Fig. 6: Different regulatory mechanisms in ATC.

a Plant ATCs present a unique mechanism of regulation, where UMP binds and blocks the active site. The CP-loop (represented in magenta) alternates between an UMP-bound inhibited conformation and an active conformation that ensures the sequential and perhaps ordered firing of the active sites. b In bacteria, isolated ATC catalytic trimers are unregulated. The association of two catalytic trimers with three dimers of regulatory subunits results in a holoenzyme that undergoes large conformational changes upon binding of UTP (inhibitor) or ATP (activator) to allosteric sites in the regulatory subunits. c In eukaryotes other than plants, ATC is fused together with CPS and DHO into a single multienzymatic protein named CAD that oligomerizes into hexamers where ATC trimers are proposed to occupy apical positions. The CPS and ATC activities are regulated by the binding of UTP to a regulatory region within the CPS domain.