Capping and alpha-helix stability.
The first and last four residues of alpha-helices differ from the rest by not being able to make the intrehelical hydrogen bonds between the backbone greater than C=O groups of one turn and the greater than NH groups of the next. Physico-chemical arguments and statistical analysis suggest that there is a preference for certain residues at the C and N termini (The C- and N-caps) that can fulfil the hydrogen bonding requirements. We have tested this hypothesis by constructing a series of mutations in the two N-caps of barnase (Bacillus amyloliquefaciens ribonuclease, positions Thr 6 and Thr 26) and determining the change in their stability. The N-cap is found to stabilize the protein by up to approximately 2.5 kcal mol(-1). The presence of a negative charge of the N-cap adds some 1.6 kcal mol(-1) of stabilization energy because of the interaction with the macroscopic electrostatic dipole of the helix.
Serrano
L
L
Department of Chemistry, University of Cambridge, UK.
Fersht
A R
AR
D001407:Bacillus / Q000201:enzymology*; D004252:DNA Mutational Analysis; D006860:Hydrogen Bonding; D008958:Models, Molecular; D011487:Protein Conformation; D012260:Ribonucleases / Q000235:genetics; D013329:Structure-Activity Relationship; D013816:Thermodynamics
D012260:Ribonucleases