Thermodynamics of zinc binding to rhodanese

NH2-terminal sequence truncation decreases the stability of bovine rhodanese, minimally perturbs its crystal structure, and enhances interaction with GroEL under native conditions

The NH2-terminal sequence of rhodanese influences many of its properties, ranging from mitochondrial import to folding. Rhodanese truncated by >9 residues is degraded in Escherichia coli. Mutant enzymes with lesser truncations are recoverable and active, but they show altered active site reactivities (Trevino, R. J., Tsalkova, T., Dramer, G., Hardesty, B., Chirgwin, J. M., and Horowitz, P. M. (1998) J. Biol. Chem. 273, 27841-27847), suggesting that the NH2-terminal sequence stabilizes the overall structure. We tested aspects of the conformations of these shortened species. Intrinsic and probe fluorescence showed that truncation decreased stability and increased hydrophobic exposure, while near UV CD suggested altered tertiary structure. Under native conditions, truncated rhodanese bound to GroEL and was released and reactivated by adding ATP and GroES, suggesting equilibrium between native and non-native conformers. Furthermore, GroEL assisted folding of denatured mutants to the same extent as wild type, although at a reduced rate. X-ray crystallography showed that Delta1-7 crystallized isomorphously with wild type in polyethyleneglycol, and the structure was highly conserved. Thus, the missing NH2-terminal residues that contribute to global stability of the native structure in solution do not significantly alter contacts at the atomic level of the crystallized protein. The two-domain structure of rhodanese was not significantly altered by drastically different crystallization conditions or crystal packing suggesting rigidity of the native rhodanese domains and the stabilization of the interdomain interactions by the crystal environment. The results support a model in which loss of interactions near the rhodanese NH2 terminus does not distort the folded native structure but does facilitate the transition in solution to a molten globule state, which among other things, can interact with molecular chaperones.

The use of microcalorimetry to measure thermodynamic parameters of the binding of ligands to insulin

Flow microcalorimetry was used to measure the free energies, enthalpies, and entropies of interactions between the hormone insulin and small ligand molecules or ions. Measurable amounts of heat were obtained for binding of four phenolic preservative molecules--phenol, meta-cresol, resorcinol, and methylparaben--to both two-zinc and zinc-free insulin and for binding of zinc ions to zinc-free insulin. All of the reactions were spontaneous, but the phenolic binding was driven by enthalpy, while that of zinc was entropy-driven. A combination of van der Waals interactions, hydrophobic effects, and protein conformational changes appeared to be involved in binding of the phenolic ligands. Zinc ions displayed two types of binding to insulin, both involving ion-dipole interactions.

Differences in the binding of sulfate, selenate and thiosulfate ions to bovine liver rhodanese, and a description of a binding site for ammonium and sodium ions. An X-ray diffraction study

The binding of sulfate, selenate and thiosulfate by the sulfur-transferase rhodanese (EC 2.8.1.1) in the crystalline state has been studied by X-ray analysis at resolutions between 0.23 nm and 0.4 nm. The three ions appear to occupy a common site between the N eta atoms of Arg-29 and the main-chain NH group of Glu-148 at the surface of the enzyme molecule. A second binding site for the three ions is situated at the entrance to the active centre, between the side chains of Arg-186 and Lys-249. Selenate and thiosulfate are bound equally well at both anion-binding sites. Sulfate, however, binds better at the first position, near Arg-29, than at the second site near Arg-186. In the complex of sulfur-rhodanese with thiosulfate, the outer sulfur atom of the anion near the active centre points towards the extra sulfur atom which is bound as a persulfide to the S gamma of the essential Cys-247. The distance between the outer sulfur atom of the thiosulfate ion and the persulfide sulfur atom appears to be about 0.3 nm. The thiosulfate difference Fourier also shows a distinct, localized conformational change involving residues 71, 72 and 249. This is the result of the replacement of an ammonium ion in the sulfate and selenate media by a sodium ion in the sodium thiosulfate solution. Rhodanese is apparently able to accomodate ions with different radii at this cation-binding site by minor structural alterations.

Thermal behavior of HeLa and KB cells in suspension and attached to glass

HeLa S-3 and KB cells were grown in a LKB Batch Microcalorimeter under a variety of nutrient medium conditions amd mixing intervals. These conditions produced rather large apparent endothermic and exothermic responses on mixing that could be correlated with the presence of suspended cells (unattached) as well as cells attached to the glass calorimeter vessel. Cells capable of being resuspended upon mixing of the calorimeter vessel produces first an endothermic followed by an exothermic signal while attached cells produced only an apparent endothermic response. The exothermic response is believed to be associated with increased metabolic heat on suspending the cells followed by partial suppression of the steady state metabolic heat on cell settling. Rates of cell settling correlated well with the rate of decay of the exothermic signal. The rapid appearance of endothermicity on mixing suggests it is associated with rapid events such as binding of nutrients to cell surfaces. The response in the endothermic direction on mixing is discussed in terms of the disruption of mechanisms which tend to exclude nutrients from the surface of the cell.