STRUCTURE-FUNCTION RELATIONSHIPS IN ABNORMAL FIBRINOGEN WITH Bβ14 ARG→CYS SUBSTITUTION: FIBRINOGENS SEATTLE I AND CHRISTCHURCH II
Genetically abnormal, dysfunctional fibrinogen variants may be used as unique models for studies of structure-function relationships both in vitro and in vivo. Out of the over kO so far structurally elucidated abnormal fibrinogens only 4 have an amino acid substitution in the Bg-chain. These variants are named Fibrinogen Pontoise, Nev York I, Christchurch II and Seattle I.Fibrinogens Seattle I and Christchurch II are slow-clotting fibrinogens which on thrombin-treatment release only half the normal amount of fibrinopeptide B and therefore were expected to contain an amino acid substitution close to the thrombin cleavage site in the Bβ-chain. In order to sequence the abnormal Bg-chains the fibrinogens were cleaved with thrombin and cyanogen bromide. The abnormal Bβ-chain components were isolated from the mercapto-lysed-pyridylethylated N-terminal disulfide knots. After pyroglu-tamyl-peptidase digestion the Bβ 14 Arg→Cys substitutions could be demonstrated for both variants by direct N-terminal sequence analysis. The form of the cyst(e)ine residue was determined by amino acid analysis of the alkylated native fibrinogen. As no alkylated cysteine was detected it was concluded that Bβ 14 Cys participates in a disulfide bridge.Fibrinogens Seattle I and Christchurch II are the first two elucidated fibrinogens with substitutions at the Bβ-chain thrombin cleavage site. Surprisingly, both the thrombin and Reptilase times are prolonged. It may be assumed that the half-cystine residues in position 14 of the two Bβ-chains within one fibrinogen molecule are disulfide-linked to each other, in an analogous way to that already established for the half-cystine residues in position 16 of the Aα-chains of several abnormal fibrinogens.This additional disulfide bridge might change the conformation and charge in the N-terminal region sufficiently to explain the prolonged clotting times. Furthermore, this bridge would provide the evidence for the parallel arrangement of the Bβ-chains at the fibrinogen N-terminus in a similar way as previously shown for the Aα-chains.