HExxH

HEXXH is a conserved amino acid sequence motif found in zinc-dependent metalloproteases. The designation refers to the pattern H E X X H, where H is histidine, E is glutamate, and X represents any amino acid. In these enzymes the two histidines, along with a catalytic glutamate, coordinate a catalytic Zn2+ ion and help activate a water molecule for peptide bond hydrolysis.

HEXXH occurs in a broad superfamily known as metzincins, including thermolysin-like proteases (M4), astacins (M12), and various matrix metalloproteinases (MMPs). It is commonly located within the catalytic domain and is essential for proteolytic activity. The motif is frequently accompanied by additional conserved residues that form the substrate-binding site and the downstream ligand to zinc.

Biological significance: Metalloproteases bearing the HEXXH motif are involved in digestion, tissue remodeling, extracellular matrix degradation, and virulence in pathogens. Examples include thermolysin, collagenases, neprilysin, and human matrix metalloproteinases. In bioinformatics and biochemistry, HEXXH serves as a signature used to identify protease genes in genomes and to classify enzymes or to design inhibitors that target the active site. Mutations within the motif can abolish activity, underscoring its critical role in catalysis.