Glycolysis

• Class I Reactions: Generation of Precursor Metabolites and Energy
SECTION A1 • CENTRAL METABOLISM

Glycolysis
DAN G. FRAENKEL

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INTRODUCTION
The Embden-Meyerhof and pentose phosphate (pentose-P) pathways are the two trunk routes of intermediary sugar metabolism in enteric bacteria (Fig. 1). A third route, the Entner-Doudoroff pathway, is also mentioned, as are some of the connections(Fig. 2) with carboxylic acid pathways. The text mainly considers function, and there is a final section on general problems. Table 1 lists some key or recent papers on the genetics, enzymology, and molecular biology of the individual reactions.

EMBDEN-MEYERHOF PATHWAY
Glucokinase and Phosphoglucomutase Although the phosphoenolpyruvate (PEP) phosphotransferase system (see chapter 75) is theusual route of glucose uptake and phosphorylation, enteric bacteria also contain a glucokinase ( glk). The normal role of this enzyme is unclear; it might act on glucose generated internally (e.g., from lactose). The slow growth on glucose of mutants defective in phosphotransferase components depends on glk, since its further loss then causes almost complete inability to grow on glucose (57, 83).The slow growth on glucose of phosphofructokinase (pfkA) mutants also depends on glucokinase (174). In a wild-type background, glk mutants grow almost normally on glucose (57). Catabolism of galactose and maltose involves formation of glucose 1-phosphate (glucose-1-P), which is converted to glucose-6-P by phosphoglucomutase (pgm). In growth on other carbon sources, the same reaction is usedbiosynthetically to make glucose-1-P (and hence UDP-glucose). pgm mutants are impaired on galactose and stain blue with iodine (3, 142). Phosphoglucose Isomerase and Phosphomannose Isomerase The interconversion of glucose-6-P and fructose-6-P is catalyzed by phosphoglucose isomerase. Mutants (pgi) grow slowly on glucose using the pentose-P pathway (Fig. 1), with oxidative formation of pentose-P fromglucose-6-P and with nonoxidative reactions yielding fructose-6-P and glyceraldehyde-3-P (86). Phosphoglucose isomerase is needed for synthesis of glucose-6-P in growth on substances that do not provide it by catabolism (glycerol, gluconate, etc.), but growth is adequate because glucose-6-P and derivatives are apparently not essential, as shown by lipopolysaccharide analysis (87), DNA glucosylation(111), or acid hydrolyzable glucose content (207). Normal location of some phosphoglucose isomerase to the periplasm was reported (91). Phosphomannose isomerase (manA, pmi), like phosphoglucomutase, is used in both sugar catabolism (growth on mannose) and polysaccharide synthesis. Mutants lacking the isomerase are blocked in growth on mannose and are defective in mannose-containing polysaccharides(144, 175).

.1/74-  Glycolysis. Where applicable, all compounds are D-configuration. Gene symbols are those used (or, in parentheses, suggested) for E. coli. The trunk route is indicated by heavy arrows. Other reactions may be peripheral, inducible, gluconeogenic, etc. (see the text). Phosphofructokinase (Fructose-6-P 1-Kinase) and Fructose-1,6-Bisphosphatase The main phosphofructokinase ofEscherichia coli, Pfk-1, is an allosteric enzyme activated by nucleoside diphosphates and inhibited by PEP (26), and it has been a major subject of structural and kinetic studies. pfkA mutants are impaired in their growth on substances catabolized via fructose-6-P (174, 206). E. coli K-12 also has a minor phosphofructokinase activity (ca. 10% of the total), Pfk-2, encoded by pfkB. This activity isneeded for the residual growth of pfkA mutants but has unknown function in pfkA strains. The pfkB1 mutation, which suppresses the glucose negativity of a pfkA mutant, increases the expression of Pfk-2 by ca. 25-fold (8), and is a point mutation in the promoter (58). Originally thought to be nonallosteric, Pfk-2 was later shown (unlike Pfk-1) to be inhibited by ATP (133). Strains with high levels...