B beta Glu397 and B beta Asp398 but not B beta Asp432 are required for "B:b" interactions

We synthesized three fibrinogen variants, BbetaE397A, BbetaD398A, and BbetaD432A, with substitutions at positions identified in crystallographic studies as critical for binding the "B" peptide, Gly-His-Arg-Pro-amide (GHRPam), to the "b" polymerization site. We examined thrombin- and batroxobin-catalyzed polymerization by turbidity measurements and found that BbetaE397A and BbetaD398A were impaired while BbetaD432A was normal. Changes in polymerization as a function of calcium were similar for variant and normal fibrinogens. We determined crystal structures of fragment D from the variant BbetaD398A in the absence and presence of GHRPam. In the absence of peptide, the structure showed that the alanine substitution altered only specific local interactions, as alignment of the variant structure with the analogous normal structure resulted in an RMSD of 0.53 A over all atoms. The structure also showed reduced occupancy of the beta2 calcium-binding site that includes the side chain carbonyl of BbetaD398, suggesting that calcium was not bound at this site in our polymerization studies. In the presence of peptide, the structure showed that GHRPam was not bound in the "b" site and the conformational changes associated with peptide binding to normal fragment D did not occur. This structure also showed GHRPam bound in the "a" polymerization site, although in two different conformations. Calcium binding was associated with only one of these conformations, suggesting that calcium binding to the gamma2-site and an alternative peptide conformation were induced by crystal packing. We conclude that BbetaE397 and BbetaD398 are essential for the "B:b" interaction, while BbetaD432 is not.