Hydrophilic patch

Another modification easily assessed by immersion microcalorimetry is the change in hydrophobicity of a surface, e.g. by oxidizing a graphitized carbon surface. The energy of immersion in water was shown to increase almost linearly with decrease in hydrophobicity (Young et al., 1954) and the energies of immersion of hydrophobic and hydrophilic patches were estimated to be 31 and 730 J m-2, respectively (Healy et ah, 1955). 

The conserved polar residue at position 13 is adjacent to position 126 within the cavity, and because of hydrogen bond formation may be necessary for preserving the structure at 126. This residue contributes to a hydrophilic patch that exists in this part of the cavity. The functions of basic residues 37 and 130 (ALBP numbering) are difficult to explain at first sight. They all have solvent-exposed side chains and are not interacting with other conserved residues. Residue 37 occurs at the C terminus of the second a helix. Basic residues at such a location are frequently observed and may be due to an interaction with the dipole moment of the helix. The role of the basic residue at 130 has been mentioned earlier and involves an interaction with the ring of the highly conserved Trp-8. 

Fig. 7. Lid movements accompanying ATP binding, (a) Conformation of the lid segment (residues 100-118 in yeast Hsp90) in the ADP-bound form observed in crystal structures (Prodromou et al., 1997a). Residue Thr-101 is buried in this open-lid conformation while Ala-107 is fully exposed, (b) Model of the closed-lid conformation believed to be favored by ATP binding. In this conformation, Thr-101 becomes fully exposed, while Ala-107 becomes buried against a hydrophilic patch formed by AsnAO and Asp-43. Mutation of Thr-101 to the more hydrophobic isoleucine disfavors lid closure and results in diminished N-terminal association and ATPase activity. Mutation of Ala-107 to the more hydrophilic asparagine favors lid closure and enhances N-terminal association and ATPase activity (Prodromou et al., 1997a).

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