Structural and functional significance of disulfide bonds in saxatilin, a 7.7 kDa disintegrin.

Saxatilin is a 7.7 kDa disintegrin that belongs to a family of homologous protein found in several snake venoms. Six disulfide bond locations of the disintegrin were determined by enzymatic cleavage and matrix-assisted-laser-desorption-ionization time-of-flight mass spectrometry (MALDI-TOF). Functional implications of the disulfide bonds related to the biological activity of saxatilin were investigated with recombinant protein species produced by site-directed mutagenesis of saxatilin. Several lines of experimental evidence indicated that three disulfide bonds, Cys21-Cys35, Cys29-Cys59, and Cys47-Cys67, of the disintegrin are closely associated with its biological function such as its ability to block the binding of integrin GPIIb-IIIa and alpha(v)beta(3) with fibrinogen and extracellular matrix. Those disulfide linkages were also revealed to be important for maintaining the functional structure of the protein molecule. On the other hand, the disulfide bridges of Cys6-Cys15 and Cys8-Cys16 do not appear to be critical for the molecular structure and function of saxatilin.