Introduction
The activation of factor X is one of the steps in the coagulation cascade that is
driven by the assembly of an activated serine protease with a membrane-bound cofactor.
In the initial phase of coagulation, factor X is activated by the complex of activated
factor VII (factor VIIa) and tissue factor. Subsequently, during the so-called propagation
phase, factor X activation is catalyzed by the complex of activated factor IX (factor
IXa) and activated factor VIII (factor VIIIa). In these complexes, factor VIIa and
factor IXa are the factor X-activating enzymes, whereas tissue factor and factor VIIIa
serve as non-enzymatic cofactors.1 Factors VIIa and IXa are highly homologous to other cofactor-dependent enzymes, such
as activated factor X (factor Xa) and activated protein C, both in amino acid sequence,
domain organization, and three-dimensional structure.2 Factor VIIa and IXa further share low or negligible activity towards their natural
substrate factor X, unless in complex with their physiological cofactors.
Although tissue factor and factor VIIIa serve similar roles as biological amplifiers,
they are structurally different. Tissue factor is a small, transmembrane protein with
an extracellular part comprising 219 amino acids. Factor VIII is much larger (2,332
amino acids), circulates in plasma, and requires proteolytic processing to exert its
biological activity.3 When cofactors are assembled with their respective enzymes, a dramatic increase in
enzymatic activity occurs. The underlying molecular mechanism, however, remains poorly
understood.
During the past few years, remarkable progress has been made in understanding the
molecular details of enzyme-cofactor assembly within the coagulation cascade. Crystallography
has provided high-resolution structures of tissue factor4 and the various cofactor-dependent coagulation enzymes.2 Moreover, the crystal structure of the factor VIIa—tissue factor complex has been
resolved and has allowed the identification of the molecular sites involved in enzyme-cofactor
interaction.5,6 Such details are still lacking, however, for the factor IXa—factor VIIIa complex.
Current views are derived from three-dimensional models generated by homology modeling
based on structurally-related proteins, such as nitrite reductase,7 ceruloplasmin,8 and galactose oxidase.9 Despite their inherent limitations, these models greatly facilitate the interpretation
of previous functional studies on factor X activation. As such, the availability of
molecular models may be considered an important step toward resolving the structure
of the factor IXa—factor VIIIa complex and understanding the role of complex assembly
and defects thereof. This chapter provides an overview of the current developments
in this field.
