Sequence and context dependence of EF-hand loop dynamics. An 15N relaxation study of a calcium-binding site mutant of calbindin D9k

The influence of amino acid sequence and structural context on the backbone dynamics of EF-hand calcium-binding loops was investigated using 15N spin relaxation measurements on the calcium-free state of the calbindin D9k mutant (A14D+A15Delta+P20Delta+N21G+P43M), in which the N-terminal pseudo-EF-hand loop, characteristic of S100 proteins, was engineered so as to conform with the C-terminal consensus EF-hand loop. The results were compared to a previous study of the apo state of the wild-type-like P43G calbindin D9k mutant. In the helical regions, the agreement with the P43G data is excellent, indicating that the structure and dynamics of the protein core are unaffected by the substitutions in the N-terminal loop. In the calcium-binding loops, the flexibility is drastically decreased compared to P43G, with the modified N-terminal loop showing a motional restriction comparable to that of the surrounding helixes. As in P43G, the motions in the C-terminal loop are less restricted than in the N-terminal loop. Differences in key hydrogen-bonding interactions correlate well with differences in dynamics and offer insights into the relationship between structure and dynamics of these EF-hand loops. It appears that the entire N-terminal EF-hand is built to form a rigid structure that allows calcium binding with only minor rearrangements and that the structural and dynamical properties of the entire EF-hand--rather than the loop sequence per se--is the major determinant of loop flexibility in this system.

Characterization of recombinant epidermal growth factor (EGF)-like modules from vitamin-K-dependent protein S expressed in Spodoptera cells--the cofactor activity depends on the N-terminal EGF module in human protein S

Epidermal growth factor (EGF)-like modules in protein S, a physiological anticoagulant protein that functions as a cofactor to activated protein C, have been expressed in Spodoptera cells using baculovirus. EGF modules 1-3, 1-4, 2-3 and 2-4 were produced on a preparative scale. The isolated modules were more than 95% homogeneous, as judged by sequence determination. 45Ca2+-ligand blotting experiments indicated that recombinant proteins that contained the fourth EGF module, i.e. EGF 1-4 and 2-4, bound Ca2+ with high affinity. The 45Ca2+-ligand blotting results, together with results of competitive binding experiments using monoclonal antibodies as structural probes, indicated that the recombinant proteins had been folded to a native conformation. EGF modules 1-3 and 1-4 inhibited the interaction between activated protein C and protein S, whereas modules 2-3 and 2-4 had no effect on this interaction. It is thus apparent that EGF module 1 is crucial for the interaction between protein S and activated protein C. Moreover, EGF modules 1-4 were approximately 10-fold more effective in inhibiting the interaction than modules 1-3, suggesting a very weak interaction between module 4 and activated protein C or that this module is important to keep module 1 in a conformation that is optimal for interaction with activated protein C.

The high affinity calcium-binding sites in the epidermal growth factor module region of vitamin K-dependent protein S

Vitamin K-dependent protein S, a cofactor of the anticoagulant enzyme-activated protein C, has four epidermal growth factor (EGF)-like modules, all of which have one partially hydroxylated Asp (EGF 1; beta-hydroxyaspartic acid) or Asn (EGF 2, 3, and 4; beta-hydroxyasparagine) residue. The three C-terminal modules have a typical Ca2+ binding sequence motif that is usually present in EGF modules with hydroxylated Asp/Asn residues. Using the chromophoric Ca2+ chelators Quin 2 and 5,5'-Br2BAPTA, we have now determined the Ca2+ affinity of recombinant fragments containing EGF modules 1-3, 1-4, 2-3, and 2-4. EGF modules 1-4 and 2-4 each contains two very high affinity Ca2+-binding sites, i.e. with dissociation constants ranging from 10(-10) to 10(-8) M in the absence of salt and from 10(-8) to 10(-6) M in the presence of 0.15 M NaCl. In contrast, in EGF 1-3 and EGF 2-3, the Ca2+ affinity is 2-4 orders of magnitude lower. EGF 4 thus appears to have the highest Ca2+ affinity, and furthermore it seems to influence the Ca2+ affinity of its immediate N-terminal neighbor EGF 3 by a factor of approximately 230. In addition, EGF 4 seems to influence the Ca2+ affinity of EGF 2 by a factor of approximately 25. The Ca2+ affinity of the binding sites in EGF modules 3 and 4 in fragments EGF 1-4 and EGF 2-4 is 10(3)-10(5)-fold higher than in the corresponding isolated modules, implying important contributions to the Ca2+ affinity of each module from interactions with neighboring modules. This difference is much higher than the approximately 10-fold difference previously found in similar comparisons of EGF modules from fibrillin. However, the modules studied in protein S and fibrillin appear to have the similar Ca2+ ligands. The structural basis for the difference in Ca2+ affinity is not yet understood.

Calcium-binding properties of the third and fourth epidermal-growth-factor-like modules in vitamin-K-dependent protein S

Protein S is a plasma glycoprotein requiring vitamin K for normal biosynthesis and functioning as a cofactor of activated protein C, a regulator of blood coagulation. Protein S contains four modules that are similar to the epidermal growth factor (EGF) precursor. Qualitative Ca2+-binding experiments have indicated that the EGF-module region of bovine protein S harbors high-affinity Ca2+-binding sites. We have chemically synthesized the third and fourth EGF modules from human protein S, which both have the sequence motif associated with Ca2+-binding and Asp/Asn beta-hydroxylation. Both modules were folded to a native conformation, as judged by immunochemical experiments and NMR spectroscopy. Ca2+ binding to the modules was monitored with 1H-NMR spectroscopy. At physiological pH and 0.15 M NaCl, each module was found to have a single Ca2+-binding site with low affinity, i.e. Kd values of 6.1 mM for the third and 8.6 mM for the fourth EGF module. At low salt conditions the Ca2+ affinities are 5.2 mM and 0.6 mM, respectively. This Ca2+ affinity is similar to that of the isolated N-terminal EGF module from coagulation factors IX and X. The very high affinity Ca2+ binding to the EGF-module region of protein S thus appears to be due to the influence of neighboring modules.

Transferred nuclear Overhauser effect spectroscopy study of a peptide from the PapG pilus subunit bound by the Escherichia coli PapD chaperone

Interaction of the Escherichia coli PapD chaperone with the synthetic peptide PapG308-314 (Thr-Met-Val-Leu-Ser-Phe-Pro), corresponding to the seven C-terminal residues of the PapG pilus subunit, was studied by transferred nuclear Overhauser effect (TRNOE) spectroscopy. The observation of cross-peaks corresponding to either intraresidue or sequential C(alpha)H/NH and C(beta)H/NH TRNOEs and the absence of sequential NH(i)/NH(i+1) TRNOEs indicate that the peptide binds to PapD in an extended conformation. In addition, line-broadening effects gave information of the peptide's mode of interaction with PapD. These observations were in excellent agreement with a recent crystal structure of a PapG peptide complexed with PapD.

Phosphorylation controls the three-dimensional structure of plant light harvesting complex II

The most abundant chlorophyll-binding complex in plants is the intrinsic membrane protein light-harvesting complex II (LHC II). LHC II acts as a light-harvesting antenna and has an important role in the distribution of absorbed energy between the two photosystems of photosynthesis. We used spectroscopic techniques to study a synthetic peptide with identical sequence to the LHC IIb N terminus found in pea, with and without the phosphorylated Thr at the 5th amino acid residue, and to study both forms of the native full-length protein. Our results show that the N terminus of LHC II changes structure upon phosphorylation and that the structural change resembles that of rabbit glycogen phosphorylase, one of the few phosphoproteins where both phosphorylated and non-phosphorylated structures have been solved. Our results indicate that phosphorylation of membrane proteins may regulate their function through structural protein-protein interactions in surface-exposed domains.

Solution structure of the albumin-binding GA module: a versatile bacterial protein domain

The albumin-binding GA module is found in a family of surface proteins of different bacterial species. It comprises 45 amino acid residues and represents the first known example of contemporary module shuffling. Using 1H NMR spectroscopy we have determined the solution structure of the GA module from protein PAB, a protein of the anaerobic human commensal and pathogen Peptostreptococcus magnus. This structure, the first three-dimensional structure of an albumin-binding protein domain described, was shown to be composed of a left-handed three-helix-bundle. Sequence differences between GA modules with different affinities for albumin indicated that a conserved region in the C-terminal part of the second helix and the flexible sequence between helices 2 and 3 could contribute to the albumin-binding activity. The effect on backbone amide proton exchange rates upon binding to albumin support this assumption. The GA module has a fold that is strikingly similar to the immunoglobulin-binding domains of staphylococcal protein A but it shows no resemblance to the fold shared by the immunoglobulin-binding domains of streptococcal protein G and peptostreptococcal protein L. When the gene sequences, binding properties and thermal stability of these four domains are analysed in relation to their global folds an evolutionary pattern emerges. Thus, in the evolution of novel binding properties mutations are allowed only as long as the energetically favourable global fold is maintained.

Three-dimensional structures of three engineered cellulose-binding domains of cellobiohydrolase I from Trichoderma reesei

Three-dimensional solution structures for three engineered, synthetic CBDs (Y5A, Y31A, and Y32A) of cellobiohydrolase I (CBHI) from Trichoderma reesei were studied with nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. According to CD measurements the antiparallel beta-sheet structure of the CBD fold was preserved in all engineered peptides. The three-dimensional NMR-based structures of Y31A and Y32A revealed only small local changes due to mutations in the flat face of CBD, which is expected to bind to crystalline cellulose. Therefore, the structural roles of Y31 and Y32 are minor, but their functional importance is obvious because these mutants do not bind strongly to cellulose. In the case of Y5A, the disruption of the structural framework at the N-terminus and the complete loss of binding affinity implies that Y5 has both structural and functional significance. The number of aromatic residues and their precise spatial arrangement in the flat face of the type I CBD fold appears to be critical for specific binding. A model for the CBD binding in which the three aligned aromatic rings stack onto every other glucose ring of the cellulose polymer is discussed.

Cello-oligosaccharide hydrolysis by cellobiohydrolase II from Trichoderma reesei. Association and rate constants derived from an analysis of progress curves

The hydrolysis of soluble cello-oligosaccharides, with a degree of polymerisation of 4-6, catalysed by cellobiohydrolase II from Trichoderma reesei was studied using 1H-NMR spectroscopy and HPLC. The experimental progress curves were analysed by fitting numerically integrated kinetic equations, which provided cleavage patterns and kinetic constants for each oligosaccharide. This analysis procedure accounts for product inhibition and avoids the initial slope approximation. No glucose was detected at the beginning of the reaction indicating that only the internal glycosidic linkages are attacked. For cellotetraose only the second glycosidic linkage was cleaved. For cellopentaose and cellohexaose the second and the third glycosidic linkage from the non-reducing end were cleaved with approximately equal probability. The degradation rates of these cello-oligosaccharides, 1-12 s-1 at 27 degrees C, are about 10-100 times faster than for the 4-methylumbelliferyl substituted analogs or for collotriose. No intermediate products larger than cellotriose were released. The degradation rate for cellotetraose were higher than its off-rate, which accounts for the processive degradation of cellohexaose. A high cellohexaose/enzyme ratio caused slow reversible inactivation of the enzyme.

The relative orientation of Gla and EGF domains in coagulation factor X is altered by Ca2+ binding to the first EGF domain. A combined NMR-small angle X-ray scattering study

Coagulation factor X is a serine protease containing three noncatalytic domains: an N-terminal gamma-carboxyglutamic acid (Gla)1 domain followed by two epidermal growth factor (EGF)-like domains. The isolated N-terminal EGF domain binds Ca2+ with a Kd of 10(-3) M. When linked to the Gla domain, however, its Ca2+ affinity is increased 10-fold. In this paper, we present the NMR solution structure of the factor X Gla-EGF domain pair with Ca2+ bound to the EGF domain, as well as small angle X-ray scattering (SAXS) data on the Gla-EGF domain pair with and without Ca2+. Our results show that Ca2+ binding to the EGF domain makes the Gla and EGF domains fold toward each other using the Ca2+ site as a hinge. Presumably, a similar mechanism may be responsible for alterations in the relative orientation of protein domains in many other extracellular proteins containing EGF domains with the consensus for Ca2+ binding. The results of the NMR and SAXS measurements reported in this paper confirm our previous result that the Gla domain is folded also in its apo state when linked to the EGF domain [Sunnerhagen, M., et al. (1995) Nat. Struct. Biol. 2, 504-509]. Finally, our study clearly demonstrates the powerful combination of NMR and SAXS in the study of modular proteins, since this enables reliable evaluation of both short-range (NMR) and long-range interactions (SAXS).

Solution structure of nodularin. An inhibitor of serine/threonine-specific protein phosphatases

The three-dimensional solution structure of nodularin was studied by NMR and molecular dynamics simulations. The conformation in water was determined from the distance and dihedral data by distance geometry and refined by iterative relaxation matrix analysis. The cyclic backbone adopts a well defined conformation but the remote parts of the side chains of arginine as well as the amino acid derivative Adda have a large spatial dispersion. For the unusual amino acids the partial charges were calculated and nodularin was subjected to molecular dynamic simulations in water. A good agreement was found between experimental and computational data with hydrogen bonds, solvent accessibility, molecular motion, and conformational exchange. The three-dimensional structure resembles very closely that of microcystin-LR in the chemically equivalent segment. Therefore, it is expected that the binding of both microcystins and nodularins to serine/threonine-specific protein phosphatases is similar on an atomic level.

Ability of a beta-casein phosphopeptide to modulate the precipitation of calcium phosphate by forming amorphous dicalcium phosphate nanoclusters

The ability of casein in the form of colloidal-sized casein micelles to modulate the phase separation of calcium phosphate during milk secretion is adapted to produce nanometre-sized particles of calcium phosphate stabilized by a casein phosphopeptide (nanoclusters). The nanoclusters were prepared from an undersaturated solution of salts and the peptide by raising the pH homogeneously from about 5.5 to 6.7 with urea plus urease. Chemical analysis and IR spectroscopy showed that they comprise an amorphous dicalcium phosophate bound to the phosphopeptide. Multinuclear NMR spectroscopy of the cluster solutions showed that the small ions and free peptide in the solution were in a state of dynamic exchange with the nanoclusters. The peptide is linked to the calcium phosphate through its sequence of phosphorylated residues, but, in a proportion of adsorbed conformational states, the termini retain the conformational freedom of the unbound peptide. The ability of casein to form nanoclusters in milk suggests a more general mechanism for avoiding pathological calcification and regulating calcium flow in tissues and biological fluids exposed to or containing high concentrations of calcium.

Backbone dynamics of a domain of protein L which binds to immunoglobulin light chains

Protein L is a multidomain protein expressed at the surface of some strains of the anaerobic bacterial species Peptostreptococcus magnus. The molecule interacts with the variable domain of immunoglobulin (Ig) light chains through five repeated homologous domains denoted B1 to B5. The fold of the Ig-light-chain-binding B1 domain of protein L (PLB1) has been shown to comprise an alpha-helix packed against a four-stranded beta-sheet and therefore resembles the structure of the IgG-binding domains of streptococcal protein G. In the present study, amide-proton exchange and 15N-relaxation NMR measurements were performed on the B1 domain to investigate its backbone mobility. It was shown that the folded portion of PLB1 is rigid with no regions of significantly higher flexibility than average. The N-terminus, however, is highly flexible consistent with earlier studies on the solution structure of PLB1. Comparison of the amide-proton-exchange data with similar measurements performed on the IgG-binding domains of protein G indicates that the two proteins have different exchange behaviors in their second beta-strands. Both protein G and L employ this region of their structures for binding to immunoglobulins since the interaction of protein G and protein L with IgG Fab and the Ig light chain, respectively, involves residues from the second beta-strand.

Structure of a cyclic peptide with a catalytic triad, determined by computer simulation and NMR spectroscopy

We report the design of a cyclic, eight-residue peptide that possesses the catalytic triad residues of the serine proteases. A manually built model has been relaxed by 0.3 ns of molecular dynamics simulation at room temperature, during which no major changes occurred in the peptide. The molecule has been synthesised and purified. Two-dimensional NMR spectroscopy provided 35 distance and 7 torsion angle constraints, which were used to determine the three-dimensional structure. The experimental conformation agrees with the predicted one at the beta-turn, but deviates in the arrangement of the disulphide bridge that closes the backbone to a ring. A 1.2 ns simulation at 600 K provided extended sampling of conformation space. The disulphide bridge reoriented into the experimental arrangement, producing a minimum backbone rmsd from the experimental conformation of 0.8 A. At a later stage in the simulation, a transition at Ser3 produced more pronounced high-temperature behaviour. The peptide hydrolyses p-nitrophenyl acetate about nine times faster than free histidine.

Effect of Ca2+ on the structure of vitamin K-dependent coagulation factors

Coagulation factors VII, IX, X, and protein C contain an N-terminal module with 9-12 gamma-carboxyglutamic acid (Gla) residues. It is followed by two modules that are homologous to the epidermal growth factor (EGF) and a C-terminal serine protease module. Upon calcium binding to the Gla module the side chains of three hydrophobic residues are exposed in a manner indicating that they interact with biological membranes. The calcium-binding site in the first EGF-like module appears to be required for proper orientation of the Gla and EGF-like modules relative to each other. A single calcium-binding site is also present in the serine protease module. The properties of these calcium-binding sites are briefly reviewed.

Kinetic and stereochemical studies of manno-oligosaccharide hydrolysis catalysed by beta-mannanases from Trichoderma reesei

The two beta-mannanases from Trichoderma reesei with pI of 4.6 and 5.4, respectively, have been characterised by NMR spectroscopy. Following the kinetics of manno-oligosaccharide degradation with complete progress-curve analysis the stereospecificity and degradation pattern have been delineated. It was found that degradation of mannotriose and mannopentaose proceeds with retention of the anomeric configuration. Mannotriose degradation proceeds by almost random release of mannose. For mannopentaose there is initially no mannose formed showing that only the two middle mannosidic linkages are attacked. Progress-curve analysis shows that there is preference (70%) for cleavage of mannopentaose in such a way that mannobiose is released from the reducing end. The final product composition from the mannotriose degradation showed that transglycosylation has to be taken into account. Model calculation and progress-curve analysis showed that the transglycosylation rate is the fastest of all the rates in this system, 15 s-1 compared with mannohexaose and mannotetraose hydrolysis rates of 2 s-1 and mannotriose hydrolysis rate of 0.03 s-1 at 50 degrees C.

The GA module, a mobile albumin-binding bacterial domain, adopts a three-helix-bundle structure

We present the first study of the secondary structure and global fold of an albumin-binding domain. Our data show that the GA module from protein PAB, an albumin-binding protein from the anaerobic bacterial species Peptostreptococcus magnus, is composed of a left-handed three-helix bundle. The helical regions were identified by sequential and medium range NOEs, values of NH-C alpha H coupling constants, chemical shift indices, and the presence of slowly exchanging amide protons, as determined by NMR spectroscopy. In addition, circular dichroism studies show that the module is remarkably stable with respect to both pH and temperature.

Kinetic analysis of cyclophilin-catalyzed prolyl cis/trans isomerization by dynamic NMR spectroscopy

To investigate the kinetics of the prolyl peptide bond cis/trans isomerization of N-succinyl-Ala-Phe-Pro-Phe-(4)-nitroanilide catalyzed by peptidyl prolyl cis/trans isomerases (PPIases), one-dimensional dynamic 1H NMR spectroscopy was employed. To this end line shape analyses of proton signals were performed at various concentrations of both cytosolic porcine kidney cyclophilin (Cyp18) and peptide substrate. Catalysis of the cis/trans isomerization by Cyp18 is best described by a four-site exchange model, where the four sites represent the cis and trans isomers free in solution and bound to the enzyme. Combination of dynamic NMR spectroscopy with the classical protease-coupled PPIase assay allowed determination of the complete set of the microscopic rate constants describing the four site exchange model. The comparison of the rate constants of cis-->trans isomerization of the peptide free in solution and bound to cyclophilin yields an acceleration factor of 3.5 x 10(5). Dissociation of the Michaelis complexes are of the same order of magnitude as the isomerization rates on the enzyme. Therefore, all microscopic rate constants contribute to the steady state parameters. For the first time, the kcat (620 s-1) and KM (220 microM) value for the trans isomer in addition to the values of the cis isomer (kcat = 680 s-1, KM = 80 microM) could be determined under reversible conditions at pH 6.0 and 10 degrees C. The affinity of Cyp18 for the cis isomer is 4 times higher than for the trans isomer. This results in a shift of the cis/trans equilibrium toward the cis isomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-induced structural changes and domain autonomy in calmodulin

We have determined the solution structures of the apo and (Ca2+)2 forms of the carboxy-terminal domain of calmodulin using multidimensional heteronuclear nuclear magnetic resonance spectroscopy. The results show that both forms adopt well-defined structures with essentially equal secondary structure. A comparison of the structures of the two forms shows that Ca2+ binding causes major rearrangements of the secondary structure elements with changes in inter-residue distances of up to 15 A and exposure of the hydrophobic interior of the four-helix bundle. Comparisons with previously determined high-resolution X-ray structures and models of calmodulin indicate that this domain is structurally autonomous.

Characterisation of 4-deoxy-beta-L-threo-hex-4-enopyranosyluronic acid attached to xylan in pine kraft pulp and pulping liquor by 1H and 13C NMR spectroscopy

A new acidic sidegroup in xylans, from both kraft pulp and pulping liquor, was identified by NMR spectroscopy. Unmodified oligosaccharides from kraft pulp xylan were obtained by enzymatic hydrolysis with xylanase (Trichoderma reesei). The acidic oligosaccharides were separated from the natural forms on an anion exchange resin. The new acidic sidegroup was identified as 4-deoxy-beta-L-threo-hex-4-enopyranosyluronic acid (hexenuronic acid) by 1H and 13C NMR spectroscopy. Hexenuronic acid is a beta-elimination product of 4-O-methylglucuronic acid and is formed during kraft pulping. HMBC and NOESY experiments showed that hexenuronic acid is attached beta-(1 --> 2) to xylose. The NOESY data further indicated that hexenuronic acid protrudes from the main xylan chain. The pKa values for hexenuronic acid (3.03) and 4-O-methylglucuronic acid (3.14) attached (1 --> 2) to xylose were determined from pH-dependent chemical shifts.

Mapping of the immunoglobulin light chain-binding site of protein L

Protein L is a cell surface protein expressed by some strains of the anaerobic bacterial species Peptostreptococcus magnus. The molecule binds specifically and with high affinity to immunoglobulins (Ig) of a wide range of animal species. The Ig-binding activity is mediated through five highly homologous domains, each 72 to 76 amino acid residues long, which interact with framework regions in the variable domain of Ig light chains. The interaction does not interfere with the antigen binding capacity of the antibody. The fold of the Ig light chain-binding domains of Protein L is comprised of an alpha-helix packed against a four stranded beta-sheet and is similar to the fold of the IgG heavy chain-binding domains of streptococcal protein G, despite the fact that the two proteins show no significant sequence homology. In the present work, heteronuclear NMR spectroscopy has been utilized to define the interaction between the N-terminal Ig-binding domain of Protein L and the variable domain of a human Ig kappa light chain. The Ig-binding region of the Protein L domain involves most of the residues in the second beta-strand, the C-terminal residues of the alpha-helix and the loop connecting the alpha-helix with the third beta-strand. The Ig light chain-binding surface of Protein L thus resembles the surface of Protein G which binds to the C gamma 1 domain of IgG, but is different from the portion of Protein G involved in the contact with the C gamma 2-C gamma 3 interface region. The data suggest that the global fold shared by the Ig-binding domains of Proteins L and G provide bacteria with a flexible template for the evolution of surface structures capable of interacting with different conserved parts of Ig molecules of the infected host.

Progress-curve analysis shows that glucose inhibits the cellotriose hydrolysis catalysed by cellobiohydrolase II from Trichoderma reesei

NMR spectroscopy and HPLC were used to investigate the hydrolysis of cellotriose by cellobiohydrolase II from Trichoderma reesei. Substrate and product concentrations were followed as a function of time. Progress curves were calculated by forward numerical integration of the full kinetic equations and were fitted to the experimental data. Binding and rate constants were obtained from this fit, whereby no initial slope or Michaelis-Menten approximation was used. The progress curves from a single experiment sufficed to produce agreement with the Michaelis-Menten model (eight experiments). The absence of a kinetic isotope effect was proven. The progress-curve analysis showed that a simple degradation model cannot describe the experimental time-courses at substrate concentrations greater than 1 mM. A model containing competitive inhibition from cellobiose as well as non-competitive inhibition from glucose was developed. This four-parameter model accurately reproduces about 1000 experimental data points covering five orders of magnitude in oligosaccharide concentrations. Glucose binding to the enzyme/cellotriose complex retards, in a non-competitive fashion, cellotriose hydrolysis by at least a factor of 30. A structural model for the non-competitive inhibition is discussed. The NMR experiment also produced individual progress curves for the alpha and beta anomers. The beta anomer of cellotriose was degraded 2.5-times faster than the alpha anomer.

Structure of the Ca(2+)-free Gla domain sheds light on membrane binding of blood coagulation proteins

Reversible membrane binding of gamma-carboxyglutamic acid (Gla)-containing coagulation factors requires Ca(2+)-binding to 10-12 Gla residues. Here we describe the solution structure of the Ca(2+)-free Gla-EGF domain pair of factor x which reveals a striking difference between the Ca(2+)-free and Ca(2+)-loaded forms. In the Ca(2+)-free form Gla residues are exposed to solvent and Phe 4, Leu 5 and Val 8 form a hydrophobic cluster in the interior of the domain. In the Ca(2+)-loaded form Gla residues ligate Ca2+ in the core of the domain pushing the side-chains of the three hydrophobic residues into the solvent. We propose that the Ca(2+)-induced exposure of hydrophobic side chains is crucial for membrane binding of Gla-containing coagulation proteins.

Identification of functionally important amino acids in the cellulose-binding domain of Trichoderma reesei cellobiohydrolase I

Cellobiohydrolase I (CBHI) of Trichoderma reesei has two functional domains, a catalytic core domain and a cellulose binding domain (CBD). The structure of the CBD reveals two distinct faces, one of which is flat and the other rough. Several other fungal cellulolytic enzymes have similar two-domain structures, in which the CBDs show a conserved primary structure. Here we have evaluated the contributions of conserved amino acids in CBHI CBD to its binding to cellulose. Binding isotherms were determined for a set of six synthetic analogues in which conserved amino acids were substituted. Two-dimensional NMR spectroscopy was used to assess the structural effects of the substitutions by comparing chemical shifts, coupling constants, and NOEs of the backbone protons between the wild-type CBD and the analogues. In general, the structural effects of the substitutions were minor, although in some cases decreased binding could clearly be ascribed to conformational perturbations. We found that at least two tyrosine residues and a glutamine residue on the flat face were essential for tight binding of the CBD to cellulose. A change on the rough face had only a small effect on the binding and it is unlikely that this face interacts with cellulose directly.