The Role of Proteolytically Inactive Serine Proteases from Sarcoptes scabiei in Complement Evasion - Proteinases as Drug Targets - page 110

Antimicrobial Protein 37 kDa (CAP37). It resembles neutrophil elastase, but
substitutions of the catalytic histidine and serine residues render it inactive. 111
HBP is a multifunctional protein actively involved in host defense and
inflammation. 112 In addition to its heparin-binding abilities, 113 HBP has been
shown to display antibiotic activity against Gram-negative bacteria, 114 and the
ability to chemo-attract and activate monocytes and T-cells. 115
These examples demonstrate that inactive proteases have evolved to perform
a large variety of functions. The mode of action, however, remains elusive for
many of these proteases. A role for SMIPP-Ss in immune evasion has also been
demonstrated, 43 and elucidating the underlying mechanism is one of our
group's research goals.
4.5.2 Mode of Action of Inactive Enzymes
4.5.2.1 Binding via the Inactive Catalytic Site
Some inactive proteases with mutated catalytic sites are still able to use the
binding pocket for protein-protein interactions. Sonic hedgehog is an impor-
tant signaling molecule in the Hedgehog pathway involved in embryogenesis
and tissue regeneration. The N-terminal domain of Sonic hedgehog is
responsible for short- and long-range signaling and acts as a ligand for mem-
brane-bound receptors. This signaling domain contains an inactive catalytic
site that is instead used for regulation. 116 It has been shown that hedgehog
regulatory proteins such as hedgehog-interacting protein bind to this site and
an adjacent calcium-binding site. 117,118
Inactivated catalytic sites can be utilized as a means of regulating activation,
inhibition, or even binding anity. The tumour suppressor kinase LKB1 is
allosterically activated by the pseudokinase STe20 Related Adaptor (STRAD),
which is an inactive kinase that is missing the catalytic residues necessary for
kinase activity. 119 Activated LKB1 is found in cells complexed in a 1:1:1 ratio
with STRAD and a scaffold protein. 120 Studies show that the scaffold protein
binds STRAD, enabling STRAD to adopt an active conformation. In this
state, the region that would normally make up the substrate-binding site of
STRAD is utilized for binding to the kinase domain of LKB1. Hence, although
unable to activate LKB1 by catalysis, STRAD is able to enforce a conforma-
tional change on LKB1 consistent with an active conformation. Once activated,
LKB1 is able to perform its cellular functions. 121
4.5.2.2 Glycan Binding
Glycans have been previously shown to play a biological role in proteins by
contributing to the stability of a proteins structure, 122 altering biological
properties such as anticoagulant activities, 123 and inducing pharmokinetic
changes. 124 The contribution of glycans to protein function in the human HBP
(Section 4.5.1.4.) was explored by Iversen et al. 125 A mutant was produced by
site-directed mutagenesis that was devoid of the three N-linked glycosylation