The initial sequence of reactions, in which a molecule of glucose is converted to two molecules of pyruvate , is called glycolysis (Figures 6.16 and 6.17). In the first phase of glycolysis, glucose is phosphorylated and its six-carbon ring structure rearranged, before being cleaved into two three-carbon molecules. In the second phase, each of these undergoes oxidation, resulting in pyruvate.
Also known as the Embden-Meyerhof pathway, glycolysis is used for the metabolism of simple sugars not just by microorganisms, but by most living cells. The pathway, which takes place in the cytoplasm, comprises a series of 10 linked reactions, in which each molecule of the six-carbon glucose is converted to two molecules of the three-carbon pyruvate, with a net gain of two molecules of ATP.
The full pathway of reactions is shown in Figure 6.17. Note how, in terms of energy, glycolysis can be divided conveniently into a 'sowing' phase, in which two molecules of ATP are expended per molecule of glucose, followed by a 'reaping' phase which yields four molecules of ATP. The overall energy balance is therefore a gain of two molecules of ATP for each molecule of glucose oxidised to pyruvate. In addition, the second phase features the conversion of two molecules of NAD+ to NADH, which, as we will see, act as an important source of reducing power in subsequent pathways.
The reactions by which ATP is generated from ADP in the second phase of glycolysis are examples of substrate-level phosphorylation, so-called because the phosphate group is transferred directly from a donor molecule.
What happens next to the pyruvate produced by glycolysis depends on the organism concerned, and on whether the environment is aerobic or anaerobic; we shall look at these possibilities in due course.
Glycolysis is not the only way to metabolise glucose
Although glycolysis is widespread in both the microbial and nonmicrobial worlds, several bacterial types use alternative pathways to oxidise glucose. For certain Gramnegative groups, notably the pseudomonads (see Chapter 7), the main route used is the Entner-Doudoroff pathway, producing a mixture of pyruvate and glyceraldehyde-3-phosphate (Figure 6.18). The former, like that produced in glycolysis, can enter a number of pathways, while the latter can feed into the later stages of glycolysis. The net result of catabolism by the Entner-Doudoroff pathway is the production of one molecule each of ATP, NADH and NADPH per molecule of glucose degraded.
A secondary pathway, which may operate in tandem with glycolysis or the Entner-Doudoroff pathway, is the pentose phosphate pathway, sometimes known as the hexose monophosphate shunt (Figure 6.19). Like glycolysis, the pathway can operate in the presence or absence of oxygen. Although glyceraldehyde-3-phosphate can once again enter the glycolytic pathway and lead to ATP generation, for most organisms the pathway has a mainly anabolic (biosynthetic) function, acting as a source of precursor molecules for other metabolic pathways. The pentose phosphate pathway is a useful
At physiological pH, carboxylic acids such as pyruvic acid and citric acid are found in their ionised form (pyruvate, citrate); however long-established traditions persist, and you may well find reference elsewhere to the '-ic acid' forms.
Glucose hexokinase V ATP
Glucose-phosphate phosphohexose isomerase
Fructose phosphate -ATP
Glyceraldehyde 3-phosphate triose phosphate isomerase
triose phosphate isomerase glyceraldehyde 3-phosphate dehydrogenase
1,3-Bisphosphoglycerate ADP ^
3-Phosphoglycerate phosphoglycerate mutase x 2
Figure 6.17 Glycolysis: a more detailed look. Glycolysis comprises 10 separate enzyme-catalysed reactions. Of the two 3-carbon compounds formed in the first stage, dihydroxyace-tone phosphate cannot directly enter the later part of the pathway, but must first be converted to its isomer glyceraldehyde-3-phosphate. For each molecule of glucose, two molecules of each compound are therefore produced from this point onwards, and the yield of ATP and NADH is likewise doubled
Glucose 6-phosphate A_NADP+
Steps 6-10 of glycolysis (x 2)
Figure 6.18 The Entner-Doudoroff pathway: an alternative way to metabolise glucose. The products of the pathway are pyruvate and glyceraldeyde-3-phosphate (G3-P). There is a loss of one molecule of ATP in the opening reaction, however when the G3-P joins the later stages of glycolysis, two ATPs are generated, giving a net balance of +1. In addition, the pathway yields one molecule each of NADH and NADPH. The intermediate compound 6-phosphogluconate can enter the pentose phosphate pathway and be decarboxylated to the 5-carbon compound ribulose-5-phosphate (see Figure 6.19)
source of reducing power in the form of NADPH. In addition it acts as an important source of precursors in the synthesis of essential molecules; ribose-5-phosphate is an important precursor in the synthesis of nucleotides, while the four-carbon erythrose 4-phosphate is required for the synthesis of certain amino acids and ribulose-5-phosphate is an intermediate in the Calvin cycle of carbon fixation (see later in this chapter).
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