| Literature References | 1. RAWLINGS, N.D. AND BARRETT, A.J.
Evolutionary families of metallopeptidases.
METHODS ENZYMOL. 248 183-228 (1995).
2. HAEGGSTROM, J.Z., WETTERHOLM, A., SHAPIRO, R., VALLEE, B.L.
AND SAMUELSSON, B.
Leukotriene A4 hydrolase: a zinc metalloenzyme.
BIOCHEM.BIOPHYS.RES.COMMUN. 172 965-970 (1990).
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| Documentation | Metalloproteases are the most diverse of the four main types of protease,
with more than 30 families identified to date [1]. Of these, around
half contain the HEXXH motif, which has been shown in crystallographic
studies to form part of the metal-binding site [1]. The HEXXH motif is
relatively common, but can be more stringently defined for metallo-
proteases 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 [1].
Metalloproteases can be split into five groups on the basis of their metal-
binding residues: the first three contain the HEXXH motif, the other two
do not [1]. In the first group, a glutamic acid completes the active site -
these are termed HEXXH+E: all families in this group show some sequence
relationship and have been assigned to clan MA [1]. The second group, which
have a third histidine as the extra metal-binding residue, are termed
HEXXH+H and are grouped into clan MB on the basis of their inter-relation-
ship[1]. In the third group, the additional metal-binding residues are
unidentified. The fourth group is diverse - the metal-binding residues are
known but do not form the HEXXH motif. And the fifth group comprises the
remaining families where the metal-binding residues are as yet unknown [1].
The membrane alanyl aminopeptidase family (M1) is part of the HEXXH+E
group; it consists entirely of aminopeptidases, spread across a wide
variety of species [1]. The enzymes are varied in specificity, removing
N-terminal neutral, acidic and basic residues; one family member (leuko-
triene-A4 hydrolase) is known to hydrolyse the epoxide leukotriene-A4
to form an inflammatory mediator [1]. This hydrolase has been shown to
have aminopeptidase activity [2], and the zinc ligands of the M1 family
were identified by site-directed mutagenesis on this enzyme [1].
ALADIPTASE is a 5-element fingerprint that provides a signature for the
membrane alanyl dipeptidase (M1) family of metallopeptidases. The
fingerprint was derived from an initial alignment of 12 sequences: the
motifs were drawn from conserved regions within the central portion of
the alignment - motif 4 includes the region encoded by PROSITE pattern
ZINC_PROTEASE (PS00142), which contains the HEXXH motif, the 2 histidines
of which are zinc ligands and the glutamate of which is the catalytic
residue; motif 5 also contains a conserved glutamate, which provides the
third zinc ligand. Three iterations on OWL29.3 were required to reach
convergence, at which point a true set comprising 51 sequences was
identified. Four partial matches were also found: D909165 is a protein from
Synechocystis that fails to make significant matches with motifs 2 and 3;
and SSAPN, I57748 and AMPE_RAT are fragments that match only 2 motifs.
An update on SPTR37_9f identified a true set of 56 sequences, and 2
partial matches.
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