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Date of Award

Fall 2012

Degree Type

Thesis

Degree Name

Master of Health Sciences (MSc)

Department

Medical Sciences

Supervisor

Bill Sheffield

Language

English

Committee Member

Jeffrey Weitz, Patricia Liaw

Abstract

The serpin α-1 proteinase inhibitor (API) normally only impacts the coagulation cascade through its ability to inactivate factor XIa. However, the point mutation (Met to Arg) at position 358 results in a potent thrombin inhibitor, API M358R. This mutation also enhances this serpin’s ability to inhibit the anticoagulant protein, activated protein C (APC) and hence this property limits its therapeutic potential. As a result, various modifications to this protein have been engineered in order to enhance its specificity towards thrombin. Previously, the Heparin Cofactor II (HCII) N-terminal tail, HCII 1-75, which binds exosite 1 of thrombin, was tethered to the N-terminus of API M358R, creating HAPI M358R. Although this change did not alter anti-APC activity, it did augment the anti-thrombin activity of API M358R. In addition, further changes in the reactive center loop, the region that interacts with the thrombin active site, resulted in a significant reduction in APC activity while maintaining antithrombotic activity similar to HAPI M358R; this variant was termed HAPI RCL5.

Preliminary experiments were performed with the C-terminal tridecapeptide of Hirudin Variant 3 (HV3) to determine its exosite 1 binding capacity compared to HCII 1-75. Three different variants of this peptide were tested: one with a hexahistidine tag (H6HV354-66), another that also had a hexa-glycine C-terminal addition (H6HV354-66G6) and a third without either addition. All were found to bind exosite 1 with a greater affinity than HCII 1-75. Thus, the H6HV354-66G6 peptide was fused to API M358R and API RCL5 in hopes of creating an inhibitor with heightened specificity compared to HAPI M358R and HAPI RCL5, respectively.

HV3API M358R and HV3API RCL5 were expressed in a bacterial system and purified by nickel-chelate and ion exchange chromatography. Second order rate constants for the inhibition of thrombin and APC by the API variants and fusion proteins were determined. The K2 values for α-thrombin inhibition ranged from 186 M-1min-1 to 22 M-1min-1 with an order of inhibitory potency observed as follows: HAPI M358R > HAPI RCL5 > HV3API M358R > HV3API RCL5>API RCL5 > API M358R.

The ability of recombinant chimeric serpins to bind thrombin exosite 1 in a manner independent of RCL-thrombin active site interactions was also investigated through competitive inhibition of the binding of active site-inhibited thrombin to immobilized HCII 1-75. It was found that the order of exosite 1 binding affinity was HV3API RCL5 > H6HV354-66G6> HCII 1-75 > HAPI RCL5. Our results indicate that fusing the C-terminal tridecapeptide of HV3 to API variants enhanced their ability to inhibit thrombin, but to a lesser extent than fusing the N-terminal 75 residues of HCII. This finding likely reflects a requirement for the exosite 1-binding motif of the fusion protein to bind exosite 1 in a way that allows for subsequent optimal active site attack on the RCL by the serpin moiety of the fusion protein. In general, this work provides a second novel example of how the activity of a thrombin-inhibitory serpin can be enhanced by fusion to an exosite-1 binding motif.

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