serine proteinase : inhibitor complex formation is a pH-dependent entropy-driven process. RTI-III isoinhibitors do not show any similarity to other serine proteinase inhibitors except the low molecular mass white mustard trypsin isoinhibitor, isolated from Sinapis alba L. seed (MTI-2). Therefore, RTI-III and MTI-2 isoinhibitors could be members of a new class of plant serine proteinase inhibitors.

Amino Acid Sequence↗
PubMed1999

Homology modeling of the multicopper oxidase Fet3 gives new insights in the mechanism of iron transport in yeast.

Fet3, the multicopper oxidase of yeast, oxidizes extracellular ferrous iron which is then transported into the cell through the permease Ftr1. A three-dimensional model structure of Fet3 has been derived by homology modeling. Fet3 consists of three cupredoxin domains joined by a trinuclear copper cluster which is connected to the blue copper site located in the third domain. Close to this site, which is the primary electron acceptor from the substrate, residues for a potential iron binding site could be identified. The surface disposition of negatively charged residues suggests that Fet3 can translocate Fe(3+) to the permease Ftr1 through a pathway under electrostatic guidance.

Amino Acid Sequence↗
PubMed1999

The T-knot motif revisited.

The T-knot scaffold, a disulphide-reinforced structural motif shared by several proteins with very different biological functions, has been defined as 'a stretch of the protein chain which comprises two strands of a beta-sheet and three loops, knotted by two disulphides into the shape of the letter T'. In this communication we show that the presence of a central beta-sheet is not a required structural feature for proteins sharing the T-knot topology. Moreover, superposition of the three-dimensional structures of representative members of the T-knot family highlights a common and structurally well-defined core, formed by the two knotted disulphides, substituting for a larger residue-based hydrophobic core. These results suggest that folding and stability of the T-knot scaffold mainly depend on the geometry of the two knotted disulphides and on the loop length, and that the secondary structure elements are not a prerequisite for motif formation. Accordingly, a redefinition of the T-knot motif is proposed.

Amino Acid Sequence↗
PubMed1998

Easy method to predict solvent accessibility from multiple protein sequence alignments.

An easy and uncomplicated method to predict the solvent accessibility state of a site in a multiple protein sequence alignment is described. The approach is based on amino acid exchange and compositional preference matrices for each of three accessibility states: buried, exposed, and intermediate. Calculations utilized a modified version of the 3D_ali databank, a collection of multiple sequence alignments anchored through protein tertiary structural superpositions. The technique achieves the same accuracy as much more complex methods and thus provides such advantages as computational affordability, facile updating, and easily understood residue substitution patterns useful to biochemists involved in protein engineering, design, and structural prediction. The program is available from the authors; and, due to its simplicity, the algorithm can be readily implemented on any system. For a given alignment site, a hand calculation can yield a comparative prediction.

Amino Acid Sequence↗
PubMed1998

Fast-reacting thiols in rat hemoglobins can intercept damaging species in erythrocytes more efficiently than glutathione.

The S-conjugation rates of the free-reacting thiols present on each component of rat hemoglobin with 5,5-dithio-bis(2,2-nitrobenzoic acid) (DTNB) have been studied under a variety of conditions. On the basis of their reactivity with DTNB (0.5 mM), three classes of thiols have been defined as follows: fast reacting (fHbSH), with t1/2 <100 ms; slow reacting (sHbSH), with t1/2 30-50 s; and very slow reacting (vsHbSH), with t1/2 180-270 s. Under paraphysiological conditions, fHbSH (identified with Cys-125beta(H3)) conjugates with DTNB 100 times faster than glutathione and approximately 4000 times more rapidly than (v)sHbSH (Cys-13alpha(A11) and Cys-93beta(F9)). Such characteristics of fHbSH reactivity that are independent of the quaternary state of hemoglobin are mainly due to the following: (i) its low pK (approximately 6.9, the cysteinyl anion being stabilized by a hydrogen bond with Ser-123beta(H1)) and (ii) the large exposure to the solvent (as measured by analysis of a model of the molecular surface) and make these thiols the kinetically preferred groups in rat erythrocytes for S-conjugation. In addition, because of the high cellular concentration (8 mM, i.e. four times that of glutathione), fHbSHs are expected to intercept damaging species in erythrocytes more efficiently than glutathione, thus adding a new physiopathological role (direct involvement in cellular strategies of antioxidant defense) to cysteinyl residues in proteins.

Animals↗
PubMed1998

The structure of serine hydroxymethyltransferase as modeled by homology and validated by site-directed mutagenesis.

We describe a model for the three-dimensional structure of E. coli serine hydroxymethyltransferase based on its sequence homology with other PLP enzymes of the alpha-family and whose tertiary structures are known. The model suggests that certain amino acid residues at the putative active site of the enzyme can adopt specific roles in the catalytic mechanism. These proposals were supported by analysis of the properties of a number of site-directed mutants. New active site features are also proposed for further experimental testing.

Amino Acid Sequence↗
PubMed1998

Leech antihemostatic proteins share the T-knot scaffold, a disulfide-reinforced structural motif.

The occurrence of similar topologies among unrelated proteins is an emerging theme in structural biology. Here we report that the T-knot scaffold, a disulfide-reinforced structural motif shared by knottins and EGF-like proteins, is also present in leech antihemostatic proteins. Our finding emphasizes the versatile nature of this small structural motif, representing a compact structural unit suitable for the diverse biological functions performed by knottins, EGF-like proteins and leech antihemostatic proteins.

Animals↗
PubMed1997

Stability of aspartate aminotransferase from Sulfolobus solfataricus.

Aspartate aminotransferase from Sulfolobus solfataricus (SsAspAT) is an extremely thermophilic and thermostable dimeric enzyme which retains its structure and reaches maximal activity at 100 degrees C. The structural stability of this protein was investigated by coupling isothermally and thermally induced denaturation studies to molecular modeling. Gel filtration analysis indicated that SsAspAT unfolds with an N2 reversible 2D mechanism. In the molecular model, a cluster of hydrophobic residues was shown at the interface between the subunits of SsAspAT and suggested this cluster as a structural feature stabilizing the enzyme quaternary structure. At 25 degrees C, SsAspAT is less resistant to guanidinium chloride-induced denaturation than the cytosolic aspartate aminotransferase from pig heart (cpAspAT), which was chosen as a mesophilic counterpart in the thermodynamic analysis since it shares with SsAspAT the two-state unfolding mechanism. Therefore, in the case of aspartate aminotransferases, thermal stability does not correlate with the stability against chemical denaturants. Isothermal denaturation curves at 25 degrees C and melting profiles recorded in the presence of guanidinium chloride showed that the delta G degrees (H2O) at 25 degrees C of SsAspAT exceeds that of cpAspAT by roughly 15 kJ/mol; the parameter delta n, related to the number of binding sites for the denaturant differentially exposed in unfolded and folded states, is higher for SsAspAT than for cpAspAT; and delta Cp is lower for the thermophilic enzyme than for the mesophilic one by 8 kJ/K.mol. These results are indicative of a less hydrophobic core for SsAspAT than cpAspAT. In agreement with this, the molecular model predicts that some charged side chains are buried in SsAspAT and interact to form an H-bond/ion-pair network.

Amino Acid Sequence↗
PubMed1996

BSTI, a trypsin inhibitor from skin secretions of Bombina bombina related to protease inhibitors of nematodes.

From skin secretions of the European frog Bombina bombina, a new peptide has been isolated that contains 60 amino acids, including 10 cysteine residues. Its sequence was determined by automated Edman degradation and confirmed by analysis of the cDNA encoding the precursor. A search in the databanks demonstrated that the pattern of cysteine residues in this skin peptide is similar to the ones found in protease inhibitors from Ascaris and in a segment of human von Willebrand factor. The 3D structure of the trypsin inhibitor from Ascaris suum could be used as a template to build a model of the amphibian peptide. In addition, we have demonstrated that this constituent of skin secretion is indeed an inhibitor of trypsin and thrombin, with K(i) values in the range of 0.1 to 1 microM. The new peptide was thus named BSTI for Bombina skin trypsin/thrombin inhibitor.

Amino Acid Sequence↗
PubMed1995

Active site model for gamma-aminobutyrate aminotransferase explains substrate specificity and inhibitor reactivities.

A homology model for the pig isozyme of the pyridoxal phosphate-dependent enzyme gamma-aminobutyrate (GABA) aminotransferase has been built based mainly on the structure of dialkylglycine decarboxylase and on a multiple sequence alignment of 28 evolutionarily related enzymes. The proposed active site structure is presented and analyzed. Hypothetical structures for external aldimine intermediates explain several characteristics of the enzyme. In the GABA external aldimine model, the pro-S proton at C4 of GABA, which abstracted in the 1,3-azaallylic rearrangement interconverting the aldimine and ketimine intermediates, is oriented perpendicular to the plane of the pyridoxal phosphate ring. Lys 329 is in close proximity and is probably the general base catalyst for the proton transfer reaction. The carboxylate group of GABA interacts with Arg 192 and Lys 203, which determine the specificity of the enzyme for monocarboxylic omega-amino acids such as GABA. In the proposed structure for the L-glutamate external aldimine, the alpha-carboxylate interacts with Arg 445. Glu 265 is proposed to interact with this same arginine in the GABA external aldimine, enabling the enzyme to act on omega-amino acids in one half-reaction and on alpha-amino acids in the other. The reactivities of inhibitors are well explained by the proposed active site structure. The R and S isomers of beta-substituted phenyl and p-chlorophenyl GABA would bind in very different modes due to differential steric interactions, with the reactive S isomer leaving the orientation of the GABA moiety relatively unperturbed compared to that of the natural substrate. In our model, only the reactive S isomer of the mechanism-based inhibitor vinyl-GABA, an effective anti-epileptic drug known clinically as Vigabatrin, would orient the scissile C4-H bond perpendicular to the coenzyme ring plane and present the proton to Lys 329, the proposed general base catalyst of the reaction. The R isomer would direct the vinyl group toward Lys 329 and the C4-H bond toward Arg 445. The active site model presented provides a basis for site-directed mutagenesis and drug design experiments.

4-Aminobutyrate Transaminase↗
PubMed1994

Similarity between pyridoxal/pyridoxamine phosphate-dependent enzymes involved in dideoxy and deoxyaminosugar biosynthesis and other pyridoxal phosphate enzymes.

A multiple sequence alignment among aspartate aminotransferase, dialkylglycine decarboxylase, and serine hydroxymethyltransferase (DAS) was used for profile databank search. The DAS profile could detect similarities to other pyridoxal or pyridoxamine phosphate-dependent enzymes, like several gene products involved in dideoxysugar and deoxyaminosugar synthesis. The alignment among DAS and such gene products shows the conservation of aspartate 222 and lysine 258, which, in aspartate aminotransferase, interacts with the N1 of the coenzyme pyridine ring and forms the internal Schiff base, respectively. The lysine is replaced by histidine in the pyridoxamine phosphate-dependent gene products. The alignment indicates also that the region encompassing the coenzyme binding site is the most conserved.

Amino Acid Sequence↗