Ornithine carbamoyltransferase (OTC) catalyzes the reversible transfer of the carbamoyl group from carbamoyl phosphate (CP) to the Nε atom of L-ornithine (ORN) to produce L-citrulline. There are two types of the enzyme – anabolic (aOTC) and catabolic (cOTC). Anabolic OTCs participate in the urea cycle and L-arginine biosynthesis. Catabolic OTCs are part of the catabolic arginine deiminase pathway found in a number of microorganisms. The reported structure is for the anabolic enzyme from pathogen Vibrio vulnificus. The structure has three monomers in the asymmetric unit which form the physiologically active trimer. Two of the three enzyme subunits are entrapped in a complex with its first substrate carbamoyl phosphate. That complex provides additional structural insights in the enzyme mechanism of action because it proves that binding of CP causes closure of interdomain cleft and change of 55-60 catalytic loop conformation, whereas catalytic loops B2–H3 and SMG can adopt conformations different from the Michaelis complex of be disordered.