Hysteretic behavior of methionine adenosyltransferase III - Methionine switches between two conformations of the enzyme with different specific activity

Methionine adenosyltransferase III (MATIII) catalyzes S-adenosylmethionine (AdoMet) synthesis and, as part of its reaction mechanism, it also hydrolyz es tripolyphosphate. Tripolyphosphatase activity was linear over time and h ad a slightly sigmoidal behavior with an affinity in the low micromolar ran ge. On the contrary, AdoMet synthetase activity showed a lag phase that was independent of protein concentration but decreased at increasing substrate concentrations. Tripolyphosphatase activity, which appeared to be slower t han AdoMet synthesis, was stimulated by preincubation with ATP and methioni ne so that it matched AdoMet synthetase activity. This stimulation process, which is probably the origin of the lag phase, represents the slow transit ion between two conformations of the enzyme that could be distinguished by their different tripolyphosphatase activity and sensitivity to S-nitrosylat ion. Tripolyphosphatase activity appeared to be the rate-determining reacti on in AdoMet synthesis and the one inhibited by S-nitrosylation. The methio nine concentration necessary to obtain half-maximal stimulation was in the range of physiological methionine fluctuations. Moreover, stimulation of MA T activity by methionine was demonstrated in vivo. We propose that the hyst eretic behavior of MATIII, in which methionine induces the transition to a higher specific activity conformation, can be considered as an adaptation t o the specific functional requirements of the liver.