Structure and function of factor X: properties, activation, and activity in prothrombinase. A retrospective in a historical context.

The evolution of our understanding of the formation of thrombin from the postulated thrombokinase of Morawitz to activated Factor X and prothrombinase occurred during a period of nearly 100 years. During this time structure-function relationships have emerged and the roles of phospholipid surfaces, the accessory factor, Factor V and its activated form have been clarified. This paper summarizes this story with particular acknowledgement of the seminal contributions of Haskell Milstone.

Low molecular weight heparins differ substantially: impact on developing biosimilar drugs.

Generic drugs are an important component for meaningful health-care reform currently being debated in the United States. Aside from defining the period of drug exclusivity, however, there is a critical need to ensure that generics of biologic medicines (biosimilars) are safe and effective. For low molecular weight heparins (LMWHs), the standard of care for management of venous thromboembolism, their complex structure and polypharmacological actions make producing a generic LMWH more challenging than a generic small molecule medicine. Because biosimilar LMWHs will be used interchangeably with their branded product, inherent variability between products could lead to important differences in potency, safety, or effectiveness, including unanticipated immune responses. Awareness of the specific problems associated with biosimilar LMWH development led to new recommendations from several expert bodies. This article discusses the implications of these differences for the production of biosimilar LMWHs and provides recommendations to address the limitations in the pending U.S. Congress legislation, a well-intentioned undertaking but one that must preserve the health and welfare of citizens who require these critical care medications.

Models for reaction mechanisms in haemostasis--contributions from the study of prothrombin activation.

A brief history of the development of the contemporary model for the mechanism of prothrombin activation is presented. The focus is on the advances in understanding structure-function relationships in the molecules that comprise "prothrombinase" that occurred primarily during the 1970s. A link between the "classical theory" of hemostasis and the conceptual development of activation complexes as the activators of the precursors of coagulation proteases is developed. It is argued that advances occurred when new ideas arose that could be tested and new technologies enabled more definitive experiments to be performed.

Interpretation of the temperature dependence of equilibrium and rate constants.

The objective of this review is to draw attention to potential pitfalls in attempts to glean mechanistic information from the magnitudes of standard enthalpies and entropies derived from the temperature dependence of equilibrium and rate constants for protein interactions. Problems arise because the minimalist model that suffices to describe the energy differences between initial and final states usually comprises a set of linked equilibria, each of which is characterized by its own energetics. For example, because the overall standard enthalpy is a composite of those individual values, a positive magnitude for DeltaH(o) can still arise despite all reactions within the subset being characterized by negative enthalpy changes: designation of the reaction as being entropy driven is thus equivocal. An experimenter must always bear in mind the fact that any mechanistic interpretation of the magnitudes of thermodynamic parameters refers to the reaction model rather than the experimental system. For the same reason there is little point in subjecting the temperature dependence of rate constants for protein interactions to transition-state analysis. If comparisons with reported values of standard enthalpy and entropy of activation are needed, they are readily calculated from the empirical Arrhenius parameters.

Interpretation of the temperature dependence of rate constants in biosensor studies.

A comparison is made between Arrhenius and transition-state analyses of the temperature dependence of rate constants reported in four published biosensor studies. Although the Eyring transition-state theory seemingly affords a more definitive solution to the problem of characterizing the activation energetics, the analysis is equivocal because of inherent assumptions about reaction mechanism and the magnitude of the transmission coefficient. In view of those uncertainties it is suggested that a preferable course of action entails reversion to the empirical Arrhenius analysis with regard to the energy of activation and a preexponential factor. The former is essentially equivalent to the enthalpy of activation, whereas the magnitude of the latter indicates directly the extent of disparity between the frequency of product formation and the universal frequency factor (temperature multiplied by the ratio of the Boltzmann and Planck constants) and hence the likelihood of a more complicated kinetic mechanism than that encompassed by the Eyring transition-state theory.

Temperature dependence of the thrombin-catalyzed proteolysis of prothrombin.

Measurement of the temperature-dependence of thrombin-catalyzed cleavage of the Arg(155)-Ser(156) and Arg(284)-Thr(285) peptide bonds in prothrombin and prothrombin-derived substrates has yielded Arrhenius parameters that are far too large for classical mechanistic interpretation in terms of a simple hydrolytic reaction. Such a difference from the kinetic behavior exhibited in trypsin- and chymotrypsin-catalyzed proteolysis of peptide bonds is attributed to contributions by enzyme exosite interactions as well as enzyme conformational equilibria to the magnitudes of the experimentally determined Arrhenius parameters. Although the pre-exponential factor and the energy of activation deduced from the temperature-dependence of rate constants for proteolysis by thrombin cannot be accorded the usual mechanistic significance, their evaluation serves a valuable role by highlighting the existence of contributions other than those emanating from simple peptide hydrolysis to the kinetics of proteolysis by thrombin and presumably other enzymes of the blood coagulation system.

The preferred pathway of glycosaminoglycan-accelerated inactivation of thrombin by heparin cofactor II.

Thrombin (T) inactivation by the serpin, heparin cofactor II (HCII), is accelerated by the glycosaminoglycans (GAGs) dermatan sulfate (DS) and heparin (H). Equilibrium binding and thrombin inactivation kinetics at pH 7.8 and ionic strength (I) 0.125 m demonstrated that DS and heparin bound much tighter to thrombin (K(T(DS)) 1-5.8 microm; K(T(H)) 0.02-0.2 microm) than to HCII (K(HCII(DS)) 236-291 microm; K(HCII(H)) 25-35 microm), favoring formation of T.GAG over HCII.GAG complexes as intermediates for T.GAG.HCII complex assembly. At [GAG] << K(HCII(GAG)) the GAG and HCII concentration dependences of the first-order inactivation rate constants (k(app)) were hyperbolic, reflecting saturation of T.GAG complex and formation of the T.GAG.HCII complex from T.GAG and free HCII, respectively. At [GAG] >> K(HCII(GAG)), HCII.GAG complex formation caused a decrease in k(app). The bell-shaped logarithmic GAG dependences fit an obligatory template mechanism in which free HCII binds GAG in the T.GAG complex. DS and heparin bound fluorescently labeled meizothrombin(des-fragment 1) (MzT(-F1)) with K(MzT(-F1)(GAG)) 10 and 20 microm, respectively, demonstrating a binding site outside of exosite II. Exosite II ligands did not attenuate the DS-accelerated thrombin inactivation markedly, but DS displaced thrombin from heparin-Sepharose, suggesting that DS and heparin share a restricted binding site in or nearby exosite II, in addition to binding outside exosite II. Both T.DS and MzT(-F1).DS interactions were saturable at DS concentrations substantially below K(HCII(DS)), consistent with DS bridging T.DS and free HCII. The results suggest that GAG template action facilitates ternary complex formation and accommodates HCII binding to GAG and thrombin exosite I in the ternary complex.

A critical evaluation of the prothrombin time for monitoring oral anticoagulant therapy.

The Quick prothrombin time is the most common clotting test performed, principally for monitoring oral anticoagulant therapy. The International Normalized Ratio (INR) for comparing patient results from prothrombin time measurements and the International Standardized Index (ISI) for achieving greater consistency of results using different thromboplastins have made it possible to compare the results of vitamin K antagonist drug therapy that was impossible before the introduction of the INR and ISI. However, INR values obtained from the same patient plasma sample using different thromboplastins are significantly different. This is so even when the thromboplastins have nearly the same ISI values. We suggest that investigation of patient-specific differences can provide a means by which the INR discrepancies can be identified and understood and thus lead to better methods for monitoring oral anticoagulant therapy.

Thermodynamic non-ideality as an alternative source of the effect of sucrose on the thrombin-catalyzed hydrolysis of peptide p-nitroanilide substrates.

The inhibitory effect of sucrose on the kinetics of thrombin-catalyzed hydrolysis of the chromogenic substrate S-2238 (D-phenylalanyl-pipecolyl-arginoyl-p-nitroanilide) is re-examined as a possible consequence of thermodynamic non-ideality-an inhibition originally attributed to the increased viscosity of reaction mixtures. However, those published results may also be rationalized in terms of the suppression of a substrate-induced isomerization of thrombin to a slightly more expanded (or more asymmetric) transition state prior to the irreversible kinetic steps that lead to substrate hydrolysis. This reinterpretation of the kinetic results solely in terms of molecular crowding does not signify the lack of an effect of viscosity on any reaction step(s) subject to diffusion control. Instead, it highlights the need for development of analytical procedures that can accommodate the concomitant operation of thermodynamic non-ideality and viscosity effects.