Purification, Metal Cofactor, N‐Terminal Sequence and Subunit Composition of a 5‐Aminolevulinic Acid Dehydratase from the Unicellular Green Alga Scenedesmus obliquus , Mutant C‐2A′

5‐Aminolevulinic acid dehydratase was purified to apparent homogeneity from Scenedesmus obliquus , mutant C‐2A′, starting with serial affinity chromatography according to Wang et al. [Wang, W.‐Y., Gough, S. P. & Kannangara, C. G. (1981) Carlsberg Res. Commun. 46 , 243–257], followed by separation on DEAE‐Cellulose DE 52, TSKgel Toyopearl HW‐55 and FPLC on Mono Q. The enzyme was purified 117‐fold compared with the initial crude soluble enzyme preparation and showed a final specific activity of 9.17 μkat/kg protein at pH 8.2 at a total recovery of 7%. Mg 2+ was determined to be the metal cofactor of the enzyme. It can, to a certain extent, be substituted by other divalent cations. From the purified enzyme the first 15 amino acids of the N‐terminus could be determined, showing a moderate similarity to 5‐aminolevulinic acid dehydratases from spinach, pea, Escherichia coli and yeast. The molecular mass of the native protein was determined by gel filtration to be 282±5 kDa. 42±1 kDa were ascertained for the subunit size by SDS/PAGE. These investigations, supported by electron microscopy, revealed that the enzyme from Scenedesmus consists of six subunits arranged in a six‐membered ring. Additionally, there is some evidence that two of the rings form a sandwich‐like complex.