We investigated the possible role of residues at the Ccap position in an alpha-helix on protein stability. A set of 431 protein alpha-helices containing a C'-Gly from the Protein Data Bank (PDB) was analyzed, and the normalized frequencies for finding particular residues at the Ccap position, the average fraction of buried surface area, and the hydrogen bonding patterns of the Ccap residue side-chain were calculated. We found that on average the Ccap position is 70% buried and noted a significant correlation (R=0.8) between the relative burial of this residue and its hydrophobicity as defined by the Gibbs energy of transfer from octanol or cyclohexane to water. Ccap residues with polar side-chains are commonly involved in hydrogen bonding. The hydrogen bonding pattern is such that, the longer side-chains of Glu, Gln, Arg, Lys, His form hydrogen bonds with residues distal (>+/-4) in sequence, while the shorter side-chains of Asp, Asn, Ser, Thr exhibit hydrogen bonds with residues close in sequence (<+/-4), mainly involving backbone atoms. Experimentally we determined the thermodynamic propensities of residues at the Ccap position using the protein ubiquitin as a model system. We observed a large variation in the stability of the ubiquitin variants depending on the nature of the Ccap residue. Furthermore, the measured changes in stability of the ubiquitin variants correlate with the hydrophobicity of the Ccap residue. The experimental results, together with the statistical analysis of protein structures from the PDB, indicate that the key hydrophobic capping interactions between a helical residue (C3 or C4) and a residue outside the helix (C", C3' or C4') are frequently enhanced by the hydrophobic interactions with Ccap residues. Study holds ProTherm entries: 15658, 15659, 15660, 15661, 15662, 15663, 15664, 15665, 15666, 15667, 15668, 15669, 15670, 15671, 15672, 15673, 15674, 15675, 15676, 15677, 15678, 15679, 15680, 15681, 15682, 15683, 15684, 15685, 15686, 15687, 15688, 15689, 15690, 15691, 15692, 15693 Extra Details: R 63 K substituted pseude wild type. helix termination; C-capping motifs; statistical analysis; hydrophobic interactions; differential scanning calorimetry
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:46 p.m.
|Number of data points||141|
|Proteins||E3 ubiquitin-protein ligase RSP5 ; Ubiquitin-binding protein CUE2|
|Assays/Quantities/Protocols||Experimental Assay: dG_H2O ; Experimental Assay: dCp temp:65.0 C ; Experimental Assay: dHcal temp:65.0 C ; Experimental Assay: dG ; Experimental Assay: dHcal ; Experimental Assay: Tm ; Experimental Assay: dCp ; Derived Quantity: ddG_H2O ; Derived Quantity: ddG ; Derived Quantity: dTm|
|Libraries||Mutations for sequence A:NDDHESKLSILMDMFPAISKSKLQVHLLENNNDLDLTIGLLLKENDDKS/B:MQIFVKTLTGKTITLEVESSDTIDNVKSKIQDKEGIPPDQQRLIFAGKQLEDGRTLSDYNIQKESTLHLVLRLRGG|