Histidine relative hydrophobicity

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

As stated earlier, lipases act at the interface between hydrophobic and hydrophilic regions, a characteristic that distinguishes lipases from esterases. Similar to serine proteases, lipases share the nucleophile-histidine-acidic residue catalytic triad that manifests itself as either a Ser-His-Asp triad or a Ser-His-Glu triad. The enzyme s catalytic site often is buried within the protein structure, surrounded by relatively hydrophobic residues. An a-helical polypeptide structure acts as a cover, making the site inaccessible to solvents and substrates. For the lipase to be active, the a-helical lid structure has to open so that the active site is accessible to the substrate. The phenomenon of interfacial activation is often associated with reorientation of the lid, increasing the hydrophobicity of the surface in the vicinity of the active site and exposing it. The opening of the lid structure may be initiated on interaction with an oiFwater interface. 

The third class of host defense peptides, the extended peptide class, is defined by the relative absence of a defined secondary structure. These peptides normally contain high proportions of amino acids such as histidine, tryptophan, or proline and tend to adopt an overall extended conformation upon interaction with hydrophobic environments. Examples of peptides belonging to the extended class include indolicidin, a bovine neutrophil peptide, and the porcine peptide fragment, tritpticin. These structures are stabilized by hydrogen bonding and van der Waals forces as a result of contact with lipids in contrast to the intramolecular stabilization forces found in the former peptide classes. 

Figure 7. Liposome-assisted catalysis. (A) Dependency of the initial hydrolysis rate of C16-O Np (nitrophenyl-pamitate) catalyzed by 1 mM carbobenzoxy-Phe-His-Leu-OH on the substrate concentration, in 0.05 M borate buffer pH 8.5. The filled circles are relative to the self-hydrolysis (no peptide, no liposomes). Open triangles are without liposomes, open squares with liposomes. (B) The pseudo-enzymatic turnover of the catalytically active liposomes. The catalytic activity results primarily from the binding (and solubilization) of a very hydrophobic histidin-containing peptide and the very hydrophobic substrate.

The active site in myoglobin is a heme tightly bound to the proteins through about 80 hydrophobic interactions as well as by one close histidine and one more remote histidine interaction with the central iron ion. The unsymmetrical substitution with two imidazole ligands produces a high-spin Fe(II) ion with relatively high... 

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