New carbohydrate site in mutant antithrombin (7 Ile→Asn) with decreased heparin affinity

A mutant antithrombin was isolated from the plasma of a patient with pulmonary embolism. The new protein, which accounted for 55% of the antithrombin, had decreased heparin affinity and contained two components when analysed on the basis of either charge or molecular mass. Sialidase and endo‐β‐ N ‐acetylglucosaminidase F treatment suggested that this heterogeneity was due to a partial glycosylation occurring at a new carbohydrate attachment sequence. Peptide mapping by reverse‐phase HPLC showed that the abnormality involved the N‐terminal tryptic peptide. Sequence analysis demonstrated that the underlying mutation was 7 Ile→Asn which introduces a new Asn‐Cys‐Thr glycosylation sequence. This new oligosaccharide attachment site occupies the base of the proposed heparin‐binding site, and the finding explains the consequent decrease in heparin affinity.