Topographical relationships between the monovalent and divalent cation binding sites of des-1-41-light chain bovine plasma protein C and des-1-41-light chain-activated bovine plasma protein C

Jahn-Teller dynamics in a series of high-symmetry Co(II) chelates determine paramagnetic relaxation enhancements

NMR paramagnetic relaxation enhancements (PREs) of a series of structurally characterized, trigonal bis-trispyrazolylborate (Tp) chelates of high-spin Co(II), spanning 100-850 MHz in field, are reported. Prior knowledge of the metal-nucleus distances allows numerical extraction of position-dependent electron spin relaxation rates (τ(c)(-1)) from direct measurement of the individual PREs of the four symmetry distinct protons in Co(Tp)(2), using available closed-form expressions. The data for this electronically complex system where spin-orbit coupling defines the ground state electronic structure are analyzed in terms of the Solomon-Bloembergen-Morgan (SBM) relations, as well as available zero-field splitting limit theories. A simple angular correction is shown to be sufficient to reconcile the individual τ(c)(T) data for the four classes of protons. The data identify a previously unrecognized dynamic Jahn-Teller effect in these historically important complexes, with a barrier of ~230 cm(-1), pointing to a level of dynamics in trispyrazolylborate chemistry that has not been described before, and further show that it is the Jahn-Teller that is responsible for the PREs in fluid solution. A field-dependent component is also identified for the two protons nearest g(//), which is suggested to arise due to Zeeman mixing of excited state character into the ground level.

Identification and characterization of the sodium-binding site of activated protein C

Activated protein C (APC) requires both Ca2+ and Na+ for its optimal catalytic function. In contrast to the Ca2+-binding sites, the Na+-binding site(s) of APC has not been identified. Based on a recent study with thrombin, the 221-225 loop is predicted to be a potential Na+-binding site in APC. The sequence of this loop is not conserved in trypsin. We engineered a Gla domainless form of protein C (GDPC) in which the 221-225 loop was replaced with the corresponding loop of trypsin. We found that activated GDPC (aGDPC) required Na+ (or other alkali cations) for its amidolytic activity with dissociation constant (Kd(app)) = 44.1 +/- 8.6 mM. In the presence of Ca2+, however, the requirement for Na+ by aGDPC was eliminated, and Na+ stimulated the cleavage rate 5-6-fold with Kd(app) = 2.3 +/- 0.3 mM. Both cations were required for efficient factor Va inactivation by aGDPC. In the presence of Ca2+, the catalytic function of the mutant was independent of Na+. Unlike aGDPC, the mutant did not discriminate among monovalent cations. We conclude that the 221-225 loop is a Na+-binding site in APC and that an allosteric link between the Na+ and Ca2+ binding loops modulates the structure and function of this anticoagulant enzyme.