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

Cellular and molecular studies of Marfan syndrome mutations identify co-operative protein folding in the cbEGF12-13 region of fibrillin-1

Human fibrillin-1 is an extra-cellular matrix glycoprotein with a modular organisation that includes 43 calcium-binding epidermal growth factor-like (cbEGF) domains arranged as multiple tandem repeats interspersed with transforming growth factor beta binding protein-like (TB) domains. We have studied Marfan syndrome-causing mutations which affect calcium binding to cbEGF13, and demonstrate that in human fibroblast cells they cause unexpected endoplasmic reticulum retention, indicative of a folding defect. Biochemical and biophysical studies of in vitro refolded fragments from the TB3-cbEGF14 region indicate long-range and unidirectional effects of these substitutions on the adjacent N-terminal domain cbEGF12. In contrast, only short-range effects of a pathogenic mutation affecting calcium binding to cbEGF19 are observed, and secretion of this mutant protein occurs. Further NMR studies on wild-type cbEGF12-13 and cbEGF12-14 identify a co-operative dependence of domain folding where calcium binding to cbEGF13 is required before cbEGF12 can adopt a native Ca(2+)-dependent fold. These data demonstrate that during biosynthesis of fibrillin-1, multiple tandem repeats of cbEGF domains may not necessarily fold independently and therefore missense mutations resulting in identical substitutions may have different effects on the fate of the mutant protein. Complex folding of modular proteins should therefore be considered when interpreting the molecular pathology of single-gene disorders.

Binding of calcium to anticoagulant protein S: role of the fourth EGF module

Protein S is an anticoagulant protein containing a Gla (enclosing gamma-carboxyglutamic acids) module, a TSR (thrombin sensitive region) module, four EGF (epidermal growth factor)-like modules, and a SHBG (sex hormone binding globulin)-like region. Protein S is a cofactor to activated protein C (APC) in the degradation of coagulation factors Va and VIIIa but also has APC-independent activities. The function of the fourth EGF module (EGF4) in protein S has so far not been clear. We have now investigated this module through studies of recombinant wild-type protein S and a naturally occurring mutant (Asn217Ser). The mutant has essentially normal APC anticoagulant activity and a previously reported secretion defect. In the wild-type protein, Asn217 is normally beta-hydroxylated. The binding of calcium to wild-type protein S is characterized by four high-affinity binding sites with K(D) values ranging from 10(-)(7) to 10(-)(9) M. Three of these binding sites are located in EGF modules. Using surface plasmon resonance, competition with a calcium chelator, and antibody-based methods, we found that one high-affinity binding site for calcium was lost in protein S Asn217Ser but that the mutation also affected the calcium-dependent conformation of EGF1. We conclude that binding of calcium to EGF4 of protein S, involving Asn217, is important for the maintenance of the structure of protein S. Also, the abolition of binding of calcium to EGF4, related to Asn217, impairs both the structure and function of EGF1.

Solution structure of the Ca2+-Binding EGF3-4 pair from vitamin K-dependent protein S: identification of an unusual fold in EGF3

Vitamin K-dependent protein S is a cofactor of activated protein C, a serine protease that regulates blood coagulation. Deficiency of protein S can cause venous thrombosis. Protein S has four EGF domains in tandem; domains 2-4 bind calcium with high affinity whereas domains 1-2 mediate interaction with activated protein C. We have now solved the solution structure of the EGF3-4 fragment of protein S. The linker between the two domains is similar to what has been observed in other calcium-binding EGF domains where it provides an extended conformation. Interestingly, a disagreement between NOE and RDC data revealed a conformational heterogeneity within EGF3 due to a hinge-like motion around Glu186 in the Cys-Glu-Cys sequence, the only point in the domain where flexibility is allowed. The dominant, bent conformation of EGF3 in the pair has no precedent among calcium-binding EGF domains. It is characterized by a change in the psi angle of Glu186 from 160 degrees +/- 40 degrees , as seen in ten other EGF domains, to approximately 0 degrees +/- 15 degrees . NOESY data suggest that Tyr193, a residue not conserved in other calcium-binding EGF domains (except in the homologue Gas6), induces the unique fold of EGF3. However, SAXS data, obtained on EGF1-4 and EGF2-4, showed a dominant, extended conformation in these fragments. This may be due to a counterproductive domain-domain interaction between EGF2 and EGF4 if EGF3 is in a bent conformation. We speculate that the ability of EGF3 to adopt different conformations may be of functional significance in protein-protein interactions involving protein S.

Structural consequences of cysteine substitutions C1977Y and C1977R in calcium-binding epidermal growth factor-like domain 30 of human fibrillin-1

The largest group of disease-causing mutations affecting calcium-binding epidermal growth factor-like (cbEGF) domain function in a wide variety of extracellular and transmembrane proteins is that which results in cysteine substitutions. Although known to introduce proteolytic susceptibility, the detailed structural consequences of cysteine substitutions in cbEGF domains are unknown. Here, we studied pathogenic mutations C1977Y and C1977R, which affect cbEGF30 of human fibrillin-1, in a recombinant three cbEGF domain fragment (cbEGF29-31). Limited proteolysis, 1H NMR, and calcium chelation studies have been used to probe the effect of each substitution on cbEGF30 and its flanking domains. Analysis of the wild-type fragment identified two high affinity and one low affinity calcium-binding sites. Each substitution caused the loss of high affinity calcium binding to cbEGF30, consistent with intradomain misfolding, but the calcium binding properties of cbEGF29 and cbEGF31 were surprisingly unaffected. Further analysis of mutant fragments showed that domain packing of cbEGF29-30, but not cbEGF30-31, was disrupted. These data demonstrate that C1977Y and C1977R have localized structural effects, confined to the N-terminal end of the mutant domain, which disrupt domain packing. Cysteine substitutions affecting other cbEGF disulfide bonds are likely to have different effects. This proposed structural heterogeneity may underlie the observed differences in stability and cellular trafficking of proteins containing such changes.

N-Terminal Domain Linkage Modulates the Folding Properties of Protein S Epidermal Growth Factor-like Modules

Protein S interacts with activated protein C to play a crucial role in blood anticoagulation, and protein S deficiency is associated with increased risk of thrombosis. Despite the large volume of functional data available for this protein, no atomic resolution structure data have yet been reported. This is due at least in part to difficulties encountered when trying to produce fragments dissected from the intact protein; however, a few successful strategies have been described. In this research we have expressed a number of constructs containing protein S epidermal growth factor-like (EGF) domains 1 and 2 in Escherichia coli and Pichia pastoris. None of the proteins produced was stably folded as assayed by solution nuclear magnetic resonance spectroscopy. We therefore constructed a series of non-native protein S EGF concatemers to investigate the role of pairwise domain linkage in domain folding. Our results demonstrate that N-terminal domain linkage can either positively or negatively impact on the refolding of an adjacent domain. Furthermore, analysis of the NMR data for EGF3-4 reveals the expected interdomain NOEs that are characteristic of an extended arrangement of calcium-binding EGF domains and a similar average [(1)H]-(15)N heteronuclear NOE value for each of the two domains. These results provide the first data in support of protein S EGF3-4 adopting the same extended domain orientation as observed for the functionally distinct proteins fibrillin-1 and the low-density lipoprotein receptor. The results also have important implications for future studies, particularly when a dissection approach is used, of tandem EGF domains from protein S and other proteins.

Functional analysis of the EGF-like domain mutations Pro55Ser and Pro55Leu, which cause mild hemophilia B

We studied the functional role of two mutations, Pro55Ser and Pro55Leu, located in the N-terminal Epidermal Growth Factor-like domain (EGF1) of coagulation factor (F) IX. Both mutations cause mild hemophilia B with habitual FIX coagulant activities of 10-12% and FIX antigen levels of 50%. We found that activation by FVIIa/TF and FXIa was normal for FIXPro55Ser, but resulted in proteolysis of FIXPro55Leu at Arg318-Ser319 with a concomitant loss of amidolytic activity, suggesting intramolecular communication between EGF1 and the serine protease domain in FIX. This was further supported by experiments using an anti-EGF1 monoclonal antibody. Activation of FX by FIXaPro55Ser was impaired in both the presence and the absence of phospholipid or FVIIIa, indicating that Pro55 is not directly involved in binding to FVIIIa. We also studied the effect of the two Pro55 mutations on Ca2+ affinity and found only small changes. Thus, the Pro55Ser mutation causes hemophilia primarily through to an impaired ability to activate FX whereas at least in vitro the Pro55Leu defect interferes with the activation of FIX.

Deletion or replacement of the second EGF-like domain of protein S results in loss of APC cofactor activity

Human protein S (PS), a cofactor of anticoagulant-activated protein C (APC), is a modular protein containing 4 epidermal growth factor (EGF)-like domains. EGF1 appears to mediate PS interaction with APC, but the roles of EGFs 2, 3, and 4 are less clear. We synthesized PS variants lacking single EGF domains (EGF2, 3, or 4) and assessed their APC cofactor activity in a factor Va inactivation assay. The variant lacking EGF2 (variant 134) showed the most dramatic loss of activity (approximately 10% of recombinant wild-type PS activity). Replacement of EGF2 by an additional EGF3 (variant 1334) resulted in a comparable loss of activity, suggesting that the loss of a specific rather than "spacer" function of EGF2 was responsible. We confirmed that the variant 134 had a functional gamma-carboxyglutamic acid (Gla) domain and that EGF1 was correctly folded. This is the first clear evidence that EGF2 is required for the expression of PS activity.

Functional properties of the sex-hormone-binding globulin (SHBG)-like domain of the anticoagulant protein S

Protein S (PS) possesses a sex-hormone-binding globulin (SHBG)-like domain in place of the serine-protease domain found in other vitamin K-dependent plasma proteins. This SHBG-like domain is able to bind a complement fraction, C4b-binding protein (C4b-BP). To establish whether the PS SHBG-like domain can fold normally in the absence of other domains, and to obtain information on the specific functions of this region, we expressed the PS SHBG-like domain alone or together with its adjacent domain EGF4. The folding of the two recombinant modules was studied by analyzing their binding to C4b-BP. The apparent dissociation constants of this interaction indicated that both recombinant modules adopted the conformation of native PS, indicating that the PS SHBG-like region is an independent folding unit. We also obtained the first direct evidence that the SHBG-like domain alone is sufficient to support the interaction with C4b-BP. In addition, both recombinant modules were able to bind Ca2+ directly, as shown by the migration shift in agarose gel electrophoresis in the presence of Ca2+, together with the results of equilibrium dialysis and the functional effect of Ca2+ on the C4b-BP/PS interaction, confirming the presence of one Ca2+ binding site within the SHBG-like domain. Neither recombinant module exhibited activated protein C (aPC) cofactor activity in a clotting assay, suggesting that the PS SHBG-like region must be part of the intact molecule for it to contribute to aPC cofactor activity, possibly by constraining the different domains in a conformation that permits optimal interaction with aPC.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

Effects of the N2144S mutation on backbone dynamics of a TB-cbEGF domain pair from human fibrillin-1

The calcium-binding epidermal growth factor-like (cbEGF) module and the transforming growth factor beta-binding protein-like (TB) module are the two major structural motifs found in fibrillin-1, the extracellular matrix (ECM) protein defective in the Marfan syndrome (MFS). An MFS-causing mutation, N2144S, which removes a calcium ligand in cbEGF32, does not detectably affect fibrillin-1 biosynthesis, rate of secretion, processing, or deposition of reducible fibrillin-1 into the ECM. Since the residue at position 2144 is normally engaged in calcium ligation, it is unable to mediate intermolecular interactions. We have shown previously that this mutation does not affect the folding properties of the TB or cbEGF domains in vitro, but does decrease calcium-binding in cbEGF and TB-cbEGF domain constructs. Here, we use NMR spectroscopy to probe the effects of the N2144S mutation on backbone dynamic properties of TB6-cbEGF32. Analysis of the backbone (15)N relaxation data of wild-type TB6-cbEGF32 has revealed a flexible inter-domain linkage. Parallel dynamics analysis of the N2144S mutant has shown increased flexibility in the region joining the two domains as well as in the calcium-binding site at the N terminus of cbEGF32. This research demonstrates that a small change in peptide backbone flexibility, which does not enhance proteolytic susceptibility of the domain pair, is associated with an MFS phenotype. Flexibility of the TB-cbEGF linkage is likely to contribute to the biomechanical properties of fibrillin-rich connective tissue microfibrils, and may play a role in the microfibril assembly process.

An anti-EGF monoclonal antibody that detects intramolecular communication in factor IX

Coagulation factor IX contains a gamma-carboxyglutamic acid (Gla) module, two epidermal growth factor-like (EGF) modules, and a serine protease region. We have characterized a mouse monoclonal antibody that binds the N-terminal EGF-like module of human factor IX with high affinity. Studies of recombinant factor IX mutants indicated that the epitope is located in the C-terminal end of the EGF-like module, which is consistent with the binding being non-Ca(2+)-dependent. The antibody bound factor IXa (K(D) = 7.6 x 10(-10) M) with about 10-fold higher affinity than factor IX (K(D) = 6.2 x 10(-9) M). Binding of the antibody to factor IXa did not affect the amidolytic activity of the protein, nor was binding affected by active site inhibition of factor IXa. These results are consistent with long-range interactions between the serine protease region and the N-terminal EGF-like module in factor IX.

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.

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.

Calcium-binding EGF-like modules in coagulation proteinases: function of the calcium ion in module interactions

Epidermal growth factor (EGF)-like modules are involved in protein-protein interactions and are found in numerous extracellular proteins and membrane proteins. Among these proteins are enzymes involved in blood coagulation, fibrinolysis and the complement system as well as matrix proteins and cell surface receptors such as the EGF precursor, the low density lipoprotein receptor and the developmentally important receptor, Notch. The coagulation enzymes, factors VII, IX and X and protein C, all have two EGF-like modules, whereas the cofactor of activated protein C, protein S, has four EGF-like modules in tandem. Certain of the cell surface receptors have numerous EGF modules in tandem. A subset of EGF modules bind one Ca(2+). The Ca(2+)-binding sequence motif is coupled to a sequence motif that brings about beta-hydroxylation of a particular Asp/Asn residue. Ca(2+)-binding to an EGF module is important to orient neighboring modules relative to each other in a manner that is required for biological activity. The Ca(2+) affinity of an EGF module is often influenced by its N-terminal neighbor, be it another EGF module or a module of another type. This can result in an increase in Ca(2+) affinity of several orders of magnitude. Point mutations in EGF modules that involve amino acids which are Ca(2+) ligands result in the biosynthesis of biologically inactive proteins. Such mutations have been identified, for instance, in factor IX, causing hemophilia B, in fibrillin, causing Marfan syndrome, and in the low density lipoprotein receptor, causing hypercholesterolemia. In this review the emphasis will be on the coagulation factors.

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.

EGF-like domain calcium affinity modulated by N-terminal domain linkage in human fibrillin-1

Calcium binding epidermal growth factor-like domains (cbEGFs) are present in many extracellular proteins, including fibrillin-1, Notch-3, protein S, factor IX and the low density lipoprotein (LDL) receptor, which perform a diverse range of functions. Genetic mutations that cause amino acid changes within these proteins have been linked to the Marfan syndrome (MFS), CADASIL, protein S deficiency, haemophilia B and familial hypercholesterolaemia, respectively. A number of these mutations disrupt calcium binding to cbEGFs, emphasising the critical functional role of calcium in these proteins. We have determined the calcium binding affinity of two sites within a cbEGF pair (cbEGF12-13) from human fibrillin-1 using two-dimensional nuclear magnetic resonance (NMR) and fluorescence techniques. Fibrillin-1 is a mosaic protein containing 43 cbEGF domains, mainly arranged as tandem repeats. Our results show that the cbEGF13 site in the cbEGF12-13 pair possesses the highest calcium affinity of any cbEGF investigated from fibrillin-1. A comparative analysis of these and previously reported calcium binding data from fibrillin-1 demonstrate that the affinity of cbEGF13 is enhanced more than 70-fold by the linkage of an N-terminal cbEGF domain. In contrast, comparison of calcium binding by cbEGF32 in isolation relative to when linked to a transforming growth factor beta-binding protein-like domain (TB6-cbEGF32) reveals that the same enhancement is not observed for this heterologous domain pair. Taken together, these results indicate that fibrillin-1 cbEGF Ca2+ affinity can be significantly modulated by the type of domain which is linked to its N terminus. The cbEGF12-13 pair is located within the longest contiguous section of cbEGFs in fibrillin-1, and a number of mutations in this region are associated with the most severe neonatal form of MFS. The affinities of cbEGF domains 13 and 14 in this region are substantially higher than in the C-terminal region of fibrillin-1. This increased affinity may be important for fibrillin assembly into 10-12 nm connective tissue microfibrils and/or may contribute to the biomechanical properties of the microfibrillar network.

The epidermal growth factor precursor. A calcium-binding, beta-hydroxyasparagine containing modular protein present on the surface of platelets

Various human body fluids and secretions contain a soluble form of the epidermal growth factor (EGF) precursor. The EGF precursor molecule contains eight EGF modules in addition to EGF itself. Using monoclonal antibodies specific for the EGF modules 7 and 8, we have purified the soluble form of the EGF precursor from human urine to homogeneity. The protein was shown to have a molecular mass of about 160 kDa and the N-terminal sequence SAPNHWSXPE. EGF modules 2, 7 and 8 of the precursor have the consensus sequence for post-translational beta-hydroxylation of Asp/Asn residues. We identified the presence of erythro-beta-hydroxy-aspartic acid (Hya) in acid hydrolysates of the EGF precursor (2.4 M.M protein-1). As the DNA sequence encodes Asn in the corresponding position, the Hya represents erythro-beta-hydroxyasparagine (Hyn). The Hyn-containing modules have a consensus calcium-binding motif immediately N-terminal of the first Cys residue. The synthetic EGF module 2 (residues 356-395) of the EGF precursor was found to bind calcium with low affinity, Kd approximately 3.5 mM, i.e. similar to the affinity of other isolated calcium-binding EGF modules. EGF module 7, when part of the intact protein, was found to bind Ca2+ with a Kd approximately 0.2 microM, i.e. approximately 10(4)-fold higher than that of isolated EGF modules presumably due to the influence of neighboring modules. We have detected EGF precursor in platelet-rich plasma and demonstrated it to be associated to platelets. The platelets were found to have 30-160 EGF molecules each.

Chemical synthesis of human protein S thrombin-sensitive module and first epidermal growth factor module

Human plasma protein S is a nonenzymatic cofactor for activated protein C (APC) in the inactivation of coagulation factors Va and VIIIa, and helps to provide an essential negative feedback on blood coagulation. Previous indirect evidence suggested that the thrombin-sensitive region (TSR: residues 47-75, 1 disulfide) and the first epidermal growth factorlike region (EGF1: residues 76-116, 3 disulfides) of protein S may be functionally important for expression of its APC cofactor activity. To study the functional importance of these modules directly, access to the isolated TSR and EGF1 modules would be preferred. Recombinant expression of protein S intact TSR and correctly folded EGF1 has not been possible. Here we describe the synthesis of both TSR and EGF1 modules by stepwise solid phase peptide synthesis using the in situ neutralization/2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluron ium hexafluorophosphate activation procedure for tert-butoxycarbonyl chemistry. For the TSR, correct intramodular disulfide bonding was confirmed. To overcome folding difficulties with the EGF1, a two-step oxidation procedure was used in which the cysteines involved in the middle, crossing, disulfide bond (Cys85-Cys102) remained protected with acetamidomethyl (Acm) groups after hydrogen fluoride treatment of the peptide resin. Selective formation of the first two disulfide bonds (Cys80-Cys93 and Cys104-Cys113) was followed by release of the Acm groups and subsequent formation of the third disulfide bond (Cys85-Cys102). CD studies revealed 54% of beta-sheet/turn in the EGF1 that is characteristic for EGF modules. Deuterium exchange studies suggested a very tightly packed core in EGF1 that is not accessible to the bulk solvent, likely a result from the compact structure caused by its three disulfide bonds. The 30% beta-sheet structure observed in the TSR involved amide protons that could be readily exchanged by deuterons, likely reflecting a more flexible structure of the TSR loop in contrast to the rigid structure of EGF1. The establishment of synthetic access to the TSR and EGF1 of protein S provides a versatile tool to study interactions of these modules with the blood coagulation components of the anticoagulant plasma protein C pathway.

Chemical protein synthesis

Chemical protein synthesis is a field in transition. Previously, the synthetic accomplishment itself was the major focus of work in this field. Increasingly, chemical synthesis is now being applied to understanding how biological function originates in the structure of the protein molecule. A novel approach--'chemical ligation', which is the chemoselective reaction of unprotected peptide segments in water at pH7--has made the total synthesis of proteins a robust and practical route to the study of structure-function relationships. For certain protein families, chemical protein synthesis is the most effective way to obtain functional proteins direct from genome sequence data.