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Studies of Enzyme Mechanism Using Isotopic Probes
The isotope partitioning studies of the Ascaris suum NAD-malic enzyme reaction were examined with five transitory complexes including E:NAD, E:NAD:Mg, E:malate, E:Mg:malate, and E:NAD:malate. Three productive complexes, E:NAD, E:NAD:Mg, and E:Mg:malate, were obtained, suggesting a steady-state random mechanism. Data for trapping with E:14C-NAD indicate a rapid equilibrium addition of Mg2+ prior to the addition of malate. Trapping with 14C-malate could only be obtained from the E:Mg2+:14C-malate complex, while no trapping from E:14C-malate was obtained under feasible experimental conditions. Most likely, E:malate is non-productive, as has been suggested from the kinetic analysis. The experiment with E:NAD:malate could not be carried out due to the turnover of trace amounts of malate dehydrogenase in the pulse solution. The equations for the isotope partitioning studies varying two substrates in the chase solution in an ordered terreactant reaction were derived, allowing a determination of the relative rates of substrate dissociation to the catalytic reaction for each of the productive transitory complexes. NAD and malate are released from the central complex at an identical rate, equal to the catalytic rate
Crystallization and oligomeric structure of rat liver arginase
Rat liver arginase, a manganese-metalloenzyme, has been crystallized from polyethylene glycol 8000 in N,N-bis(2-hydroxyethyl)glycine (Bicine) buffer at pH 8.5. Crystals form as either cubes or pyramids and belong to space group P31 (or P32) with hexagonal unit cell dimensions a = b = 88·9 Å, c = 114·8 Å, or a = b = 88·5 Å, c = 104·5 Å; the variation along the c axis does not correlate with the external crystal morphology of cube or pyramidshaped. X-ray diffraction data are measured to a limiting resolution of 2·4 Å…. Given the volume constraints of the unit cell it is likely that rat liver arginase is a trimer, with three 35,000 Da monomers in the asymmetric unit. This resolves a persistent ambiguity regarding the oligomeric structure of this enzyme