title: A reexamination of the postulated charge transfer interactions at the active site of the enzyme rhodanese.
abstract: Spectral and kinetic studies of the interaction of N-methylnicotinamide chloride and nicotinamide with the enzyme thiosulphate sulphurtransferase (thiosulphate: cyanide sulfurtransferase, EC 2.8.1.1) (also known as rhodanese) have been performed and compared with previous inhibition data obtained with N-1-(4-pyridyl)pyridinium chloride (NPP). Like NPP both N-methylnicotinamide chloride and nicotinamide are competitive inhibitors of rhodanese with respect to the substrate thiosulfate. Rhodanese binding of N-methylnicotinamide chloride gives rise to no charge transfer absorbtion band. In addition, the free energy of interaction (deltaG0) of NPP with rhodanese is approximately equal to the sum of the individual deltaG0 values of MNA and NA. These compounds are analogous to the two halves of the NPP structure. We conclude that NPP and N-methylnicotinamide chloride are not bound via a charge transfer mechanism. The major stabilizing influence appears to be an ionic interaction with an anionic enzyme site with accessory apolar stabilization. It is postulated that the ionized active site sulfhydryl group in rhodanese could provide the ionic site.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004579:Electron Transport; D004735:Energy Transfer; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D009536:Niacinamide / Q000031:analogs & derivatives / Q000494:pharmacology; D011485:Protein Binding; D013466:Sulfurtransferases / Q000378:metabolism; D013884:Thiosulfate Sulfurtransferase / Q000378:metabolism
chemical_list: D009536:Niacinamide; D013466:Sulfurtransferases; D013884:Thiosulfate Sulfurtransferase