Electrostatic stabilization in four-helix bundle proteins.
Charge substitutions generated by site-directed mutagenesis at the termini of adjacent anti-parallel alpha-helices in a four-helix bundle protein were used to determine a precise value for the contribution of indirect charge-charge interactions to overall protein stability, and to simulate the electrostatic effects of alpha-helix macrodipoles. Thermodynamic double mutant cycles were constructed to measure the interaction energy between such charges on adjacent anti-parallel helices in the four-helix bundle cytochrome b562 from Escherichia coli. Previously, theoretical calculations of helix macrodipole interactions using modeled four-helix bundle proteins have predicted values ranging over an order of magnitude from 0.2 to 2.5 kcal/mol. Our system represents the first experimental evidence for electrostatic interactions such as those between partial charges due to helix macrodipole charges. At the positions mutated, we have measured a favorable interaction energy of 0.6 kcal/mol between opposite charges simulating an anti-parallel helix pair. Pairs of negative or positive charges simulating a parallel orientation of helices produce an unfavorable interaction of similar magnitude. The interaction energies show a strong dependence upon ionic strength, consistent with an electrostatic effect. Indirect electrostatic contacts do appear to confer a limited stabilization upon the association of anti-parallel packing of helices, favoring this orientation by as much as 1 kcal/mol at 20 mM K phosphate.
Department of Biochemistry, University of Illinois, Urbana 61801.
C R
CR
Robinson
S G
SG
Sligar
D001059:Apoproteins / Q000737:chemistry; D001483:Base Sequence; D002942:Circular Dichroism; D003574:Cytochrome c Group / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification; D004926:Escherichia coli / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification; D013816:Thermodynamics
D001059:Apoproteins; D003574:Cytochrome c Group; D011994:Recombinant Proteins; C083951:cytochrome c553; C022302:cytochrome C-552