Ulf Ryde-Pettersson

Soaking in Cs2SO4 reveals a caesium-aromatic interaction in bovine-liver rhodanese

Soaking crystals of rhodanese (thiosulphate:cyanide sulphurtransferase) in 2 M caesium sulphate reveals three caesium binding sites of this enzyme. One of these had been described before as a binding site for sodium ions and is located in a cleft close to the active site. In this site the monovalent cation is coordinated by five oxygen atoms. The first additional binding site seems to be quite special. The caesium ion is bound to the phenyl ring of a tryptophan residue. It is further liganded by two oxygen atoms. The third binding site is a result of crystal packing effects: caesium is liganded by four oxygen atoms, provided by two rhodanese molecules and one sulphate ion. It is likely that the binding of caesium affects the fluorescence of the tryptophan residue with which it interacts. Such possible effects should also be kept in mind when caesium ions are used as a quencher in fluorescence studies of proteins in general.

Oscillations in coupled enzymic reactions at high concentration of enzyme.

The transient-state kinetic consequences of the coupling of a single-enzyme reaction to other metabolic reactions have been examined in generalized terms. Analytical data are presented which specify under what conditions such coupling may lead to an oscillatory transient rate behaviour of a reaction system involving an enzyme operating by a Michaelian mechanism. The results indicate that the presence of enzyme in concentrations comparable to those of substrate and product may represent a hitherto unforeseen possible source of weakly damped, or even sustained, oscillations in metabolic networks. This observation has some important implications with regard to the occurrence of oscillations in biochemical reaction systems and to transient-state kinetic modelling of metabolic systems.