The inherent variability of conformational diseases is demonstrated by two families with different mutations of the same conserved aminoacid in
antithrombin.
Threonine 85 underlies the opening of the main beta-sheet of the molecule and its replacement, by the polar
lysine, in
antithrombin Wobble, resulted in a plasma deficiency of
antithrombin with an uncharacteristically severe onset of
thrombosis at 10 years of age, whereas the replacement of the same residue by a nonpolar
methionine,
antithrombin Wibble, gave near-normal levels of plasma
antithrombin and more typical adult thromboembolic disease. Isolated
antithrombin Wibble had a decreased thermal stability (Tm 56.2, normal 57.6 degreesC) but was fully stabilized by the
heparin pentasaccharide (Tm 71.8, normal 71.0 degreesC), indicating that the prime abnormality is a laxity in the transition of the main sheet of the molecule from the 5- to 6-stranded form, as was confirmed by the ready conversion of
antithrombin Wibble to the 6-stranded latent form on incubation. That this transition can occur in vivo was shown by the finding of nearly 10% of the proband's plasma
antithrombin in the latent form and also, surprisingly, of small but definitive amounts of latent
antithrombin in normal plasma. The latent transition will be predictably accelerated not only by gross mutations, as with
antithrombin Wobble, to give severe episodic
thrombosis, but also by milder mutations, as with
antithrombin Wibble, to trigger
thrombosis in the presence of other predisposing factors, including the conformational stress imposed by the raised body temperatures of
fevers. Both
antithrombin variants had an exceptional (25-fold) increase in
heparin affinity and this, together with an increased inhibitory activity against
factor Xa, provides evidence of the direct linkage of A-sheet opening to the conformational basis of
heparin binding and activation.