Calcium ions occupy low (n ≃ 10; K(d) ≃ 1 mM) and high (n = 3; K(d) ≃ 1 μM) affinity sites on fibrinogen and facilitate fibrin monomer polymerization. We have previously localized two of the three high affinity Ca2+ sites to γ311-γ336. However, optimal enhancement of fibrin monomer polymerization occurs only at physiological millimolar Ca2+ concentrations which are two orders of magnitude higher than the concentration required for occupancy of the high affinity Ca2+-binding sites. In this study, we show that removal of fibrinogen sialic acid residues results in loss of low affinity Ca2+-binding sites. Clotting of asialofibrinogen appears to be Ca2+-independent and results in fiber bundles thicker in diameter than normal fibrin bundles as determined by turbidometry and scanning and transmission electron microscopy. By using a Ca2+-sensitive electrode, free sialic acid is shown to bind Ca2+ (K(d) ≃ 1 mM). These observations suggest that the high affinity fibrinogen D-domain Ca2+-binding sites may play a role in the tertiary structure of the D-domain, whereas, sialic acid residues are low affinity sites whose occupancy by Ca2+ at physiological calcium concentration facilitates fibrin polymerization.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|State||Published - 1989|
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