Fig. 7. A biochemical model to explain the tyrosine phosphorylation events leading to sperm hyperactivation. During capacitation, a variety of factors including Ca²⁺, HCO₃ or H₂O₂ stimulate sAC. This is turn leads to production of cAMP and downstream activation of PKA. PKA is central to the induction of hyperactivation in a two step process: (1) it must phosphorylate SRC and activate this kinase. (2) PKA must also phosphorylate CSK, thereby inhibiting this enzyme and promoting further PKA activity. The activation of SRC by PKA leads to autophosphorylation and consequent tyrosine phosphorylation of a number of sperm targets, including enolase, HSP90 and tubulin.