1. RAWLINGS, N.D. AND BARRETT, A.J.
Evolutionary families of metallopeptidases.
METHODS ENZYMOL. 248 183-228 (1995).
2. OSTERMAN, A.L., GRISHIN, N.V., SMULEVITCH, S.V., MATZ, M.V., ZAGNITKO,
O.P., REVINA, L.P. AND STEPANOV, V.M.
Primary structure of carboxypeptidase T: delineation of functionally
relevant features in Zn-carboxypeptidase family.
J.PROTEIN CHEM. 11 561-570 (1992).
3. REES, D.C., LEWIS, M. AND LIPSCOMB, W.N.
Refined crystal structure of carboxypeptidase A at 1.54A resoltion.
J.MOL.BIOL. 168 367-387 (1983).
4. GUASCH, A., COLL, M., AVILES, F.X. AND HUBER, R.
Three-dimensional structure of porcine pancreatic procarboxypeptidase A. A
comparison of the A and B zymogens and their determinants for inhibition
J.MOL.BIOL. 224 141-157 (1992).
Metalloproteases are the most diverse of the four main types of protease,
with more than 30 families identified to date . Of these, around
half contain the 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 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 .
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 . 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 . 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 . 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 .
The carboxypeptidase A family (M14) can be divided into two subfamilies:
carboxypeptidase H (regulatory) and carboxypeptidase A (digestive) .
Members of the H family have longer C-termini than those of family A ,
and carboxypeptidase M (a member of the H family) is bound to the membrane
by a glycosylphosphatidylinositol anchor, unlike the majority of the M14
family, which are soluble .
The zinc ligands have been determined as two histidines and a glutamate,
and the catalytic residue has been identified as a C-terminal glutamate,
but these do not form the characteristic metalloprotease HEXXH motif [1,3].
Members of the carboxypeptidase A family are synthesised as inactive
molecules with propeptides that must be cleaved to activate the enzyme.
Structural studies of carboxypeptidases A and B reveal the propeptide to
exist as a globular domain, followed by an extended alpha-helix; this
shields the catalytic site, without specifically binding to it, while the
substrate-binding site is blocked by making specific contacts [1,4].
CRBOXYPTASEA is a 4-element fingerprint that provides a signature for the
carboxypeptidase (M14) family of metalloproteases. The fingerprint was
derived from an initial alignment of 13 sequences: the motifs were drawn
from conserved regions around the active site, motifs 2 and 4 containing
the regions encoded by PROSITE patterns CARBOXYPEPT_ZN_1 (PS00132) and
CARBOXYPEPT_ZN_2 (PS00133), which contain the first His and Glu zinc
ligands, and the second zinc-binding His respectively. Three iterations on
OWL29.3 were required to reach convergence, at which point a true set
comprising 50 sequences was identified. Five partial matches were also
found: S51739 and JC5256 are AEBP1 transcription factors with carboxy-
peptidase activity that match motifs 2 and 4 (the zinc-binding regions);
ENP1_BACSH is a gamma-D-glutamyl-L-diamino acid endopeptidase I that shows
some C-terminal similarity to the carboxypeptidase A family; and PMASPA and
ACU37756 are fragments that match only 2 motifs.
An update on SPTR37_9f identified a true set of 52 sequences, and 13