Acrosin

An enzyme involved in sperm penetration of the zona pellucida

Acrosin

Acrosin is a serine protease enzyme that plays a crucial role in the process of fertilization by aiding the sperm in penetrating the zona pellucida of the ovum. It is stored in the acrosome of spermatozoa and is released during the acrosome reaction.

Structure

Acrosin is synthesized as a zymogen called proacrosin, which is activated to acrosin upon release from the acrosome. The enzyme is characterized by a typical serine protease structure, with a catalytic triad consisting of serine, histidine, and aspartate residues. The active site of acrosin is highly conserved and is responsible for its proteolytic activity.

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Crystal structure of acrosin (PDB: 1FIW)

Function

The primary function of acrosin is to facilitate the penetration of the sperm through the zona pellucida, a glycoprotein layer surrounding the ovum. Acrosin acts by hydrolyzing peptide bonds in the proteins of the zona pellucida, thereby allowing the sperm to reach the plasma membrane of the egg.

Catalytic Mechanism

Acrosin operates through a mechanism typical of serine proteases, involving the formation of a tetrahedral intermediate and the subsequent cleavage of peptide bonds. The catalytic triad in the active site plays a crucial role in this process.

Diagram of the acrosin catalytic mechanism

Active Site

The active site of acrosin is located in a cleft on the enzyme's surface and is composed of several key residues that interact with substrate molecules. The surface of the active site is designed to accommodate the specific substrates found in the zona pellucida.

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Surface representation of the acrosin active site

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Residues in the active site of acrosin

Role in Fertilization

During fertilization, the acrosome reaction is triggered when the sperm comes into contact with the zona pellucida. This reaction involves the release of acrosin and other enzymes, which work together to digest the zona pellucida and allow the sperm to fuse with the egg.

Related pages

• Fertilization
• Spermatozoon
• Zona pellucida
• Serine protease