Ca2+ binding in proteins of the calmodulin superfamily: cooperativity, electrostatic contributions and molecular mechanisms

In a large number of intracellular regulatory proteins of the calmodulin superfamily a pair of closely interacting helix-loop-helix Ca2+ binding sites ('EF hands') constitute the functional unit--an arrangement that enables cooperative binding. We have recently made detailed experimental studies of the binding of Ca2+ ions to calmodulin, its tryptic fragments TR1C and TR2C (which each constitute a globular domain of a pair of EF hands) and calbindin D9k. Macroscopic Ca2+ binding constants have been obtained over a range of ionic strengths (0 to 0.15 M KCl). For calmodulin the measurements indicate that the two separate globular domains TR1C and TR2C retain the Ca2+ binding properties they have in the intact molecule, with positive cooperativity within each domain. The absolute value of the free energy of interaction between the two sites in each domain, a measure of the cooperativity, increases with ionic strength and is greater than or equal to 10 kJ mol-1 at 0.15 M KCl. Two-dimensional 1H NMR studies show that the addition of KCl does not alter the conformation of the protein. In the case of calbindin D9k several categories of mutants have been studied. One group encompasses the effect of protein surface charges 5 to 15 A from the Ca2+ binding sites. Two-dimensional 1H NMR shows that neither the addition of KCl, nor mutations that neutralize the surface charges, change the protein conformation. Although the global structure of calbindin D9k is largely unchanged upon binding of calcium, the structure with only one cation bound is more similar to the (Ca2+)2 form. Interestingly, the dynamical properties of the Ca(2+)-free and the (Ca2+)2-forms of calbindin differ greatly. For example, the rate of NH/ND exchange of the Ca(2+)-free form is on average 200 times faster than that of the (Ca2+)2-form. The results obtained so far point to a non-negligible entropic contribution to the observed cooperativity of Ca2+ binding.

Streptococcal M protein: structural studies of the hypervariable region, free and bound to human C4BP

Streptococcus pyogenes is a Gram-positive bacterium that causes several diseases, including acute tonsillitis and toxic shock syndrome. The surface-localized M protein, which is the most extensively studied virulence factor of S. pyogenes, has an approximately 50-residue N-terminal hypervariable region (HVR) that plays a key role in the escape of the host immunity. Despite the extensive sequence variability in this region, many HVRs specifically bind human C4b-binding protein (C4BP), a plasma protein that inhibits complement activation. Although the more conserved parts of M protein are known to have dimeric coiled-coil structure, it is unclear whether the HVR also is a coiled coil. Here, we use nuclear magnetic resonance (NMR) to study the conformational properties of HVRs from M4 and M22 proteins in isolation and in complex with the M protein binding portion of C4BP. We conclude that the HVRs of M4 and M22 are folded as coiled coils and that the folded nucleus of the M4 HVR has a length of approximately 27 residues. Moreover, we demonstrate that the C4BP binding surface of M4-N is found within a region of four heptad repeats. Using molecular modeling, we propose a model for the structure of the M4 HVR that is consistent with our experimental information from NMR spectroscopy.

Interaction of levosimendan with cardiac troponin C in the presence of cardiac troponin I peptides

The interaction between troponin C (TnC) and troponin I (TnI) is essential for the regulation of muscle contraction. There are several binding sites for TnI on TnC that are differentially occupied depending on the phase of the contraction/relaxation cycle. TnI and TnC interact in an antiparallel fashion with each other. The C-domain of cTnC and the N-domain region of cTnI(residues 33-70) always interact under physiological conditions, whereas the interaction between regulatory regions of TnC and TnI (residues 128-166) is calcium dependent. Previously, it has been shown that levosimendan, a calcium sensitizer used as a treatment for acute heart failure, can interact with both domains of isolated cTnC. To understand which interaction is relevant for the mechanism of calcium sensitization, we used a more complete troponin model obtained by complexing cTnI(32-79) and cTnI(128-180) with calcium-saturated cTnC(CS). The cTnI peptides bound to cTnC(CS) to form a 1:1:1 complex. The interaction of levosimendan with this complex was followed by 1H-(15)N heteronuclear correlation spectroscopy. It was clear that based on chemical shift changes, cTnI(32-79) blocked the levosimendan interaction sites on the C-domain, whereas cTnI(128-180) did not compete with levosimendan for the binding site on the N-domain. Hence, the effective binding site of levosimendan on cTnC resulting in the calcium-sensitizing effect is located in the regulatory domain (N-domain).

Isolated hypervariable regions derived from streptococcal M proteins specifically bind human C4b-binding protein: implications for antigenic variation

Antigenic variation in microbial surface proteins represents an apparent paradox, because the variable region must retain an important function, while exhibiting extensive immunological variability. We studied this problem for a group of streptococcal M proteins in which the approximately 50-residue hypervariable regions (HVRs) show essentially no residue identity but nevertheless bind the same ligand, the human complement regulator C4b-binding protein (C4BP). Synthetic peptides derived from different HVRs were found to retain the ability to bind C4BP, implying that the HVR corresponds to a distinct ligand-binding domain that can be studied in isolated form. This finding allowed direct characterization of the ligand-binding properties of isolated HVRs and permitted comparisons between different HVRs in the absence of conserved parts of the M proteins. Affinity chromatography of human serum on immobilized peptides showed that they bound C4BP with high specificity and inhibition experiments indicated that different peptides bound to the same site in C4BP. Different C4BP-binding peptides did not exhibit any immunological cross-reactivity, but structural analysis suggested that they have similar folds. These data show that the HVR of streptococcal M protein can exhibit extreme variability in sequence and immunological properties while retaining a highly specific ligand-binding function.

Binding of levosimendan, a calcium sensitizer, to cardiac troponin C

Levosimendan is an inodilatory drug that mediates its cardiac effect by the calcium sensitization of contractile proteins. The target protein of levosimendan is cardiac troponin C (cTnC). In the current work, we have studied the interaction of levosimendan with Ca(2+)-saturated cTnC by heteronuclear NMR and small angle x-ray scattering. A specific interaction between levosimendan and the Ca(2+)-loaded regulatory domain of recombinant cTnC(C35S) was observed. The changes in the NMR spectra of the N-domain of full-length cTnC(C35S), due to the binding of levosimendan to the primary site, were indicative of a slow conformational exchange. In contrast, no binding of levosimendan to the regulatory domain of cTnC(A-Cys), where all the cysteine residues are mutated to serine, was detected. Moreover, it was shown that levosimendan was in fast exchange on the NMR time scale with a secondary binding site in the C-domain of both cTnC(C35S) and cTnC(A-Cys). The small angle x-ray scattering experiments confirm the binding of levosimendan to Ca(2+)-saturated cTnC but show no domain-domain closure. The experiments were run in the absence of the reducing agent dithiothreitol and the preservative sodium azide (NaN(3)), since we found that levosimendan reacts with these chemicals, commonly used for preparation of NMR protein samples.

The C-terminus of dUTPase: observation on flexibility using NMR

The dynamics of the C-terminus of the dUTPases from Escherichia coli and equine infectious anaemia virus (EIAV) were studied by 1H-(15)N nuclear magnetic resonance spectroscopy. The two enzymes differ with regard to flexibility in the backbone of the 15 most C-terminal amino acid residues, some of which are conserved and essential for enzymic activity. In the bacterial enzyme, the residues closest to the C-terminus are highly flexible and display a correlation time in the nanosecond time range. No similar high flexibility could be detected for the C-terminal part of EIAV dUTPase, indicating a different time range of flexibility.

Fragment complementation studies of protein stabilization by hydrophobic core residues

Interactions that stabilize the native state of a protein have been studied by measuring the affinity between subdomain fragments with and without site-specific residue substitutions. A calbindin D(9k) variant with a single CNBr cleavage site at position 43 between its two EF-hand subdomains was used as a starting point for the study. Into this variant were introduced 11 site-specific substitutions involving hydrophobic core residues at the interface between the two EF-hands. The mutants were cleaved with CNBr to produce wild-type and mutated single-EF-hand fragments: EF1 (residues 1--43) and EF2 (residues 44--75). The interaction between the two EF-hands was studied using surface plasmon resonance (SPR) technology, which follows the rates of association and dissociation of the complex. Wild-type EF1 was immobilized on a dextran matrix, and the wild-type and mutated versions of EF2 were injected at several different concentrations. In another set of experiments, wild-type EF2 was immobilized and wild-type or mutant EF1 was injected. Dissociation rate constants ranged between 1.1 x 10(-5) and 1.0 x 10(-2) s(-1) and the association rate constants between 2 x 10(5) and 4.0 x 10(6) M(-1) s(-1). The affinity between EF1 and EF2 was as high as 3.6 x 10(11) M(-1) when none of them was mutated. For the 11 hydrophobic core mutants, a strong correlation (r = 0.999) was found between the affinity of EF1 for EF2 and the stability toward denaturation of the corresponding intact protein. The observed correlation implies that the factors governing the stability of the intact protein also contribute to the affinity of the bimolecular EF1-EF2 complex. In addition, the data presented here show that interactions among hydrophobic core residues are major contributors both to the affinity between the two EF-hand subdomains and to the stability of the intact domain.

Characterization of apo and partially saturated states of calerythrin, an EF-hand protein from S. erythraea: a molten globule when deprived of Ca(2+)

Calerythrin, a four-EF-hand calcium-binding protein from Saccharopolyspora erythraea, exists in an equilibrium between ordered and less ordered states with slow exchange kinetics when deprived of Ca(2+) and at low temperatures, as observed by NMR. As the temperature is raised, signal dispersion in NMR spectra reduces, and intensity of near-UV CD bands decreases. Yet far-UV CD spectra indicate only a small decrease in the amount of secondary structure, and SAXS data show that no significant change occurs in the overall size and shape of the protein. Thus, at elevated temperatures, the equilibrium is shifted toward a state with characteristics of a molten globule. The fully structured state is reached by Ca(2+)-titration. Calcium first binds cooperatively to the C-terminal sites 3 and 4 and then to the N-terminal site 1, which is paired with an atypical, nonbinding site 2. EF-hand 2 still folds together with the C-terminal half of the protein, as deduced from the order of appearance of backbone amide cross peaks in the NMR spectra of partially Ca(2+)-saturated states.

Characterization of the EGF-like module pair 3-4 from vitamin K-dependent protein S using NMR spectroscopy reveals dynamics on three separate time scales and extensive effects from calcium binding

Protein S, a cofactor of anticoagulant activated protein C, exhibits three high-affinity Ca(2+)-binding sites in a region comprising four EGF modules. The EGF 3-4 module pair constitutes the smallest fragment that retains one high-affinity Ca(2+)-binding site and is therefore useful for investigation of the structural basis of the unusually high-affinity Ca(2+) binding compared to other EGF-containing proteins characterized so far. Extensive chemical shift effects caused by Ca(2+) binding to the EGF 3-4 module pair are observed, particularly from Ca(2+) binding to the high-affinity site in EGF 4. Ca(2+) binding to the high-affinity site in EGF 4 and the low-affinity site in EGF 3 is associated with slow and fast exchange on the NMR time-scale, respectively. We show the presence of two isoforms, characterized by a cis or trans Lys 167-Pro 168 peptide bond, that do not convert on time scales that were accessible to the experiments (k(ex) < 0.2 s(-1)). Both conformers have similar Ca(2+) affinities and backbone dynamics. Further, broadening of (1)H resonances involving residues in the major beta-sheet of EGF 3 and (15)N exchange terms, primarily in the N-terminal part of the protein, indicate the presence of slow exchange on a microsecond to millisecond time scale. (15)N spin relaxation data suggest that the module pair has a well-defined relative orientation between EGF modules 3 and 4 and has a significantly anisotropic rotational diffusion tensor in solution.

Conformations of the regulatory domain of cardiac troponin C examined by residual dipolar couplings

Conformations of the regulatory domain of cardiac troponin C (cNTnC) were studied by means of residual dipolar couplings measured from samples dissolved in dilute liquid crystals. Changes in the main chain HN residual dipolar couplings revealed a conformational change in cNTnC due to the complexation with the second binding region (amino acids 148-163) of cardiac troponin I (cTnI). Formation of the complex is accompanied with a molecular realignment in the liquid crystal. The residual dipolar couplings measured for apo-cNTnC and the complex with TnI were in agreement with the values computed from the corresponding closed and open solution structures, whereas for the calcium-loaded conformation the correlation and quality factor were only modest. Ca2+-cNTnC may be subject to conformational exchange. The data support the model that cardiac troponin C functions as a calcium-dependent open-closed switch, such as the skeletal troponin C.

1H, 15N and (13)C assignments and secondary structure of the EGF-like module pair 3-4 from vitamin K-dependent protein S

Vitamin K-dependent protein S, which is a cofactor for activated protein C and thus important for down-regulation of the coagulation cascade, contains several Ca(2+)-binding sites with unusually high affinity. The 89 amino acid fragment constituting the third and fourth epidermal growth factor-like (EGF) modules of protein S is the smallest fragment that retains high-affinity Ca(2+) binding and is therefore useful for investigating the structural basis of this property. Heteronuclear multidimensional nuclear magnetic resonance experiments were used to obtain extensive assignments of the (1)H, 15N and (13)C resonances of the module pair with one Ca(2+) bound in EGF 4. In addition, nearly complete assignments of the (1)H resonances of the isolated Ca(2+)-free EGF 3 module were obtained. The assignment process was complicated by broadening of several resonances, spectral heterogeneity caused by cis-trans isomerisation of the peptide bond preceding Pro-168, and dimerisation. Analysis of weighted average secondary chemical shifts, (3)J(HNHalpha) coupling constants, and NOE connectivities suggest that both EGF modules in this fragment adhere to the classical secondary structure of EGF modules, consisting of one major and one minor anti-parallel beta-sheet.

NMR assignments, secondary structure, and global fold of calerythrin, an EF-hand calcium-binding protein from Saccharopolyspora erythraea

Calerythrin is a 20 kDa calcium-binding protein isolated from gram-positive bacterium Saccharopolyspora erythraea. Based on amino acid sequence homology, it has been suggested that calerythrin belongs to the family of invertebrate sarcoplasmic EF-hand calcium-binding proteins (SCPs), and therefore it is expected to function as a calcium buffer. NMR spectroscopy was used to obtain structural information on the protein in solution. Backbone and side chain 1H, 13C, and 15N assignments were obtained from triple resonance experiments HNCACB, HN(CO)CACB, HNCO, CC(CO)NH, and [15N]-edited TOCSY, and HCCH-TOCSY. Secondary structure was determined by using secondary chemical shifts and characteristic NOEs. In addition, backbone N-H residual dipolar couplings were measured from a spin-state selective [1H, 15N] correlation spectrum acquired from a sample dissolved in a dilute liquid crystal. Four EF-hand motifs with characteristic helix-loop-helix patterns were observed. Three of these are typical calcium-binding EF-hands, whereas site 2 is an atypical nonbinding site. The global fold of calerythrin was assessed by dipolar couplings. Measured dipolar couplings were compared with values calculated from four crystal structures of proteins with sequence homology to calerythrin. These data allowed us to recognize an overall similarity between the folds of calerythrin and sarcoplasmic calcium-binding proteins from the sandworm Nereis diversicolor and the amphioxus Branchiostoma lanceolatum.

EGF-like module pair 3-4 in vitamin K-dependent protein S: modulation of calcium affinity of module 4 by module 3, and interaction with factor X

Calcium-binding epidermal growth factor (EGF)-like modules are found in numerous extracellular and membrane proteins involved in such diverse processes as blood coagulation, lipoprotein metabolism, determination of cell fate, and cell adhesion. Vitamin K-dependent protein S, a cofactor of the anticoagulant enzyme activated protein C, has four EGF-like modules in tandem with the three C-terminal modules each harbouring a Ca(2+)-binding consensus sequence. Recombinant fragments containing EGF modules 1-4 and 2-4 have two Ca(2+)-binding sites with dissociation constants ranging from 10(-8) to 10(-5) M. Module-module interactions that greatly influence the Ca(2+) affinity of individual modules have been identified. As a step towards an analysis of the structural basis of the high Ca(2+) affinity, we expressed the Ca(2+)-binding EGF pair 3-4 from human protein S. Correct folding was shown by (1)H NMR spectroscopy. Calcium-binding properties of the C-terminal module were determined by titration with chromophoric chelators; binding to the low-affinity N-terminal site was monitored by (1)H-(15)N NMR spectroscopy. At physiological pH and ionic strength, the dissociation constants for Ca(2+) binding were 1.0x10(-6) M and 4. 8x10(-3) M for modules 4 and 3, respectively, i.e. the calcium affinity of the C-terminal site was about 5000-fold higher than that of the N-terminal site. Moreover, the Ca(2+) affinity of EGF 4, in the pair 3-4, was about 9000-fold higher than that of synthetic EGF 4. The EGF modules in protein S are known to mediate the interaction with factor Xa. We have now found modules 3-4 to be involved in this interaction. However, the individual modules 3 and 4 manifested no measurable activity.

Battle for the EF-hands: magnesium-calcium interference in calmodulin

The ubiquitous Ca(2+)-regulatory protein calmodulin activates target enzymes as a response to submicromolar Ca(2+) increases in a background of millimolar Mg(2+). The potential influence of Mg(2+)/Ca(2+) competition is especially intriguing for the N-terminal domain of the protein which possesses the sites with the lowest Ca(2+) specificity. The interdependence of Ca(2+) and Mg(2+) binding in the N-terminal domain of calmodulin was therefore studied using (43)Ca NMR, (1)H-(15)N NMR, and fluorescent Ca(2+) chelator techniques. The apparent affinity for Ca(2+) was found to be significantly decreased at physiological Mg(2+) levels. At Ca(2+) concentrations of an activated cell the (Ca(2+))(2) state of the N-terminal domain is therefore only weakly populated, indicating that for this domain Ca(2+) binding is intimately associated with binding of target molecules. The data are in good agreement with a two-site model in which each site can bind either Ca(2+) or Mg(2+). The Mg(2+)-Ca(2+) binding interaction is slightly positively allosteric, resulting in a significantly populated (Mg(2+))(1)(Ca(2+))(1) state. The Ca(2+) off-rate from this state is determined to be at least one order of magnitude faster than from the (Ca(2+))(2) state. These two findings indicate that the (Mg(2+))(1)(Ca(2+))(1) state is structurally and/or dynamically different from the (Ca(2+))(2) state. The (43)Ca quadrupolar coupling constant and the (1)H and (15)N chemical shifts of the (Mg(2+))(1)(Ca(2+))(1) state were calculated from titration data. The values of both parameters suggest that the (Mg(2+))(1)(Ca(2+))(1) state has a conformation more similar to the "closed" apo and (Mg(2+))(2) states than to the "open" (Ca(2+))(2) state.

A comparative study of two retaining enzymes of Trichoderma reesei: transglycosylation of oligosaccharides catalysed by the cellobiohydrolase I, Cel7A, and the beta-mannanase, Man5A

HPLC, MALDI-TOF MS and NMR spectroscopy were used to investigate the hydrolysis of cello- and mannooligosaccharides by Cel7A and Man5A from Trichoderma reesei. The experimental progress curves were analysed by fitting the numerically integrated kinetic equations, which provided cleavage patterns for oligosaccharides. This data evaluation procedure accounts for product inhibition and avoids the initial slope approximation. In addition, a transglycosylation step had to be included in the model to reproduce the experimental progress curves. For the hydrolysis of manno-oligosaccharides, Man4-6, by Man5A no mannose was detected at the beginning of the reaction showing that only the internal linkages are hydrolysed. For cellotriose and cellotetraose hydrolysis by Cel7A, the main product is cellobiose and glucose is released from the non-reducing end of the substrate. Intermediary products longer than the substrates were detected by MALDI-TOF MS when oligosaccharides (Glc4-6 or Man4-6) were hydrolysed by either Cel7A or Man5A. Interestingly, two distinct transglycosylation pathways could be observed. Cel7A produced intermediates that are one unit longer than the substrate, whereas Man5A produced intermediates that are two units longer than the substrate.

Binding of peptides in solution by the Escherichia coli chaperone PapD as revealed using an inhibition ELISA and NMR spectroscopy

PapD is the prototype member of a family of periplasmic chaperones which are required for assembly of virulence associated pili in pathogenic, gram-negative bacteria. In the present investigation, an ELISA has been developed for evaluation of compounds as inhibitors of PapD. Synthetic peptides, including an octamer, derived from the C-terminus of the pilus adhesin PapG were able to inhibit PapD in the ELISA. Evaluation of a panel of octapeptides in the ELISA, in combination with NMR studies, showed that the peptides were bound as extended beta-strands by PapD in aqueous solution. The PapD-peptide complex was stabilized by backbone to backbone hydrogen bonds and interactions involving three hydrophobic peptide side chains. This structural information, together with previous crystal structure data, provides a starting point in efforts to design and synthesize compounds which bind to chaperones and interfere with pilus assembly in pathogenic bacteria.

When size is important. Accommodation of magnesium in a calcium binding regulatory domain

The accommodation of Mg2+ in the N-terminal domain of calmodulin was followed through amide 1H and 15N chemical shifts and line widths in heteronuclear single-quantum coherence spectroscopy NMR spectra. Mg2+ binds sequentially to the two Ca2+-binding loops in this domain, with affinities such that nearly half of the loops would be occupied by Mg2+ in resting eukaryotic cells. Mg2+ binding seems to occur without ligation to the residue in the 12th loop position, previously proven largely responsible for the major rearrangements induced by binding of the larger Ca2+. Consequently, smaller Mg2+-induced structural changes are indicated throughout the protein. The two Ca2+-binding loops have different Mg2+ binding characteristics. Ligands in the N-terminal loop I are better positioned for cation binding, resulting in higher affinity and slower binding kinetics compared with the C-terminal loop II (koff = 380 +/- 40 s-1 compared with approximately 10,000 s-1 at 25 degreesC). The Mg2+-saturated loop II undergoes conformational exchange on the 100-microseconds time scale. Available data suggest that this exchange occurs between a conformation providing a ligand geometry optimized for Mg2+ binding and a conformation more similar to that of the empty loop.

Solution structure of the cellulose-binding domain of endoglucanase I from Trichoderma reesei and its interaction with cello-oligosaccharides

The solution structure of a synthetic 38-residue cellulose-binding domain (CBD) of endoglucanase I from Trichoderma reesei (CBD(EGI)) was determined by two-dimensional 1H-NMR spectroscopy. 100 structures were generated from a total of 599 NOE derived distance restraints and 28 phi and 14 chi dihedral angle restraints. For the final set of 19 selected structures, the rms deviation about the mean structure was 0.83+/-0.26 A for all atoms and 0.50+/-0.22 A for the backbone atoms. The structure of CBD(EGI) was very similar to that of CBD of cellobiohydrolase I from T reesei (CBD(CBHI)). The backbone trace of CBD(EGI) followed closely the irregular triple-stranded antiparallel beta-sheet structure of CBD(CBHI). Moreover, apart from the different side chains of Trp7 (CBD(EGI)) and Tyr5 (CBD(CBHI)), the cellulose-binding face of CBD(EGI) was similar to that of CBD(CBHI) within the precision of the structures. Finally, the interaction between CBD(EGI) and soluble sugars was investigated using cellopentaose and cellohexaose as substrates. Experiments showed that the interactions between CBD(EGI) and cellobiose units of sugars are specific, supporting the previously presented model for the CBD binding to crystalline cellulose.

Softwood hemicellulose-degrading enzymes from Aspergillus niger: purification and properties of a beta-mannanase

The enzymes needed for galactomannan hydrolysis, i.e., beta-mannanase, alpha-galactosidase and beta-mannosidase, were produced by the filamentous fungus Aspergillus niger. The beta-mannanase was purified to electrophoretic homogeneity in three steps using ammonium sulfate precipitation, anion-exchange chromatography and gel filtration. The purified enzyme had an isoelectric point of 3.7 and a molecular mass of 40 kDa. Ivory nut mannan was degraded mainly to mannobiose and mannotriose when incubated with the beta-mannanase. Analysis by 1H NMR spectroscopy during hydrolysis of mannopentaose showed that the enzyme acts by the retaining mechanism. The N-terminus of the purified A. niger beta-mannanase was sequenced by Edman degradation, and comparison with Aspergillus aculeatus beta-mannanase indicated high identity. The enzyme most probably lacks a cellulose binding domain since it was unable to adsorb on cellulose.

Solution structure of the N-terminal EGF-like domain from human factor VII

Blood coagulation is initiated by Ca(2+)-dependent binding of coagulation factor VIIa (FVIIa) to its cofactor, tissue factor (TF). The TF:FVIIa complex activates factors IX and X, ultimately leading to the formation of thrombin and the coagulation of blood. FVII consists of an N-terminal gamma-carboxyglutamic-acid-containing (Gla) domain followed by two epidermal growth factor (EGF) like domains, the first of which can bind one Ca2+ ion (Kd approximately 150 microM) and a C-terminal serine protease domain. Using 1H nuclear magnetic resonance spectroscopy, we have determined the solution structure of a synthetic N-terminal EGF-like domain (EGF1) of human FVII (residues 45-85) in the absence of Ca2+. A comparison of this structure of apo EGF1 with the Ca(2+)-bound EGF1 in the complex of FVIIa and TF [Banner, D. W., et al. (1996) Nature 380, 41-46] suggests that the structural changes in the EGF1 domain upon Ca2+ binding are minor and are concentrated near the Ca(2+)-binding site, which is facing away from the TF interaction surface. Amino acid side chains that are crucial for the binding of FVII to TF show a similar conformation in both structures and are therefore unlikely to directly influence the Ca(2+)-dependent binding of FVII to TF. As Ca2+ binding to EGF1 does not lead to a conformational change in the residues constituting the interaction surface for binding to TF, our results are consistent with the idea that the altered orientation between the Gla and EGF1 domains that result from Ca2+ binding is responsible for the increased affinity of FVII/FVIIa for TF.

Solution structure and main chain dynamics of the regulatory domain (Residues 1-91) of human cardiac troponin C

The three-dimensional structure of calcium-loaded regulatory, i.e. N-terminal, domain (1-91) of human cardiac troponin C (cNTnC) was determined by NMR in water/trifluoroethanol (91:9 v/v) solution. The single-calcium-loaded cardiac regulatory domain is in a "closed" conformation with comparatively little exposed hydrophobic surface. Difference distance matrices computed from the families of Ca2+-cNTnC, the apo and two-calcium forms of the skeletal TnC (sNTnC) structures reveal similar relative orientations for the N, A, and D helices. The B and C helices are closer to the NAD framework in Ca2+-cNTnC and in apo-sNTnC than in 2.Ca2+-sNTnC. However, there is an indication of a conformational exchange based on broad 15N resonances for several amino acids measured at several temperatures. A majority of the amides in the alpha-helices and in the calcium binding loop exhibit very fast motions with comparatively small amplitudes according to the Lipari-Szabo model. A few residues at the N and C termini are flexible. Data were recorded from nonlabeled and 15N-labeled samples, and backbone dynamics was investigated by 15N T1, T2, and heteronuclear nuclear Overhauser effect as well as by relaxation interference measurements.

Tryptophan 272: an essential determinant of crystalline cellulose degradation by Trichoderma reesei cellobiohydrolase Cel6A

Trichoderma reesei cellobiohydrolase Cel6A (formerly CBHII) has a tunnel shaped active site with four internal subsites for the glucose units. We have predicted an additional ring stacking interaction for a sixth glucose moiety with a tryptophan residue (W272) found on the domain surface. Mutagenesis of this residue selectively impairs the enzyme function on crystalline cellulose but not on soluble or amorphous substrates. Our data shows that W272 forms an additional subsite at the entrance of the active site tunnel and suggests it has a specialised role in crystalline cellulose degradation, possibly in guiding a glucan chain into the tunnel.

Conformation of desmopressin, an analogue of the peptide hormone vasopressin, in aqueous solution as determined by NMR spectroscopy

Desmopressin (1-desamino-[DArg8]vasopressin, is a synthetic analogue of the neurohypophyseal peptide hormone vasopressin which has high antidiuretic and antibleeding potency. The structure of desmopressin has been determined in aqueous solution by two-dimensional NMR techniques and molecular dynamics simulations. Both standard and time-averaged distance restraints were used in structure calculations because of the inherent flexibility in small peptides. 21 models calculated with standard restraints were compared with structures refined with time-averaged distance restraints and were found to be good representatives of the conformational ensemble of desmopressin. The macrocyclic ring forms an inverse gamma-turn centered around Gln4. Residues 1 and 2, the disulphide bridge and the three-residue acyclic tail were found to be flexible in solution. Residues 4-6 in the ensemble of calculated structures contain essentially the same backbone conformation as in the crystal structure of pressinoic acid, the cyclic moiety of vasopressin, whereas residues 2-6 superimpose on the NMR-derived conformation of oxytocin bound to neurophysin. The results presented in this work suggest that, in addition to the differences in sequence between desmopressin and vasopressin, differences in conformational and dynamic properties between the two compounds explain their pharmacological differences.