科研之友
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摘要
Proline residues have unique roles in protein folding, structure, and function. Proline and the aromatic amino acids comprise the encoded cyclic protein residues. Aromatic protein side chains are defined by their negatively charged pi faces, while the faces of the proline ring are partially positively charged. This polarity results from their two-point connection of the side chain to the electron-withdrawing protein backbone, and the lower electronegativity of hydrogen compared to carbon, nitrogen, and oxygen. The hydrogens adjacent to the carbonyl and amide nitrogen, H alpha and H delta, respectively, are the most partially positive. Proline%26apos;s side chain is also conformationally restricted, allowing for interaction with aromatic residues with minimal entropic or steric penalty. Proline and aromatic residues can interact favorably with each other, due to both the hydrophobic effect and the interaction between the pi aromatic face and the polarized C H bonds, called a CH/pi interaction. Aromatic proline interactions can occur locally, for example, to stabilize cis-amide bonds, and over larger distances, in the tertiary structures of proteins, and intermolecularly in protein protein interactions. In peptides and proteins, aromatic-proline sequences more readily adopt cis-prolyl amide bonds, where the aromatic ring interacts with the proline ring in the cis conformation. In aromatic proline sequences, Tip and Tyr are more likely to induce cis-amide bonds than Phe, suggesting an aromatic electronic effect. This result would be expected for a CH/pi interaction, in which a more electron-rich aromatic would have a stronger (more cis-stabilizing) interaction with partial positive charges on prolyl hydrogens. %26lt;br%26gt;In this Account, we describe our investigations into the nature of local aromatic proline interactions, using peptide models. We synthesized a series of 26 peptides, TXPN, varying X from electron-rich to electron poor aromatic amino adds, and found that the population of cis-amide bond (K-trans/cis) is tunable by aromatic electronics. With 4-substituted phenylalanines, we observed K-trans/cis with cis trans isomerism electronically controllable by served a Hammett correlation between aromatic electronics and 1.0 kcal/mol. All aromatic residues exhibit a higher cis population than Ala or cyclohexylalanine, with Tip showing the strongest aromatic proline interaction. In addition, proline stereoelectronic effects can modulate cis trans isomerism by an additional 1.0 kcal/mol. The aromatic proline interaction is enthalpic, consistent with its description as a CH/pi interaction. Proline aromatic sequences can also promote cis-prolyl bonds, either through interactions of the aromatic ring with the preceding cis-proline or with the H alpha prior to cis-proline. Within proline-rich peptides, sequences commonly found in natively disordered proteins, aromatic residues promote multiple cis-amide bonds due to multiple favorable aromatic-proline interactions. Collectively, we found aromatic proline interactions to be significantly CH/pi in nature, tunable by aromatic electronics. We discuss these data in the context of aromatic proline and aromatic-glycine interactions in local structure, in tertiary structure, in protein protein interactions, and in protein assemblies.
- 出版日期2013-4-16
