We have studied the kinetics of the quaternary structure change associated with the allosteric transition of aspartate transcarbamylase (ATCase) (E. coli), inducing this change by exposure to the natural substrates (carbarnyl phosphate and L-aspartate). The presence of 30% ethylene glycol slowed the quaternary structure change sufficiently for it to be followed by stopped-flow X-ray scattering at − 5°C. After adding substrates to the enzyme, the change occurred, with a half-life of a few seconds, yielding a mixture of the two standard quaternary structures (or, conceivably, a state intermediate between them). This mixture persisted until the enzyme reduced the substrate concentration below a threshold value.
