Peptidase M16, zinc-binding site (Pept_M16_Zn_BS)
Type: Binding_site

Over 70 metallopeptidase families have been identified to date. In these enzymes a divalent cation, which is usually zinc but may be cobalt, manganese or copper, activates the water molecule. The metal ion is held in place by amino acid ligands, usually three in number. In some families of co-catalytic metallopeptidases, two metal ions are observed in crystal structures ligated by five amino acids, with one amino acid ligating both metal ions. The known metal ligands are His, Glu, Asp or Lys. At least one other residue is required for catalysis, which may play an electrophilic role.

Many metalloproteases contain an HEXXH motif, which has been shown in crystallographic studies to form part of the metal-binding site . The HEXXH motif is relatively common but can be more stringently defined for metalloproteases as 'abXHEbbHbc', where 'a' is most often valine or threonine and forms part of the S1' subsite in thermolysin and neprilysin, 'b' is an uncharged residue, and 'c' a hydrophobic residue. Proline is never found in this site, possibly because it would break the helical structure adopted by this motif in metalloproteases .

A number of proteases dependent on divalent cations for their activity have been shown to belong to one family, on the basis of sequence similarity. These enzymes are listed below:

Insulinase (

) (also known as insulysin or insulin-degrading enzyme or IDE), a cytoplasmic enzyme which seems to be involved in the cellular processing of insulin, glucagon and other small polypeptides.

Escherichia coli protease III (

) (pitrilysin) (gene ptr), a periplasmic enzyme that degrades small peptides.

Mitochondrial processing peptidase (

) (MPP). This enzyme removes the transit peptide from the precursor form of proteins imported from the cytoplasm across the mitochondrial inner membrane. It is composed of two non-identical homologous subunits termed alpha and beta. The beta subunit seems to be catalytically active while the alpha subunit has probably lost its activity.

Nardilysin (

) (N-arginine dibasic convertase or NRD convertase) this mammalian enzyme cleaves peptide substrates on the N terminus of Arg residues in dibasic stretches.

Klebsiella pneumoniae protein pqqF. This protein is required for the biosynthesis of the coenzyme pyrrolo-quinoline-quinone (PQQ). It is thought to be protease that cleaves peptide bonds in a small peptide (gene pqqA) thus providing the glutamate and tyrosine residues necessary for the synthesis of PQQ.

Saccharomyces cerevisiae (Baker's yeast) protein AXL1, which is involved in axial budding .

Eimeria bovis sporozoite developmental protein.

E. coli hypothetical protein yddC and HI1368, the corresponding Haemophilus influenzae protein.

Bacillus subtilis hypothetical protein ymxG.

Caenorhabditis elegans hypothetical proteins C28F5.4 and F56D2.1.

It should be noted that in addition to the above enzymes, this family also includes the core proteins I and II of the mitochondrial bc1 complex (also called cytochrome c reductase or complex III), but the situation as to the activity or lack of activity of these subunits is quite complex:

In mammals and yeast, core proteins I and II lack enzymatic activity.

In Neurospora crassa and in potato core protein I is equivalent to the beta subunit of MPP.

In Euglena gracilis, core protein I seems to be active, while subunit II is inactive.

These proteins do not share many regions of sequence similarity; the most noticeable is in the N-terminal section. This region includes a conserved histidine followed, two residues later by a glutamate and another histidine. In pitrilysin, it has been shown that this H-x-x-E-H motif is involved in enzyme activity; the two histidines bind zinc and the glutamate is necessary for catalytic activity. Non-active members of this family have lost from one to three of these active site residues. This signature pattern only detects active members of the M16 peptidase family.