Polar groups in the proteins

The presence of water molecules near the Qb site suggests that water plays an important role in proton transfer. The positions of these waters are not established with certainty by the computational analysis. However, a strong indication of their existence is the presence in the X-ray structure of significant voids near Qb that are bordered by polar groups in the protein. Because the distances between protonatable amino acid groups are, in some cases, significantly larger than the distances for proton transfer, internal water molecules are likely to play an important role as proton donors and acceptors. 

Figure 6.8 Sketch of proposed molecular mechanism of protein-surfactant interaction for CITREM + sodium caseinate (0.5 % w/v in aqueous medium (pH = 7.2, ionic strength = 0.05 M) at 293 K. Picture (I) shows the water molecules bound with polar groups of the protein and surfactant, as w ell as w ater molecules structured as a result of hydrophobic hydration around the hydrocarbon chain of the surfactant. (For clarity, the free w ater molecules are not shown.) Picture (H) demonstrates the release of bound and structured water molecules resulting Rom the predominantly hydrophobic interactions between protein and surfactant. Reproduced Rom Semenova et al. (2006) with permission.

Film penetration studies show unequivocally that lecithin-cholesterol mixtures containing from 0 to 50 mole % cholesterol and lecithin—lactoside mixtures containing from 0 to 80 mole % Ci6-dihydroceramide lactoside have the same effect as pure lecithin. This suggests the presence of a lipid complex in which lecithin prevents the interaction of the cholesterol or ceramide lactoside with globulin. Over these ranges of composition the lipid film would consist of a mixture of the lecithin-cholesterol or the lecithin-lactoside complex with excess lecithin. One may picture two models in which the protein contact is restricted to molecules of lecithin. In one, individual polar groups of the protein interact with the excess lecithin molecules as well as with the lecithin portions of the complex. In the other model, the protein as a whole interacts with the lecithin sites of polymeric lipid structures. The latter, which could be referred to as surface micelles (I), are visualized also through the term "mono-... 

Chymotrypsin is not, as enzymes go, very specific in its action it hydrolyzes proteins, peptides, simple amides, and esters alike. There is one structural requirement, nevertheless a relatively non-polar group in the acyl moiety of the substrate,... 

---