Cooperative stabilization of Escherichia coli ribonuclease HI by insertion of Gly-80b and Gly-77-->Ala substitution.

The insertion of a Gly residue (designated as Gly-80b) between the C-cap of the αII-helix (Gln-80) and the N-cap of the αIII-helix (Trp-81) in Escherichia coli ribonuclease HI enhances the protein stability by 0.4 kcal/mol in ΔG (Kimura, S., Nakamura, H., Hashimoto, T., Oobatake, M., & Kanaya, S. (1992) J. Biol. Chem. 267, 21535-21542). Another mutation within the αII-helix, Gly-77→Ala, reduces the stability by 0.9 kcal/mol. Simultaneous introduction of these mutations enhances the stability by 0.8 kcal/mol, indicating that the effects of these mutations are cooperative and not simply independent