Tryptophans 231 and 234 in Protein C Report the Ca2+-Dependent Conformational Change Required for Activation by the Thrombin-Thrombomodulin Complex

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Abstract
Human protein C circulates as both single- and two-chain zymogens. Activation by the physiological activation complex, thrombin-thrombomodulin, generates the anticoagulant enzyme, activated protein C. Ca2+ binding to the protease domain of protein C is accompanied by 5.5 +/- 0.2% quenching of intrinsic fluorescence that correlates with the conformational change required for the rapid activation by the thrombin-thrombomodulin complex. To map which Trp residues report this Ca2+ binding, candidate Trp residues at positions 84, 115, 145, 205, 231, and 234 were changed individually to Phe within a protein C deletion mutant lacking the Gla domain (GDPC). Of these, the Trp to Phe mutation at position 231 (W231F) eliminated the Ca(2+)-induced fluorescence quenching, and the Trp 234 to Phe mutation (W234F) increased the maximum quenching in protein C to 9.4 +/- 0.4%. Upon Ca2+ binding, the fluorescence emission intensity of the W231F mutant was increased 3.4% +/- 0.6%. The Kd for this site (84 +/- 20 microM) was similar to that of GDPC (Kd = 39 +/- 4 microM). To compare the properties of single- and two-chain protein C, we replaced the Lys156-Arg157 dipeptide cleavage site in protein C with Thr and Gln to form GDPCKR/TQ. GDPCKR/TQ and the two-chain form of protein C were activated at the same rate with the thrombin-thrombomodulin complex, they exhibited similar Ca2+ dependence for both activation and fluorescence quenching, and these enzymes had the same chromogenic activity. In contrast to the zymogen form, activated human Gla-domainless protein C did not undergo a Ca(2+)-induced fluorescence change. These results indicate that the environment of Trp 231 and 234 within the Ca2+ binding loop of the protein C zymogen are perturbed by Ca2+ binding to the zymogen.

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