Biomedical Engineering Reference
In-Depth Information
sites. Recombinant proteins of both the native and mutant HBP were produced
in a baculovirus system. The lack of glycosylation did not disrupt the structure
or stability of HBP. When testing for stimulation of the immune system using in
vitro and in vivo assays, the activity was significantly reduced, indicating that
the glycans were indeed involved in the binding mechanism.
4.5.2.3 Alternative Binding Sites or Exosites
An investigation by Pils and Schultz on the evolution of the Protein Phos-
phatase Family (PTP) proposed that there was a loss of evolutionary pressure
around the catalytic centers of a subclass of inactive members, resulting in a
high rate of evolutionary change. They questioned if the affected site could still
be responsible for the observed regulatory function or whether a novel binding
site had evolved. Several sites of high conservation undergoing slow rates of
evolution were identified on the other side of the molecule, opposite the
mutated catalytic center. Pils and Schultz suggested that this could indicate a
newly evolving functional center.
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Another example of an exosite within an inactive protease is displayed by
protein Z (PZ), which is a human plasma protein and a co-factor for the serpin
Protein Z inhibitor (PZI) of the coagulation factor Xa.
126,127
PZ is structurally
related to the coagulation cascade serine proteases, namely factors VII, IX, and
X, but it is catalytically inactive due to the replacement of two of the three
catalytic residues, histidine and serine for lysine and aspartic acid.
128
PZ is
composed of an N-terminal gamma carboxyglutamic acid domain (Gla) fol-
lowed by two epidermal growth-factor-like domains and a carboxy-terminal
inactive catalytic domain.
129
The C-terminal region containing the inactive
catalytic domain has a trypsin-like serine protease fold.
130
A region adjacent to
the inactive catalytic site is the site for PZ binding to PZI, an interaction
facilitated through ionic and polar interactions.
131
Mutagenesis studies of this
region demonstrated its importance in the interaction between PZ and PZI.
When a complex is formed between PZI and PZ, the Gla domain of PZ is used
to anchor PZI to membrane surfaces in an optimal orientation for ecient
interaction with factor Xa, thereby accelerating the inhibitory activity of PZI.
131
The variation in modes of action described here demonstrates that the loss of
a primary protease mechanism such as catalysis may lead to the evolution of a
new mechanism and new functions, thereby expanding the repertoire of pro-
tease paralogues.
4.6 Conclusion
A family of scabies mite inactive serine proteases (SMIPP-Ss) that are homo-
logous to the group 3 house dust mite allergens have been identified. Functional
characterization of two SMIPP-Ss has determined that they are inhibitors of
the human complement system. The binding targets have been identified as the
complement components C1q, MBL, and properdin, and binding of these
molecules results in the inhibition of all three pathways comprising the
