text: Studies have shown that statins bind reversibly to the HGMR enzyme. The affinity of statins for HGMR enzyme is in the nanomolar range, while the natural substrate's affinity is in the micromolar range. Studies have shown that statins use the conformational flexibility of the HMGR enzyme that causes a shallow hydrophobic groove that the statins exploit and is used to accommodate their hydrophobic moieties. The specificity and the tight binding of statins is due to orientation and bonding interactions that form between the statin and the HMGR enzyme. Polar interactions are formed between the HMG-moiety and residues that are located in the cis loop of the enzyme. These polar interactions are between Ser, Asp, Lys and Lys (Figure 4). The terminal carboxylate of the HMG moiety forms a salt bridge with the cationic Lys of the enzyme. In addition to the polar interaction, Lys participates in a hydrogen bonding network with Glu, Asp and the O5 hydroxyl group of the hydroxyglutartic acid component of the statins. Van der Waals interactions are formed between the hydrophobic side chains of the enzyme, which involve the Leu, Val, Leu, Ala and Leu and the statins. Type 2 statins form polar interaction between the fluorine atom on the fluorophenyl group and the guanidinium group of Arg. In addition to these interactions atorvastatin and rosuvastatin also form unique hydrogen bonds between Ser residue and either a carbonyl oxygen atom (atorvastatin) or a sulfone oxygen atom (rosuvastatin). A unique polar interaction between the Arg side chain and the electronegative sulfone group on rosuvastatin makes it the statin that has the greatest number of bonding interactions with HGMR.