Pyramidalization of backbone carbonyl carbon atoms in proteins

The high accuracy of X-ray analyses at atomic resolution is now able to dis play subtle deformations from standard geometry of building blocks in prote ins. From the analysis of nine ultra-high resolution protein structures, we derived the first experimental evidence that a significant pyramidalizatio n at the main-chain carbonyl carbon atom occurs in proteins. Our findings a lso show that this pyramidalisation is related to the main-chain psi torsio n angle. The carbonyl carbon atoms of residues that adopt alpha (R) and ext ended conformations show a clear preference for positive and negative pyram idalization, respectively. The agreement between our data and those previou sly obtained from small molecule structures demonstrates that carbon pyrami dalization is an intrinsic property of the peptide structure. Although smal l in magnitude, the pyramidalization is well preserved in the complex folde d state of a macromolecular structure that results from the interplay of ma ny different forces. In addition, this property of the peptide group may ha ve interesting implications for the enzymatic reactions involving the carbo nyl carbon atoms.