The DeLey-Doudoroff Pathway of Galactose Metabolism in Azotobacter vinelandii

Abstract

Azotobacter vinelandii cell extracts reduced NAD + and oxidized d -galactose to galactonate that subsequently was converted to 2-keto-3-deoxy-galactonate. Further metabolism of 2-keto-3-deoxy-galactonate required the presence of ATP and resulted in the formation of pyruvate and glyceraldehyde 3-P. Radiorespirometry indicated a preferential release of CO 2 at the first carbon position of the d -galactose molecule. This suggested that Azotobacter vinelandii metabolizes d -galactose via the DeLey-Doudoroff pathway. The first enzyme of this pathway, d -galactose dehydrogenase, was partially characterized. It has a molecular weight of about 74,000 Da and an isoelectric point of 6.15. The pH optimum of the galactose dehydrogenase was about 9. The apparent K m s for NAD + and d -galactose were 0.125 and 0.56 mM, respectively. Besides d -galactose, the active fraction of this galactose dehydrogenase also oxidized l -arabinose effectively. The electron acceptor for d -galactose or l -arabinose oxidation, NAD + , could not be replaced by NADP + . These substrate specificities were different from those reported in Pseudomonas saccharophila, Pseudomonas fluorescens, and Rhizobium meliloti.