Crystallization of the NAD-dependent malic enzyme from the parasitic nematode Ascaris suum

The malic enzyme from muscle mitochondria of the parasitic nematode Ascaris suum is a tetramer of 65 kDa monomers that catalyzes the oxidative decarboxylation of malate to pyruvate and CO₂ with NAD cofactor as oxidant. This malic enzyme is critical to the nematode for muscle function under anaerobic conditions. Unlike mammalian versions of the enzyme such as that found in rat liver, which require NADP as cofactor, the nematode version is an NAD-dependent enzyme. We report the crystallization of samples of the nematode enzyme at room temperature from pH 7.5 solutions of polyethylene glycol 4000 containing magnesium sulfate, NAD and sodium tartronate. Immediately upon mixing of protein and precipitant solutions, a marked precipitation of the protein occurs. Out of this precipitate, crystals appear almost immediately, most commonly in a truncated cube form that can grow to 0.5 to 0.7 mm on a cube edge in two to three days. The crystals are trigonal, space group P3₁21 or its enantiomer, with a = b = 131.2(7) A ̊, c = 152.6(9) A ̊, and two monomers per asymmetric unit. Fresh crystals diffract X-radiation from a synchrotron source (λ = 0.95 Å) to about 3.0 Å resolution. Rotational analysis of Patterson functions indicates that the malic enzyme tetramer has 222 symmetry.