THE METABOLISM OF 2-KETO-D-GLUCONATE BY CELL-FREE EXTRACTS OF LEUCONOSTOC MESENTEROIDES

Abstract

The degradation of 2-keto-D-gluconate by Leuconostoc mesenteroides is shown to proceed according to the following pathway: 2-keto-D-gluconate → 2-keto-6-phospho-D-gluconate → 6-phospho-D-gluconate → D-ribulose-5-phosphate + CO₂ → D-xylulose-5-phosphate → acetyl-phosphate + D-glyceraldehyde-3-phosphate.Cells grown on 2-keto-D-gluconate were shown previously to possess an adaptive and specific 2-ketogluconokinase. These cells also contained a reductase that reduced 2-keto-6-phospho-D-gluconate to 6-phospho-D-gluconate in the presence of DPNH or TPNH, 6-phospho-D-gluconate-dehydrogenase, phosphoketolase, D-ribulose-5-phosphate-3-epimerase, and acetokinase. The 2-keto-6-phospho-D-gluconate-reductase which was also present in cells grown on D-gluconate shows optimal activity between pH 4.5 and 6.5 and is rapidly inactivated by heat. The 6-phospho-D-gluconate-dehydrogenase specific for DPN has an optimum pH between 7.2 and 7.7 and is stable when heated to 50 °C for 5 minutes.The production of carbon dioxide or pentulose-phosphate with extracts from cells grown on 2-keto-D-gluconate proceeded more rapidly with 2-keto-6-phospho-D-gluconate as the substrate than with 6-phospho-D-gluconate. This difference in rates was eliminated if a system to recycle hydrogen such as alcohol dehydrogenase and acetaldehyde or pyruvate to couple with lactic acid dehydrogenase was provided. Thus the existence of an alternate pathway for the catabolism of 2-keto-6-phospho-D-gluconate to carbon dioxide, acetic acid, and lactic acid does not appear to exist.