2 molecules 10,000 daltons

4 molecules 20,000 daltons


2 molecules 30,000 daltons

4 molecules 60,000 daltons

Heparin 6 platelet-binding domains/volume

Concentration at 6 molecules/volume

Half-Saturation (Kd) 30,000 daltons/volume

6 platelet-binding domains/volume 3 molecules/volume 45,000 daltons/volume

FIGURE 1 Schematic binding of heparin to platelets comparing heparin of Mr 5000 with heparin of Mr 15,000. Each "platelet-binding domain" of heparin is hypothesized to have Mr > 3000, whereas heparin-binding sites on the platelets (indicated by ++++) can bind 7000 Da heparin. Therefore, each binding site is not quite filled with Mr 5000 heparin but is too occupied to allow the binding of a second heparin molecule. In contrast, Mr 15,000 heparin has adequate length to occupy two binding sites on platelets, but physical constraints, such as limited heparin flexibility and the spacial distribution of binding sites, allow it to occupy only one site at a time. The scheme is consistent with the binding parameters shown in Table 1.

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