The breakdown of glucose for energy involves a metabolic pathway in the cytoplasm known as glycolysis. This term is derived from the Greek glykys = sweet and lysis = a loosening, and it refers to the cleavage of sugar. Glycolysis is the metabolic pathway by which glucose—a six-carbon (hexose) sugar (see fig. 2.13)—is converted into two molecules of pyruvic acid, or pyruvate. Even though each pyruvic acid molecule is roughly half the size of a glucose, glycolysis is not simply the breaking in half of glucose. Glycolysis is a metabolic pathway involving many enzymatically controlled steps.
Each pyruvic acid molecule contains three carbons, three oxygens, and four hydrogens (see fig. 5.3). The number of carbon and oxygen atoms in one molecule of glucose—C6H12O6—can thus be accounted for in the two pyruvic acid molecules. Since the two pyruvic acids together account for only eight hydrogens, however, it is clear that four hydrogen atoms are removed from the intermediates in glycolysis. Each pair of these hydrogen atoms is used to reduce a molecule of NAD. In this process, each pair of hydrogen atoms donates two electrons to NAD, thus reducing it. The reduced NAD binds one proton from the hydrogen atoms, leaving one proton unbound as H+ (chapter 4, fig. 4.17). Starting from one glucose molecule, therefore, glycolysis results in the production of two molecules of NADH and two H+. The H+ will follow the NADH in subsequent reactions, so for simplicity we can refer to reduced NAD simply as NADH.
Glycolysis is exergonic, and a portion of the energy that is released is used to drive the endergonic reaction ADP + Pi ^ ATP. At the end of the glycolytic pathway, there is a net gain of two ATP molecules per glucose molecule, as indicated in the overall equation for glycolysis:
Glucose + 2 NAD + 2 ADP + 2 Pi ^ 2 pyruvic acid + 2 NADH + 2 ATP
Although the overall equation for glycolysis is exergonic, glucose must be "activated" at the beginning of the pathway before energy can be obtained. This activation requires the addition
Cell Respiration and Metabolism
Figure 5.1 The energy expenditure and gain in glycolysis. Notice that there is a "net profit" of two ATP and two NADH for every molecule of glucose that enters the glycolytic pathway. Molecules listed by number are (1) fructose 1,6-biphosphate, (2) 1,3-biphosphoglyceric acid, and (3) 3-phosphoglyceric acid (see fig. 5.2).
of two phosphate groups derived from two molecules of ATP. Energy from the reaction ATP ^ ADP + Pi is therefore consumed at the beginning of glycolysis. This is shown as an "up-staircase" in figure 5.1. Notice that the Pj is not shown in these reactions in figure 5.1; this is because the phosphate is not released, but instead is added to the intermediate molecules of glycolysis. The addition of a phosphate group is known as phosphorylation. Besides being essential for glycolysis, the phosphorylation of glucose (to glucose 6-phosphate) has an important side benefit: it traps the glucose within the cell. This is because phosphorylated organic molecules cannot cross cell membranes.
At later steps in glycolysis, four molecules of ATP are produced (and two molecules of NAD are reduced) as energy is liberated (the "down-staircase" in fig. 5.1). The two molecules of ATP used in the beginning, therefore, represent an energy investment; the net gain of two ATP and two NADH molecules by the end of the pathway represents an energy profit. The overall equation for glycolysis obscures the fact that this is a metabolic pathway consisting of nine separate steps. The individual steps in this pathway are shown in figure 5.2.
In figure 5.2, glucose is phosphorylated to glucose 6-phos-phate using ATP at step 1, and then is converted into its isomer, fructose 6-phosphate, in step 2. Another ATP is used to form fructose 1, 6-biphosphate at step 3. Notice that the six-carbon-long molecule is split into two separate three-carbon-long molecules at step 4. At step 5, two pairs of hydrogens are removed and used to reduce two NAD to two NADH + H+. These reduced coenzymes are important products of glycolysis. Then, at step 6, a phosphate group is removed from each 1,3-biphosphoglyceric acid, forming two ATP and two molecules of 3-phosphoglyceric acid. Steps 7 and 8 are isomerizations. Then, at step 9, the last phosphate group is removed from each intermediate; this forms another two ATP (for a net gain of two ATP), and two molecules of pyruvic acid.
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