Assembly of formyltetrahydrofolate synthetase monomers. Kinetic evidence for a temperature-induced conformational...

The assembly of Clostridium acidi-urici formyltetrahydrofolate synthetase (formate: tetrahydrofolate ligase (ADP-forming), EC 6.3.4.3) monomers induced by low concentrations of CsCl to form the catalytically active tetramer shows a pronounced lag phase in the time-course progress curve. Preincubation of the monomer for 20 min at 20 degree C in a medium of either high ionic strength, 10% dimethylsulfoxide or 1 M glycerol removes the lag period. Shorter preincubation periods produce the same effect if the medium contains both high ionic strength and dimethylsulfoxide or glycerol. Preincubation at 0 degree C in the presence of these agents has little effect. The reassociation reaction is first-order with respect to monomer concentration, before preincubation at 20 degree C, but second-order after. It is proposed that two conformers of the monomer exist in equilibrium in a temperature-dependent reaction and that the conformer able to reassociate is more favored at 20 degree C than at 0 degree C. High ionic strength or the presence of the organic solvents shift the equilibrium far in the direction of the conformer capable of reassociation, changing the rate-limiting step from a first-order conformational step to a second-order monomer association step.