Science / Medicine : Clue to Animal Fertilization

The mystery of how a sperm makes it through the outer coating of an egg, one of the key steps in animal fertilization, is yielding to investigation. A single enzyme allows sperm to bind to and cut through an egg’s outer coating in pigs, according to a UC Davis biochemist. Preliminary results suggest that a similar enzyme performs the same function in frog and human sperm, Jerry L. Hedrick said.

An enzyme is a protein that carries out a specific function, typically either breaking a chemical bond in another molecule or forming a new chemical bond. Hedrick’s enzyme, called acrosin, is unusual in that it carries out two separate functions.

Acrosin has two chemically active sites on its surface, Hedrick said. One site binds to sugars on the surface of the egg, attaching the sperm to it. The second site acts like chemical scissors to cut through the protein coating of the egg, allowing the sperm’s DNA to enter and join with the egg’s DNA.

The idea that a single enzyme might provide the chemical mechanism needed for a sperm’s entry into an egg was first proposed last year in the midst of a brainstorming session at another scientific meeting, Hedrick said.