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J Bacteriol. 1971 May; 106(2): 456-467
Copyright © 1971 American Society for Microbiology. All Rights Reserved.

Mechanism for Regulating the Distribution of Glucose Carbon Between the Embden-Meyerhof and Hexose-Monophosphate Pathways in Streptococcus faecalis

^a Microbial Physiology Section, Laboratory of Microbiology, National Institute of Dental Research, Bethesda, Maryland 20014

ABSTRACT

Glucose-adapted Streptococcus faecalis produced little if any ¹⁴CO₂ from glucose-1-¹⁴C, although high levels of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) were detected in cell-free extracts. Metabolism of glucose through the oxidative portion of the hexose-monophosphate pathway was shown to be regulated in this organism by the specific inhibitory interaction of the Embden-Meyerhof intermediate, fructose-1, 6-diphosphate (FDP), with 6-phosphogluconate dehydrogenase. Glucose-6-phosphate dehydrogenase activity was unaffected by FDP. The S. faecalis 6-phosphogluconate dehydrogenase was partially purified from crude extracts by standard fractionation procedures and certain kinetic parameters of the FDP-mediated inhibition were investigated. The negative effector was shown to cause a decrease in V_max and an increase in the apparent K_m for both 6-phosphogluconate and nicotinamide adenine dinucleotide phosphate (NADP). These effects were apparently a consequence of the ligand interacting with the enzyme at a site distinct from either the substrate or the coenzyme sites. Among the evidence supporting this was the fact that ß-mercaptoethanol blocked completely FDP inhibition, but had no effect on catalytic activity. The possibility that the regulation of 6-phosphogluconate dehydrogenase activity by FDP might be of some general significance was suggested by the observation that this enzyme from several other sources was also sensitive to FDP.

¹ Present address: Department of Oral Biology, School of Dental Medicine, University of Connecticut Health Center, Farmington, Conn. 06105.

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