Crystallization, characterization, and preliminary crystallographic studies of mitochondrial carbamoyl phosphate synthetase I of Rana catesbeiana.

Carbamoyl phosphate synthetase I (ammonia; E C 6.3.4.16) was purified from the liver of Rana catesbeiana (bullfrog). Crystals of the protein have been obtained at 22 degrees C by the hanging drop vapor diffusion technique, with polyethylene glycol as precipitant. Tetragonal crystals of about 0.3 x 0.3 x 0.7 mm diffract at room temperature to at least 3.5 A using a conventional source and are stable to X-radiation for about 12 h. Therefore, these crystals are suitable for high resolution studies. The space group is P4(1)2(1)2 (or its enantiomorph P4(3)2(1)2), with unit cell dimensions a = b = 291.6 A and c = 189.4 A. Density packing considerations are consistent with the presence of 4-6 monomers (M(r) of the monomer, 160,000) in the asymmetric unit. Amino-terminal sequence of the enzyme and of a chymotryptic fragment of 73.7 kDa containing the COOH-terminus has been obtained. The extensive sequence identity with rat and human carbamoyl phosphate synthetase I indicates the relevance for mammals of structural data obtained with the frog enzyme.