Aromatic Side Chain-Porphyrin Interactions in Designed Hemoproteins
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- 作者：Dahui Liu ; David A. Williamson ; Michelle L. Kennedy ; Todd D. Williams ; Martha M. Morton ; and David R. Benson
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：December 22, 1999
- 年：1999
- 卷：121
- 期：50
- 页码：11798 - 11812
- 全文大小：249K
- 年卷期：v.121,no.50(December 22, 1999)
- ISSN：1520-5126
文摘
Aromatic amino acid side chains are commonly observed to interact with the heme cofactors ofnatural hemoproteins. These interactions are of the types previously identified for pairs or groups of aromaticamino acid side chains in proteins: offset 
-stacking and T-stacking (an edge-to-face arrangement). To evaluatehow such interactions may influence structural stability of hemoproteins, we synthesized peptide-sandwichedmesohemes (PSMs) 2 and 3 in which the alanine-4 (Ala-4) residues in 1 have been replaced by phenylalanine(Phe) and tryptophan (Trp), respectively. The Co(III) analogues of 1, 2, and 3 (1-Co, 2-Co, and 3-Co,respectively) were also prepared. Histidine (His)-to-iron coordination in 1 had previously been shown to inducehelical conformations in the peptides (helix content ~50% at 8 
deg.gif">C). Molecular modeling studies suggestedthat Trp, but not Phe, could engage in edge-to-face interactions with the porphyrin if the peptides are fullyhelical. Replacing Ala-4 with Trp, but not with Phe, was thus predicted to favor enhanced peptide helix content.Circular dichroism spectra are consistent with significantly increased helix content in 3 relative to 1, but notin 2. Hydrogen-deuterium (H/D) exchange rates determined by electrospray ionization mass spectrometry,however, decrease in the order 1 
2 > 3, while pH titrations reveal that the stability of the model proteinfolds decreases in the order 3 > 2 1. Furthermore, 1H NMR spectra of 2-Co and 3-Co indicate that thearomatic side chains in each compound are oriented within the shielding region of the porphyrin ring. Two-dimensional NOE and chemical shift data show that the helices in 3-Co are more highly organized than in1-Co and span nearly the entire peptide sequence, while in 2-Co shorter helices of intermediate stability runbetween Phe-4 and Ala-13. The combined results indicate that aromatic side chain-porphyrin interactions in2 and 3 stabilize their respective model protein folds, and suggest a similar role for the corresponding interactionsin natural hemoproteins. Finally, the chemical shift patterns of the Trp side chains in 3-Co, the different effectsof Phe and Trp on peptide architecture, and the pattern of chemical shifts exhibited by the 
-NH and -CHhydrogens in all three Co(III) PSMs demonstrate that the solution structures of these designed hemoproteinsare similar to those predicted in molecular modeling studies.
-stacking and T-stacking (an edge-to-face arrangement). To evaluatehow such interactions may influence structural stability of hemoproteins, we synthesized peptide-sandwichedmesohemes (PSMs) 2 and 3 in which the alanine-4 (Ala-4) residues in 1 have been replaced by phenylalanine(Phe) and tryptophan (Trp), respectively. The Co(III) analogues of 1, 2, and 3 (1-Co, 2-Co, and 3-Co,respectively) were also prepared. Histidine (His)-to-iron coordination in 1 had previously been shown to inducehelical conformations in the peptides (helix content ~50% at 8 deg.gif">C). Molecular modeling studies suggestedthat Trp, but not Phe, could engage in edge-to-face interactions with the porphyrin if the peptides are fullyhelical. Replacing Ala-4 with Trp, but not with Phe, was thus predicted to favor enhanced peptide helix content.Circular dichroism spectra are consistent with significantly increased helix content in 3 relative to 1, but notin 2. Hydrogen-deuterium (H/D) exchange rates determined by electrospray ionization mass spectrometry,however, decrease in the order 1 2 > 3, while pH titrations reveal that the stability of the model proteinfolds decreases in the order 3 > 2 1. Furthermore, 1H NMR spectra of 2-Co and 3-Co indicate that thearomatic side chains in each compound are oriented within the shielding region of the porphyrin ring. Two-dimensional NOE and chemical shift data show that the helices in 3-Co are more highly organized than in1-Co and span nearly the entire peptide sequence, while in 2-Co shorter helices of intermediate stability runbetween Phe-4 and Ala-13. The combined results indicate that aromatic side chain-porphyrin interactions in2 and 3 stabilize their respective model protein folds, and suggest a similar role for the corresponding interactionsin natural hemoproteins. Finally, the chemical shift patterns of the Trp side chains in 3-Co, the different effectsof Phe and Trp on peptide architecture, and the pattern of chemical shifts exhibited by the -NH and -CHhydrogens in all three Co(III) PSMs demonstrate that the solution structures of these designed hemoproteinsare similar to those predicted in molecular modeling studies.