Fibrinogen sialic acid residues are low affinity calcium-binding sites that influence fibrin assembly.
Calcium ions occupy low (n congruent to 10; Kd congruent to 1 mM) and high (n = 3; Kd congruent to 1 microM) affinity sites on fibrinogen and facilitate fibrin monomer polymerization. We have previously localized two of the three high affinity Ca2+ sites to gamma 311-gamma 336. 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+ (Kd congruent to 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.
Dang
C V
CV
Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Shin
C K
CK
Bell
W R
WR
Nagaswami
C
C
Weisel
J W
JW
D001212:Asialoglycoproteins; D001780:Blood Coagulation Tests; D002118:Calcium / Q000097:blood / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000097:blood; D005337:Fibrin / Q000378:metabolism* / Q000648:ultrastructure; D005340:Fibrinogen / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D046911:Macromolecular Substances; D009391:Nephelometry and Turbidimetry; D011956:Receptors, Cell Surface / Q000502:physiology*; D018000:Receptors, Peptide; D012794:Sialic Acids / Q000097:blood / Q000378:metabolism*
D001212:Asialoglycoproteins; D002135:Calcium-Binding Proteins; D046911:Macromolecular Substances; D011956:Receptors, Cell Surface; D018000:Receptors, Peptide; D012794:Sialic Acids; C032692:asialofibrinogen; C048650:fibrin-fibrinogen degradation products receptor; D005337:Fibrin; D005340:Fibrinogen; D002118:Calcium