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The binding structures of 11 human oxidosqualene cyclase inhibitors designed as cholesterol-lowering agents were determined for the squalene-hopene cyclase from Alicyclobacillus acidocaldarius, which is the only structurally known homologue of the human enzyme. The complexes were produced by cocrystallization, and the structures were elucidated by X-ray diffraction analyses. All inhibitors were bound in the large active center cavity. The detailed binding structures are presented and discussed in the light of the IC50 values of these 11 as well as 17 other inhibitors. They provide a consistent picture for the inhibition of the bacterial enzyme and can be used to adjust and improve homology models of the human enzyme. The detailed active center structures of the two enzymes are too different to show an IC50 correlation.

Glucokinase (GK) plays a major role in the regulation of blood glucose homeostasis in both the liver and the pancreas. In the liver, GK is controlled by the GK regulatory protein (GKRP). GKRP in turn is activated by fructose 6-phosphate (F6P) and inactivated by fructose 1-phosphate (F1P). Disrupting the GK-GKRP complex increases the activity of GK in the cytosol and is considered an attractive concept for the regulation of blood glucose. We have determined the crystal structure of GKRP in its inactive F1P-bound form. The binding site for F1P is located deeply buried at a domain interface, and H-D exchange experiments confirmed that F1P and F6P compete for this site. The structure of the inactive GKRP-F1P complex provides a starting point for understanding the mechanism of fructose phosphate-dependent GK regulation at an atomic level.

The membrane protein squalene-hopene cyclase was cocrystallized with 2-azasqualene and analyzed by X-ray diffraction to 2.13 A resolution. The conformation of this close analog was clearly established, and it agreed with the common textbook presentation. The bound squalene undergoes only small conformational changes during the formation of rings A through D, thus requiring no intermediate. However, ring E formation is hindered by an entropic barrier, which may explain its absence in the steroids. The structure analysis revealed a mobile region between the active center cavity and the membrane, which may melt, opening a passage for squalene and hopene.

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