PATHWAY OF GLYCOGEN-METABOLISM IN METHANOCOCCUS-MARIPALUDIS

Methanococcus maripaludis, a facultatively autotrophic archaebacterium that grows with H, or formate as the electron donor, does not assimil ate sugars and other complex organic substrates. However, glycogen is biosynthesized intracellularly and commonly reaches values of 0.34% of the cellular dry weight in the early stationary phase. To determine t he pathway of glycogen catabolism, specific enzymes of sugar metabolis m were assayed in cell extracts. The following enzymes were found (spe cific activity in milliunits per milligram of protein): glycogen phosp horylase, 4.4; phosphoglucomutase, 10; glucose-6-phosphate isomerase, 9; 6-phosphofructokinase, 5.6, fructose-1,6-bisphosphatase, 10; fructo se-1,6-bisphosphate aldolase, 4.2; triosephosphate isomerase, 44; glyc eraldehyde-3-phosphate dehydrogenase, 26; phosphoglycerate kinase, 20; phosphoglycerate mutase, 78; enolase, 107; and pyruvate kinase, 4.0. Glyceraldehyde-3-phosphate dehydrogenase was NADP+ dependent, and the pyruvate kinase required MnCl2. The 6-phosphofructokinase had an unusu ally low pH optimum of 6.0. Four nonoxidative pentose-biosynthetic enz ymes were found (specific activity in milliunits per milligram of prot ein): transketolase, 12; transaldolase, 24; ribulose-5-phosphate-3-epi merase, 55; and ribulose-5-phosphate isomerase, 100. However, the key enzymes of the oxidative pentose phosphate pathway, the reductive pent ose phosphate pathway, and the classical and modified Entner-Doudoroff pathways were not detected. Thus, glycogen appears to be catabolized by the Embden-Meyerhoff-Parnas pathway. This result is in striking con trast to the nonmethanogenic archaebacteria that have been examined, a mong which the Entner-Doudoroff pathway is common. A dithiothreitol-sp ecific NADP+-reducing activity was also found (8.5 mU/mg of protein). Other thiol compounds, such as cysteine hydrochloride, reduced glutath ione, and 2-mercaptoethanesulfonic acid, did not replace dithiothreito l for this activity. The physiological significance of this activity i s not known.