During the acrosome reaction, the activation enzyme present in the sperm dissolves the corona radiata and zona pellucida enabling the sperm to reach the plasma membrane of the egg. Also calcium and magnesium ions play an important role in acrosome reaction [ref]. What I fail to understand is how exactly do calcium and magnesium ions play a key role in acrosome reaction?
The process is not clearly understood. Calcium helps in capacitation: During capacitation calcium enters the spermatozoa through ion channels; blocking these channels inhibits the acrosome reaction but what calcium does inside the spermatozoa is not very clear. This paper says that calcium activates a certain tyrosine kinase in the spermatozoa which in turn activates a protein called p32. This they say occurs "concomitantly with capacitation". Other papers cited by this article talk about involvement of calcium dependent adenylate cyclase.
This article also says that protein tyrosine phosphorylations are important for capacitation but doesn't describe how.
I could not find a more precise information on this topic. I shall update the answer if I find one.