Crystallization and preliminary X-ray analysis of Citrobacter amalonaticusmethylaspartate ammonia lyase

Methylaspartate ammonia lyase (MAL) catalyses the reversible alpha,beta -el imination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mes aconic acid. Crystals of Citrobacter amalonaticus MAL have been obtained by the hanging-drop method of vapour diffusion using ammonium sulfate as the precipitant. Three crystal forms were obtained from identical crystallizati on conditions, two of which (forms A and B) diffract to high resolution, wh ilst the third form diffracted poorly. Crystals of form A diffract to beyon d 2.1 Angstrom and have been characterized as belonging to one of the enant iomorphic space groups P4(1)22 or P4(3)22, with unit-cell parameters a=b=66 .0, c=233.1 Angstrom, alpha=beta=gamma =90 degrees and a monomer in the asy mmetric unit. Crystals of form B diffract to beyond 1.5 Angstrom and belong to space group C222, with unit-cell parameters a=128.3, b=237.4, c=65.8 An gstrom, alpha=beta=gamma =90 degrees and a dimer in the asymmetric unit. De termination of the structure of MAL will be an important step in resolving current conflicts concerning the enzyme mechanism which differ between one which places MAL as a member of the superfamily of ammonia lyases whose cat alytic activity requires a cofactor formed by post-translational modificati on of the enzyme and another which links MAL to the enolase superfamily.