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#slow block

3 public questions tagged with this topic.

Regeneration -ll

Which enzyme cleaves bindin receptors during slow block?

Permanent block to polyspermy requires irreversible removal of sperm attachment sites. Cortical granule serine protease, trypsin-like enzyme activated at neutral pH upon exocytosis, cleaves extracellular domain of Egg Bindin Receptor EBR1 within vitelline envelope, releasing peptide fragments and destroying lectin-binding interface. This ensures even if fertilization envelope incompletely hardens, supernumerary sperm cannot remain bound. Acrosomal protease digests egg jelly to allow sperm penetration, ovoperoxidase crosslinks envelope proteins for hardening, phospholipase C generates IP3 and DAG for signaling but does not cleave receptors. Specific serine protease inhibitors prevent receptor loss while envelope still elevates.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: Cortical granule serine protease clipping bindin receptor EBR1.

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Sea urchin -l

Slow block to polyspermy is stabilized by:

After initial elevation triggered by mucopolysaccharide swelling, vitelline envelope transformed into fertilization envelope requires chemical hardening to provide durable mechanical barrier against supersperm and environmental stress. Two cortical granule enzymes accomplish covalent stabilization: egg-specific ovoperoxidase catalyzes oxidative crosslinking forming dityrosine bridges between adjacent envelope glycoproteins using hydrogen peroxide, while transglutaminase catalyzes formation of ε-(γ-glutamyl)lysine isopeptide bonds. Together these enzymatic crosslinks convert soluble vitelline envelope into insoluble, tough, impermeable protective coat encasing embryo. Glycosaminoglycans drive swelling but not stabilization, hyalin builds hyaline layer for cell adhesion, Na+ mediates fast electrical block. Thus stabilization depends on peroxidase-transglutaminase system.

Ref: Foerder & Shapiro, PNAS 1977, Peroxidase hardening; Gilbert, Developmental Biology, Chapter 7: Envelope crosslinking.

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Sea urchin -l

Which enzyme cleaves bindin receptors during slow block?

Permanent elimination of sperm binding capacity during slow block involves enzymatic destruction of specific recognition molecules. Cortical granules discharge large trypsin-like serine protease that cleaves peptide linkages anchoring EBR1 bindin receptor complex to vitelline envelope glycoprotein scaffold and degrades residual fertilizing sperm proteins adhering to envelope. This proteolytic destruction ensures no new bindin-receptor interactions can reform even before envelope hardening physically completes. Acrosomal protease facilitates sperm entry through jelly, ovoperoxidase catalyzes dityrosine crosslinks hardening envelope, phospholipase C generates IP3 for calcium release but does not degrade receptors. Thus enzyme responsible for cleaving bindin receptors is cortical granule serine protease.

Ref: NCBI Bookshelf, Developmental Biology, Chapter 7: Cortical granule protease cleaves bindin receptor.

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