Zinc specifically stimulates the selective binding of a peptide analog of bindin to sulfated fucans.

A synthetic nonapeptide (Leu-Arg-His-Leu-Arg-His-His-Ser-Asn) derived from the sequence of the sea urchin sperm adhesive protein, bindin, has been shown to bind sulfated fucans in high ionic strength (seawater) conditions. The binding is enhanced by approximately 100-fold in the presence of zinc ions, and no other transition metal tested demonstrates any enhancement. Bindin isolated from sperm contains zinc ion at roughly equimolar concentrations. In the presence of Zn++, the synthetic nonapeptide binds to eggs and inhibits fertilization with a half-maximal effective concentration of 300 microM. The polysaccharide binding selectivity of the peptide/Zn++ complex is similar to bindin but less stringent. Although the order of effectiveness of the inhibitory polysaccharides is the same for bindin and the synthetic peptide, polysaccharides that are only weak inhibitors of fucan binding to bindin show greater effectiveness against the peptide. The effect of chemical modification, pH, and amino acid substitution on the binding properties of the peptide suggest that arginine guanido moieties interact with the sulfated fucans, while histidine groups chelate zinc ions. Although the mechanism of zinc-specific stimulation of fucan binding is not yet clear, one potential explanation is that zinc may stabilize a peptide secondary structure that has a high affinity for fucans.