Introduction of a mutation in the shutter region of antithrombin (Phe77 --> Leu) increases affinity for heparin and decreases thermal stability

Engineering the serpin α1 -antitrypsin: A diversity of goals and techniques

α₁ -Antitrypsin (α₁ -AT) serves as an archetypal example for the serine proteinase inhibitor (serpin) protein family and has been used as a scaffold for protein engineering for >35 years. Techniques used to engineer α₁ -AT include targeted mutagenesis, protein fusions, phage display, glycoengineering, and consensus protein design. The goals of engineering have also been diverse, ranging from understanding serpin structure-function relationships, to the design of more potent or more specific proteinase inhibitors with potential therapeutic relevance. Here we summarize the history of these protein engineering efforts, describing the techniques applied to engineer α₁ -AT, specific mutants of interest, and providing an appended catalog of the >200 α₁ -AT mutants published to date.

Characterization of Protein Z-Dependent Protease Inhibitor/Antithrombin Chimeras Provides Insight into the Serpin Specificity of Coagulation Proteases

Protein Z (PZ)-dependent protease inhibitor (ZPI) and antithrombin (AT) are two physiological serpin inhibitors involved in the regulation of proteolytic activities of the blood coagulation cascade. ZPI has restricted protease specificity capable of inhibiting factors Xa (FXa) and XIa (FXIa) but exhibiting no reactivity with other coagulation proteases. Unlike ZPI, AT is a general inhibitor of all coagulation proteases and the only physiological inhibitor of factor IXa (FIXa). To understand the molecular determinants of protease specificity of the two serpins, we engineered two ZPI mutants in which the P12-P3' residues of the reactive center loop of ZPI were replaced with either P12-P3' or P12-P7' residues of AT (ZPI-AT^P12-P3' and ZPI-AT^P12-P7'). The reactivity of chimeras with FXa was improved ∼4-25-fold in the absence of PZ. Both chimeras inhibited FIXa with rate constants that were ∼2 orders of magnitude higher than the rate of the AT inhibition of the protease. PZ improved the reactivity of chimeras with FIXa by another 2 orders of magnitude, rendering the chimeras potent inhibitors of FIXa so that the PZ-mediated inhibitory activity of the ZPI-AT chimeras toward FIXa was ∼20-fold higher than that of the fondaparinux-catalyzed inhibition of FIXa by AT. Further studies revealed that the substitution of P1-Tyr of ZPI with an Arg is sufficient to convert the serpin to an effective inhibitor of FIXa. The potential therapeutic utility of the serpin chimeras as specific inhibitors of FIXa was diminished by findings that the chimeras function as effective substrates for other coagulation proteases.

High levels of latent antithrombin in plasma from patients with antithrombin deficiency

Antithrombin is an anticoagulant serpin that efficiently inhibits multiple procoagulant proteases. The cost for the structural flexibility required for this function is the vulnerability to mutations that impact its folding pathway. Most conformational mutations identified in serpins cause polymerisation. Only three mutations in SERPINC1 affecting two residues have been found to favour transformation to the latent conformation of antithrombin, another hyperstable non-anticoagulant form with strong antiangiogenic activity that constitutes 3 % of plasma antithrombin in healthy subjects. The analysis of latent antithrombin in 141 unrelated patients with antithrombin deficiency carrying 89 different SERPINC1 mutations identified four cases with higher levels than that of controls: p.Pro439Thr, p.Pro461Ser, p.Met283Val, and p.His401Tyr, the last also with circulating polymers. Heating of plasma at 42ºC exacerbated the transformation to the latent conformation in p.Pro439Thr and p.Pro461Ser. The conformational effect of p.Met283Val, the mutation associated with the highest levels of latent antithrombin detected in four members of a family, was verified in a recombinant model. Antithrombin deficiency in these cases should be classified as pleiotropic based on the impaired reactivity and low heparin affinity of the variant. Despite high levels of latent antithrombin (up to 80 µg/ml in p.Met283Val carriers), no vascular defects were described in carriers of these mutations. In conclusion, our study identifies new residues involved in the structural stability of antithrombin (and potentially of all serpins). High levels of endogenous latent antithrombin seem to play a minor antiangiogenic effect. Finally, pleiotropic deficiencies may be caused by mutations inducing transformation to the latent conformation.

The Influence of Hinge Region Residue Glu-381 on Antithrombin Allostery and Metastability

Antithrombin becomes an efficient inhibitor of factor Xa and thrombin by binding a specific pentasaccharide sequence found on a small fraction of the heparan sulfate proteoglycans lining the microvaculature. In the structure of native antithrombin, the reactive center loop is restrained due to the insertion of its hinge region into the main beta-sheet A, whereas in the heparin-activated state the reactive center loop is freed from beta-sheet A. In both structures, hinge region residue Glu-381 makes several stabilizing contacts. To determine the role of these contacts in the allosteric mechanism of antithrombin activation, we replaced Glu-381 with an alanine. This variant is less active toward its target proteases than control antithrombin, due to a perturbation of the equilibrium between the two forms, and to an increase in stoichiometry of inhibition. Pentasaccharide binding affinity is reduced 4-fold due to an increase in the off-rate. These data suggest that the main role of Glu-381 is to stabilize the activated conformation. Stability studies also showed that the E381A variant is resistant to continued insertion of its reactive center loop upon incubation at 50 degrees C, suggesting new stabilizing interactions in the native structure. To test this hypothesis, and to aid in the interpretation of the kinetic data we solved to 2.6 A the structure of the variant. We conclude that wild-type Glu-381 interactions stabilize the activated state and decreases the energy barrier to full loop insertion.