mohammad tabish ahmed - eTheses Repository - University of ...

Chapter 1
Structurally, HtpG consist of three domains and is found as a dimer when crystallised (fig
1.11). The N-terminal domain (NTD) contains the ATP binding site (lid), the carboxy
terminal domain (CTD) is needed for homodimerization and the middle domain (MD) links
the NTD and CTD together (Nemoto et al., 1995). In its substrate free state, each of the 3
domains within each chain expose hydrophobic surfaces which has been suggested to act as
substrate binding sites (Shiau et al., 2006). Hsp90 interacts with its substrates when they are
at a near native state and as such is believed to be involved in making much smaller
conformational changes to its substrate proteins than that of Hsp70 or Hsp60 (Zhao et al.,
2005, Young et al., 2001). Crystal structures of HtpG with bound nucleotide shows distinct
conformational states and it is believed that these confirmations are a part of the reaction
cycle (Shiau et al., 2006, Krukenberg et al., 2008, Harris et al., 2004). A simple
representation of the reaction cycle is as follows (fig 1.12):
- In the absence of ATP the chaperone is in an open state that has regions of
hydrophobic residues from the MD (src loop in fig 1.11) and CTD (H21 and H21‟ in
fig 1.11) are available for substrate interaction.
- The next step involves the loading of an appropriate substrate into the open cleft.
- ATP binding and hydrolysis at the active site lid then results in the binding of the
substrate. Conformational changes then closes the cleft resulting in substrate protein
remodelling. This change also hides the hydrophobic residues that were present on the
surface.
- Once ATP is hydrolysed, the resulting ADP bound HtpG then releases the substrate
and is ready for the next cycle.
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