ZP glycoprotein binding sperm (human):
Human zona pellucida contains four glycoproteins ZP1-ZP4 forming filamentous matrix. Primary binding of acrosome-intact sperm in humans involves ZP3 and ZP4, but enduring attachment of acrosome-reacted sperm requires secondary receptor ZP2, particularly its N-terminal domain. After fertilization ovastacin cleaves ZP2, destroying this binding site and preventing further sperm adhesion, contributing to zona block. Mouse ZP3 is traditionally called primary receptor, but human data highlight ZP2 as major ligand for reacted sperm. ZP1 is crosslinker structurally, ZP4 modulatory, so persistent secondary binding critical for species-specific gamete recognition centers on ZP2 integrity.
Ref: Gupta SK et al., Mol Hum Reprod 2012: Human ZP2 as secondary receptor for acrosome-reacted sperm and ZP2 cleavage block.