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Sac7d is a small chromatin protein from the hyperthermophile Sulfolobus acidocaldarius which kinks duplex DNA by approximately 66 degrees at a single base pair step with intercalation of V26 and M29 side chains. Site-directed mutagenesis coupled with calorimetric and spectroscopic data has been used to characterize the influence of the intercalating side chains on the structure and thermodynamics of the DNA complex from 5 to 85 degrees C. Two single-alanine substitutions (V26A and M29A) and five double-glycine, -alanine, -leucine, -phenylalanine, and -tryptophan substitutions of the surface residues have been created. NMR and fluorescence titrations indicated that the substitutions had little effect on the structure of the protein or DNA binding site size. Each of the mutant proteins demonstrated a temperature-dependent binding enthalpy which was correlated with a similar temperature dependence in the structure of the complex reflected by changes in fluorescence and circular dichroism. A positive heat capacity change (DeltaC(p)) for DNA binding was observed for only those mutants which also demonstrated a thermotropic structural transition in the complex, and the temperature range for the positive DeltaC(p) coincided with that observed for the structural transition. The thermodynamic data are interpreted using a model in which binding is linked to an endothermic distortion of the DNA in the complex. The results support the proposal that the unfavorable enthalpy of binding of Sac7d at 25 degrees C is due in part to the distortion of DNA.

Isothermal titration calorimetry (ITC) is a label free technique used for direct detection of biological interactions by measurement of the heat given off or taken up during the reaction. In this article we will introduce the ITC technique and review two applications of ITC in drug discovery; small molecule/protein interactions and enzyme kinetics. We will also describe the characteristics of a new miniaturized, ultrasensitive calorimeter. This new microcalorimetry system reduces the quantity of protein (or other macromolecule sample) required to obtain a complete thermodynamic profile (n, K, DeltaH and DeltaS) by up to 7-fold. The reduction in required sample quantities allows ITC to be effectively utilized at earlier stages of the drug discovery and development process.

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