Biomedical Engineering Reference
In-Depth Information
which have been shown to inhibit apoptosis of B cells (92). Therefore,
similar to AAT, SLPI, and elafin, it seems clear that TIMPs possess biolo-
gical properties that exceed their capacity to inhibit MMP activity.
VI. CYSTATINS
Cysteine protease inhibitors of the cystatin superfamily are present in a
number of human tissues and body fluids and can be grouped into three pro-
tein families (93). Family 1 members consist of cystatins A and B and are
approximately 11-12 kDa in size, do not possess disulfide bridges, and are
not expressed with a signal peptide. Cystatin A is present on chromosome
3 and cystatin B is found on chromosome 21 (94,95). Family 2 members
of the cystatin group are secreted proteins, are slightly bigger than Family
1 members (13-14 kDa), and possess two disulfide bridges. Members of this
family include cystatin C, D, S, SN, and SA and are present on chromosome
20 (96,97). The Family 3 cystatins contain Family 2 cystatin-like domains,
and members of this group are L-kininogen and H-kininogen, the latter of
which is found on chromosome 3 (98). All members of this family are
tight-binding enzyme inhibitors with specificity for cathepsin B, L, and S (99).
A. Cathepsin Inhibition
The crystal structure of chicken egg white cystatin reveals the protein is
wedge shaped and composed of a five stranded antiparallel
b
-pleated sheet
wrapped around a central long
a
-helix with a first and second
b
-hairpin loop
forming a molecular edge (100). From this model, the mode of interaction of
cystatins with their cognate enzymes was first suggested and later confirmed
by the structure elucidation of the papain-stefin B (cystatin B) complex
(101). Cystatin binds into the primed S1
0
to S4
0
subregion of papain via
the
b
-hairpin loops and the N-terminal region interacts with the nonprimed
S1 to S3 subsites in such a way as to prevent proteolytic cleavage by the
active site of papain. It was also confirmed in this model that N-terminally
truncated forms of the inhibitor could still bind to papain although with
reduced affinity.
The interaction of cystatins with cathepsin B is somewhat different
from that proposed with other cysteine proteases. Cathepsin B possesses
an ''occluding loop'' comprising residues 104-126, which partially blocks
the active site and impacts on the inhibitor-binding properties of the enzyme
(102). Further studies have revealed that inhibition of cathepsin B by cysta-
tin C occurs by a two-step mechanism in which an initial weak interaction is
followed by a conformational change. This initial interaction most likely
involves binding of the N-terminal region of the inhibitor to the S2 and
S3 subsites. Evidence from these studies also suggests that conformational
change that occurs is due to the inhibitor displacing the occluding loop of
