Peptide carbonyl groups, metal binding

An analysis of metal binding to peptide carbonyl groups (Chakrabarti, 1990), mainly calcium ions in protein crystal structures, shows that the cations tend to lie in the peptide plane near the C=0 bond direction. Generally, this binding occurs in turns in proteins or in regions with no regular secondary structures. Ca---0 distances range from 2.2 to 2.5 A, and metal ions do not deviate by more than 35° from the peptide plane. Thus, metal ions in proteins do not, Chakrabarti observed, bind in lone-pair directions. 

These results can be summarized in terms of the mechanisms shown in Figure 11 for peptide and ester hydrolysis. Assuming the peptide binds to the enzyme in the manner shown above, the exact ester analog must bind in a different manner, particularly with regard to the active site metal atom. The placement of the peptide carbonyl group on the... 

The peptide cyclo- Pro-Gly-) forms complexes with different alkali and alkaline earth metals that show spectra with sensitivity to the conformation of the peptide the arrangement of the carbonyl groups is especially of interest. The solvent plays the most important role in the development of conformation. The size of the ion-binding cavity formed by the carbonyl groups and the size and charge of the cation are only of secondary importance. 

Unlike both copper(II) andnickel(II), zinc(II) does not form very strong complexes with LHRH. Studies have shown that, at pH 7.0, only 20% of LHRH is complexed with zinc(II). In addition to binding to the imidazole of the histidine residue, the zinc(II) ion is also weakly coordinated by a carbonyl of a peptide group. NMR studies have indicated that it is the carbonyl oxygen of the His-Trp peptide bond that coordinates the metal ion. 

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