The thermal unfolding of the catabolite activator protein (CAP) of Escherichia coli and the complexes it forms with adenosine cyclic 3',5'-phosphate (cAMP) and guanosine cyclic 3',5'-phosphate (cGMP) was studied by high-sensitivity differential scanning calorimetry (DSC). The thermal denaturation of CAP at pH 7.00 gave an irreversible, symmetrical denaturation curve with a single peak. Distinctly different, more complex DSC curves were obtained for the thermal denaturation of the cAMP-protein and cGMP-protein complexes. The DSC data indicate intermolecular cooperation among CAP dimers, with the extent of oligomerization remaining unchanged during unfolding of the protein. The DSC curves for the thermal denaturation of the cAMP-protein complex and cGMP-protein complex have been resolved into three and two components, respectively, according to the model of independent two-state processes. Analysis of the DSC data suggests two and three independent domains for cGMP-protein and cAMP-protein complexes, respectively, with dissociation of mononucleotide occurring in the second component in both cases during protein denaturation. Furthermore, our studies indicate that the presence of either ligand alters the degree of oligomerization of CAP dimers, cAMP having a greater effect than cGMP.

High-sensitivity differential scanning calorimetry has been applied to the study of the reversible thermal unfolding of the lysozyme of T4 bacteriophage in which the threonine residue at position 157 has been replaced by seven different residues. High-resolution structures derived from X-ray crystallography have been reported for these and six other mutants by Alber et al. [Alber, T., Dao-Pin, S., Wilson, K., Wozniak, J. A., Cook, S. P., & Matthews, B. W. (1987) Nature 330, 41-46]. At pH 2.5 the changes relative to the wild-type protein in the standard free energy of unfolding produced by these mutations indicate apparent destabilizations of 0.6 kcal mol-1 (T157R) to 1.9 kcal mol-1 (T157I), whereas the changes in enthalpy of unfolding range from -5.8 kcal mol-1 (T157N) to 11.9 kcal mol-1 (T157E). Since the denaturations are in all cases accompanied by large changes in heat capacity amounting to 2.5 kcal K-1 mol-1, both the free energies and enthalpies are functions of temperature. An intriguing feature of the present results is the relatively large enthalpy changes and the corresponding compensating entropy changes. Our present understanding of the intramolecular energetics of proteins is insufficient to account for these changes.

Gene 32 protein (g32P), the single-stranded (ss) DNA binding protein from bacteriophage T4, is a zinc metalloprotein. The intrinsic zinc is one of the factors required for the protein to bind cooperatively to a ssDNA lattice. We have used differential scanning calorimetry to determine how the thermodynamic parameters characterizing the denaturation of g32P are affected by removal or substitution of the intrinsic zinc. Over a wide concentration range (1-10 mg/mL), the native Zn(II) protein unfolds at a tm of 55 degrees C with an associated mean enthalpy change of 139 kcal mol-1. Under the same conditions, the metal-free apoprotein denatures over a relatively broader temperature range centered at 49 degrees C, with a mean enthalpy change of 84 kcal mol-1. Substitution of Zn(II) in g32P by either Cd(II) or Co(II) does not significantly change the enthalpy of denaturation but does affect the thermal stability of the protein. All metallo forms of g32P when bound to poly(dT) undergo highly cooperative denaturational transitions characterized by asymmetric differential scanning calorimetry peaks with increases in tm of 4-5 degrees C compared to the unliganded metalloprotein. Removal of the metal ion from g32P significantly reduces the cooperativity of binding to poly(dT) [Giedroc, D. P., Keating, K. M., Williams, K. R., & Coleman, J. E. (1987) Biochemistry 26, 5251-5259], and presumably as a consequence of this, apo-g32P shows no change in either the shape or the midpoint of the thermal transition on binding to poly(dT).(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriophage T4 gene 32 encodes a single-stranded DNA (ssDNA) binding protein (gp32) required for T4 DNA replication, recombination, and repair. Previous physicochemical studies on gp32 and other ssDNA binding proteins have suggested that binding may involve hydrophobic interactions that result from the close approach of several aromatic amino acid side chains with the nucleic acid bases. In the case of gp32, five tyrosines and two phenylalanines have previously been implicated in gp32.ssDNA complex formation. Site-directed mutagenesis of T4 gene 32 was employed to produce a set of eight gp32 mutant proteins, each of which encoded a single substitution at one of the eight tyrosine residues within gp32. The mutant gp32 proteins were then subjected to physicochemical analysis to evaluate the role of each tyrosine residue in gp32 structure and function. Oligonucleotide binding studies suggest that tyrosine residues 84, 99, 106, 115, and 186 each contribute from 0.3 to 0.7 kcal/mol to ssDNA binding, which corresponds to 3-7% of the overall binding energy for gp32.ssDNA complex formation. Replacement of tyrosine residues 73 and 92 appears to lead to large structural changes that may be the result of disrupting the zinc binding subdomain within gp32.

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