Fibrinogen Lima: a homozygous dysfibrinogen with an A alpha-arginine-141 to serine substitution associated with extra N-glycosylation at A alpha-asparagine-139. Impaired fibrin gel formation but normal fibrin-facilitated plasminogen activation catalyzed by tissue-type plasminogen activator

Structural changes of proteins in liver cirrhosis and consequential changes in their function

The liver is the site of synthesis of the majority of circulating proteins. Besides initial polypeptide synthesis, sophisticated machinery is involved in the further processing of proteins by removing parts of them and/or adding functional groups and small molecules tailoring the final molecule to suit its physiological purpose. Posttranslational modifications (PTMs) design a network of molecules with the common protein ancestor but with slightly or considerably varying activity/localization/purpose. PTMs can change under pathological conditions, giving rise to aberrant or overmodified proteins. Undesired changes in the structure of proteins most often accompany undesired changes in their function, such as reduced activity or the appearance of new effects. Proper protein processing is essential for the reactions in living beings and crucial for the overall quality control. Modifications that occur on proteins synthesized in the liver whose PTMs are cirrhosis-related are oxidation, nitration, glycosylation, acetylation, and ubiquitination. Some of them predominantly affect proteins that remain in liver cells, whereas others predominantly occur on proteins that leave the liver or originate from other tissues and perform their function in the circulation. Altered PTMs of certain proteins are potential candidates as biomarkers of liver-related diseases, including cirrhosis. This review will focus on PTMs on proteins whose structural changes in cirrhosis exert or are suspected to exert the most serious functional consequences.

Structural changes of fibrinogen as a consequence of cirrhosis

Cirrhosis is a disease which may develop as a consequence of various conditions. In advanced liver disease, blood coagulation can be seriously affected. Portal hypertension, vascular abnormalities and/or a dysbalance in coagulation factors may result in bleeding disorders or in the development of thrombosis. Fibrinogen is the main protein involved in clot formation and wound healing. The aim of this work was to analyse the glycosylation pattern of the isolated fibrinogen molecules by lectin-based protein microarray, together with the carbonylation pattern of the individual fibrinogen chains, possible changes in the molecular secondary and tertiary structure and reactivity with the insulin-like growth factor-binding protein 1 (IGFBP-1) in patients with cirrhosis. The results pointed to an increase in several carbohydrate moieties: tri/tetra-antennary structures, Gal β-1,4 GlcNAc, terminal α-2,3 Sia and α-1,3 Man, and a decrease in core α-1,6 Fuc and bi-antennary galactosylated N-glycans with bisecting GlcNAc. Fibrinogen Aα chain was the most susceptible to carbonylation, followed by the Bβ chain. Cirrhosis induced additional protein carbonylation, mostly on the α chain. Spectrofluorimetry and CD spectrometry detected reduction in the α-helix content, protein unfolding and/or appearance of modified amino acid residues in cirrhosis. The amount of complexes which fibrinogen forms with IGFBP-1, another factor involved in wound healing was significantly greater in patients with cirrhosis than in healthy individuals. A more detailed knowledge of individual molecules in coagulation process may contribute to deeper understanding of coagulopathies and the results of this study offer additional information on the possible mechanisms involved in impaired coagulation due to cirrhosis.

Structural and functional changes of fibrinogen due to aging

Different factors affect coagulation process. Since fibrinogen is the main coagulation factor, the influence of aging on fibrinogen structure and function was investigated in this study. Fibrinogen was isolated from plasma obtained from healthy persons in the age range 21-83 and examined. Lectin microarray analysis demonstrated increased glycosylation of fibrinogen due to aging, with predominant increase in high-mannose or hybrid type N-glycans, as well as tri-/tetraantennary complex N-glycans with greater content of galactose and N-acetylglucosamine residues. Spectrofluorimetric analysis indicated that fibrinogen molecules have more densely packed structure, but there are no additional advanced glycation end products with increasing age. According to the results of functional analysis, fibrinogen molecules isolated from older persons exhibited reduced clotting time, with significant positive correlation with age, but there were no differences in clotting speed, maximal optical density of fibrin clot, diameter of fibrin fibres, fibrin porosity or reactivity with the insulin-like growth factor binding protein 1. Glycosylation changes of fibrinogen in healthy aging most likely affect its structure and function, namely clotting time. Structural and functional studies of proteins in relation to healthy aging contribute to deeper understanding of mechanisms responsible for longevity.

Molecular characterization of 7 patients affected by dys- or hypo-dysfibrinogenemia: Identification of a novel mutation in the fibrinogen Bbeta chain causing a gain of glycosylation

Fibrinogen is a hexameric glycoprotein consisting of two sets of three polypeptides (the Aα, Bβ, and γ chains, encoded by the three genes FGA, FGB, and FGG). It is involved in the final phase of the coagulation process, being the precursor of the fibrin monomers necessary for the formation of the hemostatic plug. Rare inherited fibrinogen disorders can manifest as quantitative deficiencies, qualitative defects, or both. In particular, dysfibrinogenemia and hypo-dysfibrinogenemia are characterized by reduced functional activity associated with normal or reduced antigen levels, and are usually determined by heterozygous mutations affecting any of the three fibrinogen genes. In this study, we investigated the genetic basis of dys- and hypo-dysfibrinogenemia in seven unrelated patients. Mutational screening disclosed six different variants, two of which novel (FGB-p.Asp185Asn and FGG-p.Asn230Lys). The molecular characterization of the FGG-p.Asn230Lys mutation, performed by transient expression experiments of the recombinant mutant protein, demonstrated that it induces an almost complete impairment in fibrinogen secretion, according to a molecular mechanism often associated with quantitative fibrinogen disorders. Conversely, the FGB-p.Asp185Asn variant was demonstrated to be a gain-of-glycosylation mutation leading to a hyperglycosylation of the Bβ chain, not affecting fibrinogen assembly and secretion. To our knowledge, this is the second gain-of-glycosylation mutation involving the FGB gene.

Elucidating the role of carbohydrate determinants in regulating hemostasis: insights and opportunities

Recent improvement in modern analytical technologies has stimulated an explosive growth in the study of glycobiology. In turn, this has lead to a richer understanding of the crucial role of N- and O-linked carbohydrates in dictating the properties of the proteins to which they are attached and, in particular, their centrality in the control of protein synthesis, longevity, and activity. Given their importance, it is unsurprising that both gross and subtle defects in glycosylation often contribute to human disease pathology. In this review, we discuss the accumulating evidence for the significance of glycosylation in mediating the functions of the plasma glycoproteins involved in hemostasis and thrombosis. In particular, the role of naturally occurring coagulation protein glycoforms and inherited defects in carbohydrate attachment in modulating coagulation is considered. Finally, we describe the therapeutic opportunities presented by new insights into the role of attached carbohydrates in shaping coagulation protein function and the promise of carbohydrate modification in the delivery of novel therapeutic biologics with enhanced functional properties for the treatment of hemostatic disorders.

Carbohydrate biomarker recognition using synthetic lectin mimics

Carbohydrate biomarkers play very important roles in a wide range of biological and pathological processes. Compounds that can specifically recognize a carbohydrate biomarker are useful for targeted delivery of imaging agents and for development of new diagnostics. Furthermore, such compounds could also be candidates for the development of therapeutic agents. A tremendous amount of active work on synthetic lectin mimics has been reported in recent years. Amongst all the synthetic lectins, boronic-acid-based lectins (boronolectins) have shown great promise. Along this line, four classes of boronolectins including peptide-, nucleic-acid-, polymer-, and small-molecule-based ones are discussed with a focus on the design principles and recent advances. We hope that by presenting the potentials of this field, this review will stimulate more research in this area.

Carbohydrate biomarkers for future disease detection and treatment

Carbohydrates are considered as one of the most important classes of biomarkers for cell types, disease states, protein functions, and developmental states. Carbohydrate "binders" that can specifically recognize a carbohydrate biomarker can be used for developing novel types of site specific delivery methods and imaging agents. In this review, we present selected examples of important carbohydrate biomarkers and how they can be targeted for the development of therapeutic and diagnostic agents. Examples are arranged based on disease categories including (1) infectious diseases, (2) cancer, (3) inflammation and immune responses, (4) signal transduction, (5) stem cell transformation, (6) embryo development, and (7) cardiovascular diseases, though some issues cross therapeutic boundaries.

Fibrinogen Yecheon: congenital dysfibrinogenemia with gamma methionine-310 to threonine substitution

This case study reports a rare fibrinogen variant, gamma Met310Thr mutation, for the first time in Korea. The case shows a point mutation from T to C in the 1,007th nucleotide of the FGG gene. This report describes a variant fibrinogen, hereinafter called "fibrinogen Yecheon", using the name after the town where the patient was living at the time of diagnosis. Fibrinogen Yecheon has a de novo heterozygous point mutation of FGG resulting in gamma Met310Thr and subsequent extra N-glycosylation at gamma Asn308. Extra N-glycosylated fibrinogen is considered a main inhibitor of normal fibrinogen activity.

Simulated Point Mutations in the Aα-Chain of Human Fibrinogen Support a Role of the αC Domain in the Stabilization of Fibrin Gel

The hydrophobicity pattern distribution in the Aalpha-, Bbeta- and gamma-chains of human fibrinogen has been studied by a nonlinear method, recurrence quantification analysis, in the wild type and in a number of naturally occurring or simulated mutants. The aim was to find a structural basis for distinguishing between silent and pathological mutants. We were successful in the case of mutations on the Aalpha-chain, thanks to the peculiar features of this chain as compared to the other two. Relevant findings concerning the point mutants of the Aalpha-chain are the following: (a) the recurrence quantification analysis-based classification of such mutants is in good agreement with the clinical classification, and (b) the location of the mutated residue on the sequence plays a more relevant role than its hydrophobic features. Artificial point mutants in the terminal zone (600-866 residues) of the extended isoform of the Aalpha-chain cluster together with the natural hemorrhagic mutants of the first (1-207) residues.

Gain-of-glycosylation mutations

Disease-causing missense (and other in-frame) mutations can exert their deleterious effects at the cellular level through multiple mechanisms. A pathogenic mechanism involves the addition of a novel N-linked glycan. Up to 1.4% of known disease-causing missense mutations are predicted to give rise to gains-of-glycosylation. For some of these mutations, the novel glycans have been shown to be both necessary and sufficient to account for the deleterious impact of the mutation. The chemical complementation of cells from patients in vitro with various modifiers of glycosylation has been demonstrated and raises the possibility of specific chemical treatments for patients bearing gain-of-glycosylation mutations.

A novel variant fibrinogen, deletion of Bbeta111Ser in coiled-coil region, affecting fibrin lateral aggregation

BACKGROUND

Functional fibrinogen concentration of a male infant showed <0.50 g/l and we speculated this patient as a dysfibrinogenemia or hypofibrinogenemia.

METHODS

We analyzed propositus and his parent by DNA sequencing and by thrombin-catalyzed fibrin polymerization for purified plasma fibrinogen.

RESULTS

Although functional fibrinogen determinations based on Clauss method showed the marked discrepancy of values among 3 sets of reagent and analyzer, we found a novel heterozygous variant fibrinogen, Kyoto IV, caused by 3-bp deletion in Bbeta-chain gene corresponding to the deletion of 111Ser located in coiled-coil region. We suggested that the discrepancy of fibrinogen values among 3 assays was caused by the difference in NaCl concentration in reagents for determination and analyzed the polymerization under the conditions of various NaCl concentrations. Although under normal physiological conditions Kyoto IV fibrinogen augmented the polymerization as compared with normal control, in 0.21 mol/l NaCl Kyoto IV fibrinogen showed abruptly impaired polymerization curve compared with normal control.

CONCLUSION

Variant fibrinogen, BbetaDelta111Ser, showed augmented lateral aggregation under normal physiological conditions and the residue located in coiled-coil region, Bbeta111Ser, plays an important role in the lateral aggregation.

Gains of glycosylation comprise an unexpectedly large group of pathogenic mutations

Mutations involving gains of glycosylation have been considered rare, and the pathogenic role of the new carbohydrate chains has never been formally established. We identified three children with mendelian susceptibility to mycobacterial disease who were homozygous with respect to a missense mutation in IFNGR2 creating a new N-glycosylation site in the IFNgammaR2 chain. The resulting additional carbohydrate moiety was both necessary and sufficient to abolish the cellular response to IFNgamma. We then searched the Human Gene Mutation Database for potential gain-of-N-glycosylation missense mutations; of 10,047 mutations in 577 genes encoding proteins trafficked through the secretory pathway, we identified 142 candidate mutations ( approximately 1.4%) in 77 genes ( approximately 13.3%). Six mutant proteins bore new N-linked carbohydrate moieties. Thus, an unexpectedly high proportion of mutations that cause human genetic disease might lead to the creation of new N-glycosylation sites. Their pathogenic effects may be a direct consequence of the addition of N-linked carbohydrate.

A novel mutation (deletion of Aα-Asn 80) in an abnormal fibrinogen

An abnormal fibrinogen was identified in a 10-year-old male with a mild bleeding tendency; several years later, the patient developed a thrombotic event. Fibrin polymerization of plasma from the propositus and his mother, as measured by turbidity, was impaired. Plasmin digestion of fibrinogen and thrombin bound to the clot were both normal. The structure of clots from both plasma and purified fibrinogen was characterized by permeability, scanning electron microscopy and rheological measurements. Permeability of patients' clots was abnormal, although some measurements were not reliable because the clots were not mechanically stable. Consistent with these results, the stiffness of patients' clots was decreased approximately two-fold. Electron microscopy revealed that the patients' clots were very heterogeneous in structure. DNA sequencing of the propositus and his mother revealed a new unique point mutation that gives rise to a fibrinogen molecule with a missing amino acid residue at Aalpha-Asn 80. This new mutation, which would disrupt the alpha-helical coiled-coil structure, emphasizes the importance of this part of the molecule for fibrin polymerization and clot structure. This abnormal fibrinogen has been named fibrinogen Caracas VI.

The effects of additional carbohydrate in the coiled-coil region of fibrinogen on polymerization and clot structure and properties: characterization of the homozygous and heterozygous forms of fibrinogen Lima (Aalpha Arg141-->Ser with extra glycosylation)

Fibrinogen Lima is an abnormal fibrinogen with an Aalpha Arg141-->Ser substitution resulting in an extra N-glycosylation at Aalpha Asn139, which seems to be responsible for the impairment of fibrin polymerization. We have studied the polymerization and properties of clots made from both plasma and purified fibrinogen of both the homozygous and heterozygous forms. The clot permeation studies with both plasma and purified protein revealed a normal flux through the network for the heterozygous form but very decreased permeation in the homozygous form. Consistent with turbidity results, the clot network of the homozygous form, seen by scanning electron microscopy, was tight and composed of thin fibers, with many branch points, while the appearance of clots from the heterozygous form was similar to that of control clots, but in both cases the fibers were more curved than those of control clots. The rheological properties of clots from the homozygous form were also altered, with rigidity being increased in plasma clots, but decreased in the purified system, a consequence of the balance between numbers of branch points and fiber curvature. From these results it seems that the extra carbohydrate moiety, located in the alpha coiled-coil region close to the betaC domains, impairs the protofibril lateral association process, giving rise to thinner, more curved fibers, with the structural anomalies being most pronounced in the clots from the homozygous plasma. These studies support a model for fibrin polymerization in which the betaC-betaC interactions are involved in lateral aggregation.

Structure and function of human fibrinogen inferred from dysfibrinogens

Fibrinogen is a 340-kDa plasma protein that is composed of two identical molecular halves, each consisting of three non-identical subunit polypeptides designated as A alpha, B beta- and gamma-chains held together by multiple disulfide bonds. Fibrinogen has a trinodular structure, i.e., one central E domain comprizing the amino-terminal regions of paired individual three polypeptides, and two identical outer D domains. These three nodules are linked by two coiled-coil regions [1,2]. After activation with thrombin, a tripeptide segment consisting of Gly-Pro-Arg is exposed at the amino-terminus of each alpha-chain residing at the center of the E domain and combines with its complementary binding site, called the 'a' site, residing in the carboxyl-terminal region of the gamma-chain in the outer D domain of another molecule. By crystallographic analysis [3], the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between the carboxyl group of gamma Asp-364 and the carboxyamide of Gln-329 in the 'a' site. Half molecule-staggered, double-stranded fibrin protofibrils are thus formed [4,5]. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280 and Ser-300 of the gamma-chain on the surface of the abutting two D domains [3]. Thereafter, carboxyl-terminal regions of the fibrin a-chains are thought to be untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and interwoven networks after appropriate branching [6-9]. Although many enigmas still remain regarding the mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In my talk, I would like to discuss these molecular interactions of fibrinogen and fibrin based on the up-date data provided by analyses of normal as well as hereditary dysfibrinogens, particularly in the latter by introducing representative molecules at each step of fibrin clot formation.

Mode of perturbation of Asahi fibrin assembly by the extra oligosaccharides

Steric hindrance by the backbone of extra oligosaccharides at gamma-Asn 308 may cause the repulsive force to widen the junction at the D:D interface, and thus, interfere with the longitudinal elongation and lateral association of protofibrils.

Hereditary disorders of fibrinogen

Fibrinogen, a 340-kDa plasma protein, is composed of two identical molecular halves each consisting of three non-identical A alpha-, B beta- and gamma-chain subunits held together by multiple disulfide bonds. Fibrinogen is shown to have a trinodular structure; that is, one central nodule, the E domain, and two identical outer nodules, the D-domains, linked by two coiled-coil regions. After activation with thrombin, a pair of binding sites comprising Gly-Pro-Arg is exposed in the central nodule and combines with its complementary binding site a in the outer nodule of another molecules. By using crystallographic analysis, the alpha-amino group of alpha Gly-1 is shown to be juxtaposed between gamma Asp-364 and gamma Asp-330, and guanidino group of alpha Arg-3 between the carboxyl group of gamma Asp-364 and gamma Gln-329 in the a site. Half molecule-staggered, double-stranded protofibrils are thus formed. Upon abutment of two adjacent D domains on the same strand, D-D self association takes place involving Arg-275, Tyr-280, and Ser-300 of the gamma-chain on the surface of the abutting two D domains. Thereafter, carboxyl-terminal regions of the alpha-chains are untethered and interact with those of other protofibrils leading to the formation of thick fibrin bundles and networks. Although many enigmas still remain concerning the exact mechanisms of these molecular interactions, fibrin assembly proceeds in a highly ordered fashion. In this review, these molecular interactions of fibrinogen and fibrin are discussed on the basis of the data provided by hereditary dysfibrinogens on introducing representative molecules at each step of fibrin clot formation.

The impaired polymerization of fibrinogen Longmont (Bbeta166Arg-->Cys) is not improved by removal of disulfide-linked dimers from a mixture of dimers and cysteine-linked monomers

This study identified a new substitution in the Bbeta chain of an abnormal fibrinogen, denoted Longmont, where the residue Arg166 was changed to Cys. The variant was discovered in a young woman with an episode of severe hemorrhage at childbirth and a subsequent mild bleeding disorder. The neo-Cys residues were always found to be disulfide-bridged to either an isolated Cys amino acid or to the corresponding Cys residue of another abnormal fibrinogen molecule, forming dimers. Removing the dimeric molecules using gel filtration did not correct the fibrin polymerization defect. Fibrinogen Longmont had normal fibrinopeptide A and B release and a functional polymerization site "a." Thus, the sites "A" and "a" can interact to form protofibrils, as evidenced by dynamic light-scattering measurements. These protofibrils, however, were unable to associate in the normal manner of lateral aggregation, leading to abnormal clot formation, as shown by an impaired increase in turbidity. Therefore, it is concluded that the substitution of Arg166-->Cys-Cys alters fibrinogen Longmont polymerization by disrupting interactions that are critical for normal lateral association of protofibrils. (Blood. 2001;98:661-666)

Thrombin inhibition by HCII in the presence of elastase-cleaved HCII and thrombin-HCII complex

The rate of thrombin inhibition by heparin cofactor II (HCII) is facilitated by heparin or dermatan sulfate in vitro. The distributions of these glycosaminoglycans (GAGs) in vivo are not the same; heparin-like substance is rich on the surface of endothelial cells and dermatan sulfate is relatively dominant in the extravascular region. When inflammation takes place, at least two other possible existent forms of HCII, the complexed form with thrombin and the cleaved form by leukocyte elastase, are assumed to be present at relatively high concentrations in a local circumstance. We examined the interactions of HCII with the two forms of HCII on thrombin inhibition in the presence of the GAGs. By HCII in complex with thrombin or cleaved by leukocyte elastase, the affinity of HCII moiety for heparin increases and that for dermatan sulfate decreases. The two forms possibly occur at relatively high concentrations in a local pathological situation, although the heparin cofactor activity for thrombin inhibition by HCII decreases and dermatan sulfate determines the cofactor activity. These results indicate efficient thrombin inhibitory activity of HCII in the extravascular region.

Fibrinogen Niigata With Impaired Fibrin Assembly: An Inherited Dysfibrinogen With a Bβ Asn-160 to Ser Substitution Associated With Extra Glycosylation at Bβ Asn-158

A novel BβAsn-160 (TAA) to Ser (TGA) substitution has been identified in fibrinogen Niigata derived from a 64-year-old asymptomatic woman, who is heterozygotic for this abnormality. The mutation creates an Asn-X-Ser–type glycosylation sequence, and a partially sialylated biantennary oligosaccharide was linked to the BβAsn-158 residue. The functional abnormality was attributed to delayed lateral association of normally formed double-stranded protofibrils based on normal cross-linking of fibrin γ-chains and tissue-type plasminogen activator-catalyzed plasmin generation by polymerizing fibrin monomers. Enzymatic removal of all the N-linked oligosaccharides from fibrinogen Niigata accelerated fibrin monomer polymerization that reached the level of untreated normal fibrin monomers, but the thrombin time was prolonged from 18.2 seconds to 113 seconds (normal: 11.2 seconds to 8.9 seconds). By scanning electron micrographic analysis, Niigata fibrin fibers were found to be more curvilinear than normal fibrin fibers. After deglycosylation, Niigata fibers became straight being similar to untreated normal fibrin fibers, whereas normal deglycosylated fibrin appeared to be less-branched than untreated normal or deglycosylated Niigata fibrin. Although normal and Niigata fibrins were similar to each other in permeation and compaction studies, deglycosylated normal and Niigata fibrins had much higher permeability and compaction values, indicating that deglycosylation had brought about the formation of more porous networks. The enzymatic deglycosylation necessitates an Asn to Asp change at position Bβ-158 that is responsible for reducing the fiber thickness because of either local repulsive forces or steric hindrance in the coiled-coil region.

Fibrinogen Niigata With Impaired Fibrin Assembly: An Inherited Dysfibrinogen With a Bβ Asn-160 to Ser Substitution Associated With Extra Glycosylation at Bβ Asn-158

A novel BβAsn-160 (TAA) to Ser (TGA) substitution has been identified in fibrinogen Niigata derived from a 64-year-old asymptomatic woman, who is heterozygotic for this abnormality. The mutation creates an Asn-X-Ser–type glycosylation sequence, and a partially sialylated biantennary oligosaccharide was linked to the BβAsn-158 residue. The functional abnormality was attributed to delayed lateral association of normally formed double-stranded protofibrils based on normal cross-linking of fibrin γ-chains and tissue-type plasminogen activator-catalyzed plasmin generation by polymerizing fibrin monomers. Enzymatic removal of all the N-linked oligosaccharides from fibrinogen Niigata accelerated fibrin monomer polymerization that reached the level of untreated normal fibrin monomers, but the thrombin time was prolonged from 18.2 seconds to 113 seconds (normal: 11.2 seconds to 8.9 seconds). By scanning electron micrographic analysis, Niigata fibrin fibers were found to be more curvilinear than normal fibrin fibers. After deglycosylation, Niigata fibers became straight being similar to untreated normal fibrin fibers, whereas normal deglycosylated fibrin appeared to be less-branched than untreated normal or deglycosylated Niigata fibrin. Although normal and Niigata fibrins were similar to each other in permeation and compaction studies, deglycosylated normal and Niigata fibrins had much higher permeability and compaction values, indicating that deglycosylation had brought about the formation of more porous networks. The enzymatic deglycosylation necessitates an Asn to Asp change at position Bβ-158 that is responsible for reducing the fiber thickness because of either local repulsive forces or steric hindrance in the coiled-coil region.

Localization of an effective fibrin beta-chain polymerization site: implications for the polymerization mechanism

To examine whether fibrin N-terminal Aalpha 17-23 and Bbeta 15-25 may contain high-affinity polymerization sites, GPRVVER and GHRPLDKKREE analogs were prepared, and their abilities to inhibit fibrin monomers from repolymerizing were compared in turbidity and clottability assays. Within Aalpha 17-23, GPR is the most active site (IC30 of 0.95-1.36 mM). Its extension into GPRVVER (IC30 of 1.75-2.3 mM) reduced activity. Within Bbeta 15-25, acyl-DKKREE (IC30 of 0.30-0.53 mM) can account for GHRPLDKKREE activity (IC30 of 0. 33-0.44 mM). Comparison of the assays showed that calcium, whose presence induces thick fibrin fibers, elicited a higher turbidity than clottability inhibition. Similarly, the lateral-association-promoting GHRP (IC30 of 1.25-1.43 mM) gave a high turbidity vs clottability inhibition ratio (137%). In contrast, low ratios were found for the linear-association-initiating GPR (73%) and for acyl-DKKREE (34%). Structure-activity correlation showed that fibrinogen-like acyl-GPRP and acyl-GHRP could inhibit D. E association at the millimolar range, but in a manner different from fibrin-related GPR peptides did, which required the NH2 as well as Arg presence. To explain Bbeta 20-25 masking, it is proposed that DKKREE in fibrinogen may engage in ionic and hydrogen bonds with KDSDW, the Aalpha 29-33 sequence implicated in thrombin binding. To explain acyl-GPRP and acyl-GHRP inhibition of D.E association, it is proposed that fibrinogen packing may be mediated by E domain association with alphaC (Aalpha 220-609) fragments of adjacent molecules, and by alphaC-alphaC association. A modified polymerization mechanism is deduced by taking into account fibrinogen N-terminal conformation as well as E domain binding to thrombin vs alphaC fragments. This model proposes the following. (1) Upon thrombin binding to fibrinogen KDSDW, DKKREE may become exposed. (2) Fibrinopeptide A cleavage further unmasks the NH2 and Arg group of GPR, leading to DKKREE and GPR initiation of polymerization. (3) The micromolar-effective thrombin-fibrin(ogen) binding may initiate a partial alphaC repulsion. Subsequent DKKREE and GPR binding to D domains of other fibrin(ogen) will lead to the formation of the trimer and bring additional molecules to fibrin N-terminal region, and the combined steric congestion may lead to a complete alphaC repulsion from the overcrowded E domain. (4) Repulsion of the large Aalpha 220-609 fragments may unmask multiple polymerization sites beyond the fibrin N-terminal region.