Biophysical studies of membrane proteins/peptides. Interaction with ...
Biophysical studies of membrane proteins/peptides. Interaction with ...
Biophysical studies of membrane proteins/peptides. Interaction with ...
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BINDING OF INHIBITORS TO A PUTATIVE BINDING<br />
DOMAIN OF V-ATPase<br />
III<br />
BINDING OF INHIBITORS TO A<br />
PUTATIVE BINDING DOMAIN<br />
OF V-ATPase<br />
1. Introduction<br />
Osteoporosis, a disease endemic in Western society, is characterized by a net loss <strong>of</strong><br />
bone mass which results from an imbalance <strong>of</strong> the bone-remodelling process, <strong>with</strong> bone<br />
resorption exceeding bone formation. Bone demineralization involves acidification <strong>of</strong><br />
the isolated extracellular microenvironment. The lowering <strong>of</strong> pH is necessary both to<br />
dissolve the bone material and to provide the acidic environment required by the<br />
collagenases to degrade the bone matrix (Teitelbaum, 2000). The acidification is due to<br />
proton translocation by the vacuolar-type H + -ATPases (V-ATPases) that are present in<br />
large numbers on the ruffled border <strong>of</strong> the osteoclast while the counterions diffuse<br />
passively through a chloride channel (Fig. III-1). Although there are a number <strong>of</strong> drugs<br />
directed to osteoporosis treatment through indirect inhibition <strong>of</strong> osteoclasts (Lark and<br />
James, 2002; Rodan and Martin, 2000), they allow only for a limited therapy as bone<br />
loss is only temporarily reduced due to weak inhibition <strong>of</strong> these cells. The osteoclast V-<br />
ATPase is, therefore, an attractive alternative target for the development <strong>of</strong> better<br />
therapeutic agents that effectively and safely reduce osteoclast activity.<br />
V-ATPases are large, multisubunit complexes organized in two domains (Fig. III.2)<br />
(Nishi and Forgac, 2002; Kawasaki-Nishi et al., 2003). The V 1 domain, which is<br />
responsible for ATP hydrolysis, comprises eight different subunits (A-H) that can be<br />
removed from the <strong>membrane</strong> in soluble form. The V o domain, which is tightly<br />
associated <strong>with</strong> the <strong>membrane</strong> and composed <strong>of</strong> five different subunits (a, c, c’, c’’, and<br />
d), is responsible for proton translocation. The enzyme functions as a rotary motor<br />
(Yokoyama et al., 2003; Harrison et al., 1997). By analogy <strong>with</strong> the mechanism<br />
proposed for the F-ATPase, ATP hydrolysis at the catalytic sites drives rotation <strong>of</strong> the<br />
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