title: Stability of monomeric Cro variants: Isoenergetic transformation of a type I' to a type II' beta-hairpin by single amino acid replacements.
abstract: The thermodynamic stabilities of three monomeric variants of the bacteriophage lambda Cro repressor that differ only in the sequence of two amino acids at the apex of an engineered beta-hairpin have been determined. The sequences of the turns are EVK-XX-EVK, where the two central residues are DG, GG, and GT, respectively. Standard-state unfolding free energies, determined from circular dichroism measurements as a function of urea concentration, range from 2.4 to 2.7 kcal/mole, while those determined from guanidine hydrochloride range from 2.8 to 3.3 kcal/mole for the three proteins. Thermal denaturation yields van't Hoff unfolding enthalpies of 36 to 40 kcal /mole at midpoint temperatures in the range of 53 to 58 degrees C. Extrapolation of the thermal denaturation free energies with heat capacities of 400 to 600 cal/mole deg gives good agreement with the parameters determined in denaturant titrations. As predicted from statistical surveys of amino acid replacements in beta-hairpins, energetic barriers to transformation from a type I' turn (DG) to a type II' turn (GT) can be quite small.
authors.affiliation: Department of Biology, Yeshiva University, New York, New York 10033, USA.
authors.forename: A K M M
authors.initials: AK
authors.lastname: Mollah
authors.forename: Rhonda L
authors.initials: RL
authors.lastname: Stennis
authors.forename: Michael C
authors.initials: MC
authors.lastname: Mossing
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D004268:DNA-Binding Proteins; D014644:Genetic Variation; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics; D013816:Thermodynamics; D014764:Viral Proteins; D054334:Viral Regulatory and Accessory Proteins
chemical_list: D004268:DNA-Binding Proteins; D012097:Repressor Proteins; D014764:Viral Proteins; D054334:Viral Regulatory and Accessory Proteins; C016649:phage repressor proteins