Probing thrombin's ability to accommodate a V34F substitution within the factor XIII activation peptide segment (28-41)*

Screening cleavage of Factor XIII V34X Activation Peptides by thrombin mutants: A strategy for controlling fibrin architecture

In blood coagulation, thrombin converts fibrinogen into fibrin monomers that polymerize into a clot network. Thrombin also activates Factor XIII by cleaving the R37-G38 peptide bond of the Activation Peptide (AP) segment. The resultant transglutaminase introduces covalent crosslinks into the fibrin clot. A strategy to modify clot architecture would be to design FXIII AP sequences that are easier or more difficult to be thrombin-cleaved thus controlling initiation of crosslinking. To aid in this design process, FXIII V34X (28-41) Activation Peptides were kinetically ranked for cleavage by wild-type thrombin and several anticoagulant mutants. Thrombin-catalyzed hydrolysis of aromatic FXIII F34, W34, and Y34 APs was compared with V34 and L34. Cardioprotective FXIII L34 remained the variant most readily cleaved by wild-type thrombin. The potent anticoagulant thrombins W215A and W215A/E217A (missing a key substrate platform for binding fibrinogen) were best able to hydrolyze FXIII F34 and W34 APs. Thrombin I174A and L99A could effectively accommodate FXIII W34 and Y34 APs yielding kinetic parameters comparable to FXIII AP L34 with wild-type thrombin. None of the aromatic FXIII V34X APs could be hydrolyzed by thrombin Y60aA. FXIII F34 and W34 are promising candidates for FXIII - anticoagulant thrombin systems that could permit FXIII-catalyzed crosslinking in the presence of reduced fibrin formation. By contrast, FXIII Y34 with thrombin (Y60aA or W215A/E217A) could help assure that both fibrin clot formation and protein crosslinking are hindered. Regulating the activation of FXIII is predicted to be a strategy for helping to control fibrin clot architecture and its neighboring environments.

Design of Factor XIII V34X activation peptides to control ability to interact with thrombin mutants

Thrombin helps to activate Factor XIII (FXIII) by hydrolyzing the R37-G38 peptide bond. The resultant transglutaminase introduces cross-links into the fibrin clot. With the development of therapeutic coagulation factors, there is a need to better understand interactions involving FXIII. Such knowledge will help predict ability to activate FXIII and thus ability to promote/hinder the generation of transglutaminase activity. Kinetic parameters have been determined for a series of thrombin species hydrolyzing the FXIII (28-41) V34X activation peptides (V34, V34L, V34F, and V34P). The V34P substitution introduces PAR4 character into the FXIII, and the V34F exhibits important similarities to the cardioprotective V34L. FXIII activation peptides containing V34, V34L, or V34P could each be accommodated by alanine mutants of thrombin lacking either the W60d or Y60a residue in the 60-insertion loop. By contrast, FXIII V34F AP could be cleaved by thrombin W60dA but not by Y60aA. FXIII V34P is highly reliant on the thrombin W215 platform for its strong substrate properties whereas FXIII V34F AP becomes the first segment that can maintain its K(m) upon loss of the critical thrombin W215 residue. Interestingly, FXIII V34F AP could also be readily accommodated by thrombin L99A and E217A. Hydrolysis of FXIII V34F AP by thrombin W217A/E217A (WE) was similar to that of FXIII V34L AP whereas WE could not effectively cleave FXIII V34P AP. FXIII V34F and V34P AP show promise for designing FXIII activation systems that are either tolerant of or greatly hindered by the presence of anticoagulant thrombins.

Effects of introducing fibrinogen Aalpha character into the factor XIII activation peptide segment

The formation of a blood clot involves the interplay of thrombin, fibrinogen, and Factor XIII. Thrombin cleaves fibrinopeptides A and B from the N-termini of the fibrinogen Aalpha and Bbeta chains. Fibrin monomers are generated that then polymerize into a noncovalently associated network. By hydrolyzing the Factor XIII activation peptide segment at the R37-G38 peptide bond, thrombin assists in activating the transglutaminase FXIIIa that incorporates cross-links into the fibrin clot. In this work, the kinetic effects of introducing fibrinogen Aalpha character into the FXIII AP segment were examined. Approximately 25% of fibrinogen Aalpha is phosphorylated at Ser3, producing a segment with improved binding to thrombin. FXIII AP ((22)AEDDL(26)) has sequence properties in common with Fbg Aalpha ((1)ADSpGE(5)). Kinetic benefits to FXIII AP cleavage were explored by extending FXIII AP (28-41) to FXIII AP (22-41) and examining peptides with D24, D24S, D24Sp, and D24Sp P27G. These modifications did not provide the same kinetic advantages that were observed with Fbg Aalpha (1-20) S3p. Such results further emphasize that FXIII AP derives most of its substrate specificity from the P(9)-P(1) segment. To enhance the kinetic properties of FXIII AP (28-41), we introduced substitutions at the P(9), P(4), and P(3) positions. Studies reveal that FXIII AP (28-41) V29F, V34G, V35G exhibits kinetic improvements that are comparable to those of FXIII AP V29F, V34L and approach those of Fbg Aalpha (7-20). Selective changes to the FXIII AP segment sequence may be used to design FXIII species that can be activated more or less readily.

Employing mutants to study thrombin residues responsible for factor XIII activation peptide recognition: a kinetic study

In the last stages of coagulation, thrombin helps to activate Factor XIII. The resultant transglutaminase introduces covalent cross-links into fibrin thus promoting clot stability. To better understand the roles of individual thrombin residues in recognition and hydrolysis of the Factor XIII activation peptide, mutations within thrombin's aryl and apolar binding site were explored. The thrombin mutants W215A, E217A, W215A/E217A, L99A, and I174A were examined through HPLC kinetics against the substrates FXIII (28-41) V34 AP and FXIII (28-41) V34L AP. Several mutants responded differently to FXIII (28-41) V34 AP vs the cardioprotective V34L AP. W215 provides an important platform for binding and directing FXIII APs for proper hydrolysis. Loss of this platform leads to decreases in kinetics, particularly to the kcat of FXIII V34L AP. E217 also plays a supporting role, but the E217A mutation is not as detrimental as W215A. W215A/E217A is unfavorable for both activation peptides and its coupling effect has been characterized. This mutant can readily bind the peptides but cannot orient them for effective hydrolysis. Kinetic studies with I174A indicate that this thrombin residue is more crucial for interactions with the larger V34L AP segment. The L99A mutation causes deleterious effects to binding and hydrolysis of both APs. The V34L, however, is able to partially compensate for the loss perhaps by increasing contact within the aryl and apolar sites. Understanding how specific FXIII and thrombin residues participate in binding and control hydrolysis may lead to the design of coagulation enzymes whose degree of activation and optimal target site can be controlled.

Probing interactions between the coagulants thrombin, Factor XIII, and fibrin(ogen)

Thrombin cleaves fibrinopeptides A and B from fibrinogen leading to the formation of a fibrin network that is later covalently crosslinked by Factor XIII (FXIII). Thrombin helps activate FXIII by catalyzing hydrolysis of the FXIII activation peptides (AP). In the current work, the role of exosites in the ternary thrombin-FXIII-fibrin(ogen) complex was further explored. Hydrolysis studies indicate that thrombin predominantly utilizes its active site region to bind extended Factor XIII AP (FXIII AP 33-64 and 28-56) leaving the anion-binding exosites for fibrin(ogen) binding. The presence of fibrin-I leads to improvements in the K(m) for hydrolysis of FXIII AP (28-41), whereas peptides based on the cardioprotective FXIII V34L sequence exhibit less reliance on this cofactor. Surface plasmon resonance measurements reveal that d-Phe-Pro-Arg-chloromethylketone-thrombin binds to fibrinogen faster than to FXIII a(2) and dissociates from fibrinogen more slowly than from FXIII a(2). This system of thrombin exosite interactions with differing affinities promotes efficient clot formation.