CRYSTALLISATION AND X-RAY DIFFRACTION ANALYSIS OF THE METALLOENZYME, ARGINASE FROM 'BACILLUS CALDOVELOX'. M. C. Bewley, J.S.Lott, M.L.Patchett & E.N. Baker. Department of Biochemistry, Massey University, Palmerston North, New Zealand

Arginine metabolism is central to life. Arginase is the enzyme which not only controls the hydrolysis of arginine to ornithine in the urea cycle but has been implicated in the regulation of nitric oxide signalling. No structure for arginase currently exists.

Arginase has been isolated from a variety of species ranging from bacteria and fungi to plants and animals. The physiological cofactor is Mn²⁺ but activation by a variety of divalent cations, such as VO²⁺, Fe²⁺, Co²⁺ and Cd²⁺, has also been reported, making arginase an attractive target for the study of metal-ion mediated catalysis, and also of the stabilisation of quarternary and tertiary structure by metal-ions.

The thermostable arginase from the thermophilic bacteria, 'Bacillus caldovelox' has been cloned, sequenced, expressed in E. coli and purified utilising the thermostable character of the protein. The amino acid sequence shows limited similarity to other arginases, but the putative metal ion binding motif (L/I)GGDHS-(14X)-DAH has been conserved.

X-ray quality crystals have been obtained which diffract to 2.2Å resolution and crystallise in an orthorhombic cell (a=89.7Å, b=146.1Å, c=154.9Å) with six molecules in the asymmetric unit. Heavy atom derivative screening is currently in progress, the results will be reported.