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Adams, Ty E.

Biochemistry

2004

Ph. D.

The crystal structure of factor Vai reveals several interesting details about the function of the cofactor, including insights into chain association, membrane binding, and prothrombinase assembly. First, the bound calcium ion necessary for association of the heavy and light chains has been identified. Coordinated completely within the A1 domain, this calcium ion orders a loop that makes key contacts with residues in the A3 domain rather than providing a direct bridge between the heavy and light chain. Second, the domain orientation of the four domains suggests that both the C1 and C2 domains may be important in membrane binding. Previous models of factor Va and factor VIIIa suggest only the C2 domain interacts with the phospholipid surface. The factor Vai structure shows the C1 and C2 domains stacked in a horizontal or "side-by-side" configuration with the C1 domain in a position to also bind membrane. Combined with evidence that all forms of factor V, including factor Va and factor Vai, bind to phospholipid vesicle with approximately equal affinity suggests that there are no large domain shifts upon activation and inactivation of the cofactor.
Based on the factor Vai crystal structure a factor Va homology model was developed using ceruloplasmin to fill in the missing A2 domain. The model is consistent with electron microscopy (EM) data and fluorescence resonance energy transfer. Mutational data for the factor Xa binding site mapped onto the homology model suggests a potential mechanism for assembly of the prothrombinase complex.