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°C by the hanging drop vapor diffusion technique, with polyethylene glycol as precipitant. Tetragonal crystals of about 0.3 × 0.3 × 0.7 mm diffract at room temperature to at least 3.5 Å using a conventional source and are stable to X‐radiation for about 12 h. Therefore, these crystals are suitablefor high resolution studies. The space group is P 4 1 2 1 2 (or its enantiomorph P 4 3 2 1 2), with unit cell dimensions a = b = 291.6 Å and c = 189.4 Å. Density packing considerations areconsistent 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. © 1995 Wiley‐Liss, Inc.