17.1.4: Central Aspect of Generating Pyruvate in Energy Metabolism - Glycolysis up close

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Glycolysis Transforms Glucose to Pyruvate

The first step in forming ATP from glucose is glycolysis (glyco=glucose,lysis=break apart), the universal pathway for glucose oxidation. Glucose metabolism is an essential energy source for all cells and particularly the brain and red blood cells. As you follow the metabolic pathway illustrated in figure 9.1.4.1, track the carbons through the process from beginning to end.

Figure 9.1.4.1. Glycolysis is a 10-step process that takes place in the cytoplasm of the cell; it converts glucose to pyruvate. Notice that glucose is depicted here as a row of six carbons, rather than a six-sided ring structure as this representation makes it easier to see the movement of carbon during metabolism.¹

Glycolysis is a 10-step anaerobic catabolic pathway that takes place in the cytoplasm of the cells. It begins with one six-carbon glucose molecule and ends with two three-carbon molecules of pyruvate and a net of two molecules of ATP.²

The initial step of glycolysis requires the use of ATP; in this reaction, one phosphate molecule is transferred from ATP to the sixth carbon of the glucose molecules, which generates glucose 6-phosphate and ADP. Once glucose 6-phosphate is formed, it continues through nine more reactions until the anaerobic stage of glycolysis is complete and pyruvate is created (see Figure 9.1.4.1).

After completing the ten steps, glycolysis produces 2 molecules of ATP and 2 molecules of pyruvate; additionally, glycolysis generates hydrogen ions (H+). As you can see in Figure 9.1.4.1, step 6, the coenzyme NAD+ picks up a hydrogen ion and reduces NAD+ to NADH. The NADH eventually carries the hydrogen ion to the final stage of energy production, the electron transport chain.

Notice that the coenzyme form of riboflavin, FAD, plays no role in glycolysis. FAD is reduced to FADH2 in the Krebs Cycle. Like NADH, FADH2 carries its hydrogen ion to the electron transport chain. There, the hydrogen ions are used to produce ATP from ADP.²

Metabolism of Fructose and Galactose

In addition to glucose, other monosaccharides, including fructose and galactose, can be used to produce ATP, but they are converted to substrates in glycolysis at different points. In muscle, fructose is first phosphorylated before it enters glycolysis as fructose 6-phosphate (see step 3 in Figure 9.1.4.1). In the liver, however, fructose passes through a more complex conversion before it becomes a substrate in glycolysis. It is first phosphorylated to form fructose 1-phosphate, which is then split into glyceraldehyde and dihydroxyacetone phosphate (DHAP), and then enters glycolysis as glyceraldehyde 3-phosphate (see steps 4 and 5 in Figure 9.1.4.1).¹

Galactose is also metabolized in the liver and must be converted to glucose before it can enter glycolysis. The first step is to add a phosphate to galactose to produce galactose 1-phosphate. After four more metabolic steps, galactose enters glycolysis as glucose 6-phosphate (see step 1 in Figure 9.1.4.1).¹

A specific enzyme directs each step of converting each monosaccharide into an intermediate substrate in glycolysis. Although rare, deficiencies in any of these enzymes due to genetic mutations have a debilitating effect on metabolism. For example, a deficiency in the enzyme that converts galactose into glucose results in a condition known as galactosemia, which can lead to mental retardation and liver damage unless dietary intake of galactose is controlled.¹

Amino Acids and Glycerol Can Yield Pyruvate

Pyruvate sits at the junction between several significant pathways in energy metabolism. Pyruvate is the end product for anaerobic glycolysis and the beginning molecule for gluconeogenesis. It can be converted to acetyl CoA and enter the Krebs cycle or provide the basis for long-chain fatty acids to be stored in the adipose tissue as a triglyceride. In other words, pyruvate serves as an intermediate substrate that helps balance the energy needs of the cells.¹

Video Animation

If the above was 'too techy' for you, please watch this short video:

Check your knowledge:

True or False: In the cytoplasm of the cell, glycolysis is a 10-step anaerobic catabolic pathway that begins with one six-carbon glucose molecule and ends with two three-carbon molecules of pyruvate and a net of two molecules of ATP.

True or False: Pyruvate is a three-carbon molecule formed from the oxidation of glucose during glycolysis.

Contributors

Claudia Kelley

References & Links

Blake, J. S., Munoz, K. D., & Volpe, S. (2019). Nutrition: From Science to You (4th ed.). Pearson.
Gropper SS, Smith JL, Groff JL. (2016) Advanced nutrition and human metabolism (7th ed.). Cengage.