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4

Glycolysis
Escherichia coli can grow on a simple medium containing glucose and
mineral salts and this bacterium can synthesize all cell constituents
using materials provided in this medium. Glucose is metabolized
through the Embden–Meyerhof–Parnas (EMP) pathway and hexose
monophosphate (HMP) pathway and the metabolic product, pyruvate, is decarboxylated oxidatively to acetyl-CoA to beoxidized
through the tricarboxylic acid (TCA) cycle. Twelve intermediates of
these pathways are used as carbon skeletons for biosynthesis
(Table 4.1). Heterotrophs that utilize organic compounds other than
carbohydrates convert their substrates into one or more of these
intermediates. For this reason, glucose metabolism through glycolysis and the TCA cycle is called central metabolism.Eukaryotes metabolize glucose through the EMP pathway to generate ATP, pyruvate and NADH, and the HMP pathway is needed to
supply the metabolic intermediates not available from the EMP pathway such as pentose-5-phosphate and erythrose-4-phosphate, and
NADPH. Most prokaryotes employ similar mechanisms, but some
prokaryotes metabolize glucose through unique pathways known
only in prokaryotes, e.g. theEntner–Doudoroff (ED) pathway and
phosphoketolase (PK) pathway. Some prokaryotes have genes
for the ED pathway in addition to the EMP pathway: genes for
these pathways are expressed at the same time in several prokaryotes
including a thermophilic bacterium (Thermotoga maritima), a thermophilic archaeon (Thermoproteus tenax) and a halophilic archaeon
(Halococcus saccharolyticus). Escherichiacoli metabolizes glucose via the
EMP pathway, but gluconate is oxidized through the ED pathway.
Modified EMP and ED pathways are quite common in archaea.
Carbohydrates are phosphorylated before they are metabolized in
most cases. It is believed that phosphorylated intermediates are less
likely to diffuse away through the cytoplasmic membrane. Some
bacteria and archaea also phosphorylateintermediates of glucose
metabolism in modified glycolytic pathways.
This chapter describes glucose oxidation to pyruvate and related
metabolic pathways. Pyruvate metabolism will be further discussed
in Chapters 5, 8 and 9.

4.1 EMP PATHWAY

Table 4.1. Metabolic intermediates used as carbon skeletons for
biosynthesis

Carbon skeleton

From

Precursor for

Glucose-6-phosphateFructose-6-phosphate
Ribose-5-phosphate
Erythrose-4-phosphate
Triose-phosphate
3-phosphoglycerate
Phosphoenolpyruvate
Pyruvate
Acetyl-CoA
2-ketoglutarate
Succinyl-CoA
Oxaloacetate

EMP
EMP
HMP
HMP
EMP
EMP
EMP
EMP
Pyruvate
TCA
TCA
TCA

polysaccharides
murein
nucleic acids
amino acids
lipids
amino acids
amino acids
amino acids
fatty acids
amino acids
amino acids
aminoacids

4.1 EMP pathway
Many anaerobic and enteric bacteria transport glucose via group
translocation (phosphotransferase system, PTS, Section 3.5) in the
form of glucose-6-phosphate. Glucose transported through active
transport is phosphorylated by hexokinase:
hexokinase
glucose + ATP

glucose-6-phosphate + ADP

Hexokinase can phosphorylate other hexoses such as mannose, and
requiresMg2þ for activity. The enzyme cannot catalyze the reverse
reaction.
Glucose-6-phosphate can also be obtained from glycogen:
phosphorylase
[glucose]n + Pi
(glycogen)

[glucose]n–1 + glucose-1-phosphate
(glycogen)
phosphoglucomutase

glucose-1-phosphate

glucose-6-phosphate

Glucose-6-phosphate is a precursor for the biosynthesis of polysaccharides as well as a substrate of the EMPpathway (Figure 4.1), which
is the commonest glycolytic pathway in all kinds of organisms.

4.1.1 Phosphofructokinase (PFK): key enzyme of the
EMP pathway
Glucose-6-phosphate is isomerized to fructose-6-phosphate before
being phosphorylated to fructose-1,6-diphosphate by the action of
phosphofructokinase (PFK). These two reactions require Mg2þ.

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GLYCOLYSIS

diphosphate

Pi...