The pyrrolidine side chain makes proline play a unique role in protein structure and function. The Cγ ring pucker preference and the cis–trans peptidyl bond ratio can be mediated via stereoelectronic effects. Here we used a compact triple-stranded antiparallel β-sheet protein, the human Pin1 WW domain, to study the consequences of implanting a preorganized Cγ ring pucker on protein structure and function. The conserved Pro37 is a key residue involved in one hydrophobic core, plays an important role in the WW domain, and adopts a Cγ-endo ring pucker in the native structure. Pro37 was replaced with Cγ-exo biased pucker derivatives: (2S,4R)-4-hydroxyproline (4R-Hyp), (2S,4R)-4-fluoroproline (4R-Flp), (2S,4R)-4-methoxyproline (4R-Mop), and Cγ-endo biased pucker derivatives: (2S,4S)-4-hydroxyproline (4S-hyp), (2S,4S)-4-fluoroproline (4S-flp), (2S,4S)-4-methoxyproline (4S-mop) to examine how a preorganized pucker affects the folding stability and ligand-binding affinity. Circular dichroism measurements indicate that among the variants, only the one with 4S-flp substitution (P37flp) is more stable than the wild type, suggesting that the stabilization effects originated from preorganization of the backbone conformation and the hydrophobicity of CF group. Analysis of ligand-binding affinity using isothermal titration calorimetry revealed that only P37flp has a stronger ligand affinity than the wild type, showing that 4S-flp can stabilize the WW domain and increase its ligand affinity. Together we have used 4-substituted proline derivatives and the WW domain to demonstrate that proline ring puckering can be a key factor in determining the folding stability of a protein but the choice of the derivative groups is also critical. Proteins 2014; 82:67–76. © 2013 Wiley Periodicals, Inc.
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