Conformational Dynamics of Prothrombin Dictate Its Ordered Cleavage by Prothrombinase

Abstract 3359

The prothrombinase complex (membranes/Xa/Va) binds prothrombin through exosite interactions which orient and present the substrate to the protease active site for cleavage. Although both cleavage sites within the substrate are accessible, prothrombin is preferentially activated by prothrombinase through sequential cleavage at R₃₂₀ followed by cleavage at R₂₇₁. We investigated the exosite-dependent interaction between prothrombin and prothrombinase using fluorescence resonance energy transfer (FRET). An acceptor fluorophore was placed onto the prothrombin cleavage site by introducing the mutation R₂₇₁C and fluorescently modifying the introduced thiol (II_C271*). The active site of Xa was covalently modified with a chloromethyl ketone conjugated to a donor fluorescent compound. Titrations of the fluorescently labeled prothrombinase complex with II_C271* revealed a maximum energy transfer efficiency of 70–80 % observed with two different donor probes. Resonance energy transfer arising from the substrate docking interaction was confirmed by equivalent changes in the excited state lifetime of the donors. The calculated distance between the probe at C₂₇₁ and the probe tethered at the active site of Xa ranged between 34 and 40 Å. Variation in the measured distance reflects the different lengths of the tethers used to connect the donor probes. The FRET measurements, validated with two donor-acceptor pairs, indicate that the 271 site within prothrombin docked to prothrombinase is positioned distant from the active site of Xa within the enzyme complex. This distant constraint prevents R₂₇₁ from effectively engaging the active site of Xa within prothrombinase, thereby providing a physical explanation for the ordered action of prothrombinase on prothrombin. It follows that initial cleavage at R₃₂₀ must reorient the substrate to allow R₂₇₁ access to the Xa active site. We pursued an intramolecular FRET approach in which both donor and acceptor probes were placed on the prothrombin molecule to investigate the putative conformational change of prothrombin as a result of initial cleavage. Orthogonal labeling of prothrombin was accomplished using a prothrombin variant containing C₂₇₁ and a LPETG extension at its C terminus. The optimized C₂₇₁ label was used for the first modification. For the second, the sequence-specific transpeptidation activity of bacterial Sortase A was used to incorporate a fluorophore at the C-terminus (II_Cterm*). To assess the new fluorescent modification, FRET studies of prothrombin binding to prothrombinase were repeated with the new II_Cterm* construct. Titrations revealed a maximal energy transfer of 16 %, confirmed by corresponding changes in excited state lifetime and yielded a Kd = 39 nM indistinguishable from the Kd= 41 nM seen for II_C271* binding. Thus, two different approaches illustrate that the affinity of prothrombin for prothrombinase is independent of active site docking by the substrate. To examine the intramolecular dynamics of prothrombin during activation, the C₂₇₁ site was labeled with BADAN (donor) and the Sortase labeling method placed the Alexa532 (acceptor) at the C terminus. The orthogonal placement of BADAN and Alexa532 on the prothrombin molecule resulted in extensive energy transfer between the two probes. The cleavage of doubly-labeled prothrombin to produce meizothrombin was associated with a change in intramolecular energy transfer. The variation of the energy transfers observed between the zymogen and protease forms can be interpreted as global rearrangement of the prothrombin molecule renders the second cleavage site accessible of active site docking. The FRET studies provide a comprehensive physical explanation for the ordered cleavage of prothrombin by prothrombinase. Initial cleavage occurs at the spatially accessible cleavage site R₃₂₀. The prothrombin molecule then significantly re-orients itself making the second R₂₇₁ cleavage site accessible to prothrombinase in order for activation to proceed and produce thrombin.