The enzymology of the beta-hydroxyaspartate pathway.

The enzymes of the 3-hydroxyaspartate pathway have been found only in Micrococcus denitrificans growing on glyoxylate or a glyoxylate precursor. The partial purification of two of these enzymes (aspartate-glyoxylate transaminase and erythxo-beta-hydroxy- aspartate dehydratase) and the development of a specific assay for the third (erythro-beta-biydroxyaspartate aldolase) has allowed a study of their properties. All three were pyridoxal phosphate enzymes, although the aldolase alone exhibited a requirement for this coenzyme in crude extract, both for activity and stability. All three were most active at pH 7 or above, but only the aldolase and dehydratase had metal ion requirements, the former needing higher concentrations of divalent metal cations for activity than the latter. The purified aspartate-glyoxylate transaminase was specific for glyoxylate as amino acceptor, but reacted with a variety of amino donors, being most active with L-aspartate and L-serine. It was shown that these activities were associated with a single enzyme protein, and that this transaminase was responsible for the formation of glycine from glyoxylate in this system since glycine dehydrogenase activity was absent from extracts of glycollate-grown organisms. erythro-beta-Hydroxyaspartate aldolase was assayed by following pyruvate production from erythro-beta-hydroxy-beta-methyl-aspartate; the dehydratase, however, only acted on erythro-L-beta-hydroxyaspartate. The variation with pH of the Vmax and Km of the dehydratase suggested both that the substrate is bound in its most anionic form, and that the breakdown of the enzyme-substrate complex into products is governed by a dissociable group of pK'a about 8.5. The three enzymes of the pathway appeared to be co-ordinately induced and repressed. Succinate and malate, but not pyruvate or glucose, acted (perhaps indirectly) as repressors, and in addition glyoxylate appeared to have a direct inductive effect. Preliminary experiments suggested the absence of further control by inhibition or activation of individual enzymes in the pathway.