FIGURE 3.
The length of Lnk2 influences the rate of prothrombin activation. The values of the specificity constant kcat/Km for activation of prothrombin by factor Xa and phospholipids in the absence (orange) or presence (blue) of cofactor Va are plotted versus the length of Lnk2. The data are from Table 2 and portray an opposite dependence of the rate of activation that increases (− cofactor Va) or decreases (+ cofactor Va) with the length of Lnk2. In the presence of phospholipids, prothrombin activation in the absence of cofactor Va is significantly enhanced when Lnk2 is <15 residues long. On the other hand, a Lnk2 of <15 residues long slightly compromises activation in the presence of cofactor. The overall effect of cofactor Va is highly dependent on the length of Lnk2 and is optimized when Lnk2 has a length of at least 20 residues, as documented in 29 different species (Fig. 1).