Susan C. Bock
Professor of Medicine and of Bioengineering and of Medicinal Chemistry
B.S. Massachusetts Institute of Technology
Ph.D. University of California, Irvine
Research
Our lab research focuses on the proteinase inhibitor antithrombin III (ATIII), an essential, endogenous anticoagulant protein. There are currently 2 NIH-funded projects with structure/function/mechanism and clinical translation themes.
Conformational Change Propagation in ATIII-Heparin
The native circulating conformation of ATIIII is an inefficient proteinase inhibitor due to partial insertion of its reactive center loop in its central A beta-sheet. For full activation, cofactor heparin must bind and induce a protein conformational change that leads to reactive loop expulsion and a ~300x increase in the fXa inhibition rate. The goals of this project are to identify ATIII structural elements and interactions that mediate conformational change transmission across the ~35 Å distance between the pentasaccharide-binding site and the reactive center loop. A model for ATIII heparin-dependent conformational change propagation was proposed and is being evaluated and refined by disrupting hypothesized, critical structural interactions, and by determining how the introduced changes affect heparin binding kinetics and affinities, allosteric activation of fXa inhibition, and the molecular structures of the mutants.
ATIII Targeting to Vascular and Biomaterial Surfaces
Activation of thrombin and fXa occurs on vascular and biomaterial surfaces and initiates coagulation, platelet, signaling and cell proliferation reactions, which may subsequently promote pathological thrombosis, occlusion and restenosis in the native circulatory system and on surfaces of implanted biomaterials and medical devices. Early intervention via the neutralization of thrombin and fXa molecules as they are generated at blood - surface interfaces would be a widely applicable strategy for reducing thrombin/fXa mediated pathologies. Implementation of this approach will require blood-borne inhibitors that target vascular and biomaterial surfaces and that remain stable under focal and systemic inflammatory conditions. We have developed candidate "super-beta ATIIIs" that inhibit thrombin and fXa with comparable efficiency to plasma-derived ATIII, but bind the pentasaccharide sequence of heparin and vascular wall heparan sulfate proteoglycans (HSPGs) with 50x greater affinity. Under flow conditions, the recombinants also load onto heparin-coated surfaces up to 7x more efficiently than does plasma ATIII, and are 10x more resistant to cleavage and inactivation by neutrophil elastase. The targeted ATIII project will investigate how mass transport and heparin binding affinity factors influence ATIII surface delivery under low-vs-high wall shear rate & steady-vs-pulsatile flow conditions, using in vitro flow model measurements and mathematical simulations. We will also evaluate super-beta-ATIII performance in vivo, using rabbit extracorporeal shunt and carotid stenosis models of thrombosis.
References
1. Dela Cruz RG, Jairajpuri MA, Bock SC (2006) Disruption of a tight cluster surrounding tyrosine-131 in the native conformation of antithrombin III activates it for factor Xa inhibition. J Biol Chem. 281:31668-31676
2. Bock SC, Picard V, Zendehrouh P (2005) Human Antithrombin IIIs and Methods Related Thereto. U.S. Patent #6,878,813
3. Bock SC (2005) Antithrombin III and Heparin Cofactor II. Chapter 13 in Hemostasis and Thrombosis, Basic Principles and Clinical Practice , 5th edition. R.W. Colman et al., eds. J.B. Lippincott, Philadelphia
4. Schedin-Weiss S, Desai UR, Bock SC, Olson ST, Björk I (2004) Roles of N-terminal region residues Lys11, Arg13 and Arg24 of antithrombin in heparin recognition and in promotion and stabilization of the heparin-induced conformational change. Biochemistry 43:675-83
5. Jairajpuri MA, Lu A, Desai U, Olson ST, Bjork I, Bock SC (2003) Antithrombin III phenylalanines 122 and 121 contribute to its high affinity for heparin and its conformational activation. J. Biol. Chem. 278:15941-50
6. Schedin-Weiss S, Desai UR, Bock SC, Gettins PGW, Olson ST, Bjork I (2002) The Importance of Lysine 125 for Heparin Binding and Activation of Antithrombin. Biochemistry 15:4779-4788
7. Chuang YJ, Richard Swanson R, Raja SM, Bock SC, Olson ST (2001) The Antithrombin P1 Residue Is Important for Target Proteinase Specificity but Not for Heparin Activation of the Serpin. Characterization of P1 Antithrombin Variants with Altered Proteinase Specificity but Normal Heparin Activation. Biochemistry 40:6670-6679
8. Bock SC (2001) Antithrombin III and Heparin Cofactor II. Chapter 14 in Hemostasis and Thrombosis, Basic Principles and Clinical Practice, 4th edition. R.W. Colman, J. Hirsh, V.J. Marder, A.W. Clowes, J.N. George, eds. J.B. Lippincott, Philadelphia
9. Desai U, Swanson R, Bock SC, Bjork I, Olson ST (2000) Role of Arginine 129 in Heparin Binding and Activation of Antithrombin. J. Biol. Chem. 275:18976-84
10. Arocas V, Turk B, Bock SC, Olson ST, Bjork I (2000) The Region of Antithrombin Interacting with Full-Length Heparin Chains Outside the High-Affinity Pentasaccharide Sequence Extends to Lys 136 But Not to Lys 139. Biochemistry 39:8512-8
