6.2: Glycolysis, a Key Pathway In Energy Flow through Life

One of the properties of life is that living things require energy. The pathways of energy flow through life are shown in Figure 6.1.

Figure 6.1: Three paths (top, middle and bottom, respectively) follow of the flow of free energy through life, from visible light to chemical energy (e.g., photosynthesis of glucose) to the high-energy intermediates like ATP (fermentation or respiration), and finally to the work of a cell (growth and metabolism).

To begin with, recall that the most common intracellular energy currency with which livings things “pay” for cellular work is ATP. The energy to make ATP on planet Earth ultimately comes from the sun via photosynthesis. Recall that light energy fuels the formation of glucose and \(O_2\) from \(CO_2\) and water in green plants, algae, cyanobacteria, and a few other bacteria. Photosynthesis even produces some ATP directly, but not enough to fuel all cellular and organismic growth and metabolism. In fact, all cells, even plant cells, use fermentation and/or respiration (anaerobic or aerobic processes, respectively) to capture nutrient free energy (mostly) as ATP.

ATP is called a high-energy intermediate because its hydrolysis releases a large amount to free energy. In the condensation reactions that make ATP, it takes about 7.3 Kcal of free energy to link a phosphate to ADP in a phosphate ester linkage.

Having captured nutrient free energy in a form that cells can use, ATP hydrolysis then releases that free energy to fuel cellular work. Cellular work includes bending cilia, whipping flagella, contracting muscles, transmitting neural information, building polymers from monomers, and more. The free energy needed to make ATP in animal cells comes exclusively from nutrients (sugars, fats, and proteins). As noted, plants get free energy directly from sunlight, but they mobilize nutrient free energy, which they make in much the same way as the rest of us get it from what we eat! The energetics of ATP hydrolysis and synthesis are summarized in Figure 6.2.

Figure 6.2: For cells, the flow of free energy involves hydrolyzing ATP (the main chemical energy currency of life on earth) to power the cellular work (upper chemical equation) and harnessing free energy from the environment to make more of it (lower chemical equation)!

In all living things, glucose oxidation releases a considerable amount of free energy— enough to synthesize many molecules of ATP (Figure 6.3).

Figure 6.3: The exergonic oxidation of one glucose molecule by glycolysis, the Krebs cycle, electron transport, and oxidative phosphorylation (upper reaction) is coupled to the synthesis of up to 36 molecules of ATP (lower reaction).

Cellular respiration—the oxidation of glucose—starts with glycolysis. Derived from the Greek glykos-(“sweet”) + -lysis (“loosening”), glycolysis is the breakdown of sugar. Otto Meyerhof and Archibald V. Hill shared a Nobel Prize in Physiology or Medicine in 1923 for isolating enzymes of glucose metabolism from muscle cells. Thanks to their efforts and the efforts of others (e.g., Gustav Embden, Otto Warburg, Gerty Cori, and Carl Cori), all the enzymes and reactions of the glycolytic pathway were known by 1940, when the pathway became known as the Embden-Meyerhof pathway. As we will see, glycolysis is an evolutionarily conserved biochemical fermentation used by all organisms to capture a small amount of nutrient free energy. Check out Fothergill-Gilmore LA [(1986) The evolution of the glycolytic pathway. Trends Biochem. Sci. 11:47-51] for more detail. The glycolytic pathway occurs in the cytosol of cells, where it breaks down each molecule of glucose (\(C_6H_{12}O_6\)) into two molecules of pyruvic acid (pyruvate: \(CH_3COCOOH\)). This occurs in two stages, capturing nutrient free energy in two ATP molecules per glucose molecule that enters the pathway. Figure 6.4 (below) gives an overview of glycolysis, highlighting its two stages.

Stage 1 of glycolysis consumes ATP. Phosphates are transferred first from ATP to glucose and then to fructose-6-phosphate, reactions catalyzed by hexokinase and phosphofructokinase, respectively. So, these Stage-1 phosphorylations consume free energy. Later, in Stage 2 of glycolysis, nutrient free energy is captured in ATP and NADH (reduced nicotinamide adenine dinucleotide). NADH forms in redox reactions, in which \(\rm NAD^{+}\) is reduced as some metabolite is oxidized. In Stage 2, it is glyceraldehyde-3-phosphate that is oxidized—but more on that later! To summarize, by the end of glycolysis, a single starting glucose molecule has been split into two molecules of pyruvate while four molecules of ATP and two molecules of NADH have been produced.

Figure 6.4: In this overview of glycolysis, a 6C glucose splits into 3C pyruvates. Two ATPs are consumed in Stage 1. The oxidation of glyceraldehyde-3-P marks the start of Stage 2, in which 4 ATPs are made.

Pyruvate will be metabolized either anaerobically or aerobically. The alternate fates of pyruvate are summarized in Figure 6.5.

Figure 6.5: Alternate Fates of Pyruvate are on the one hand, fermentation (the anaerobic reduction to alcohol, lactate, or other end-products) or on the other, respiration (the aerobic oxidation of pyruvate, using oxygen as a final electron acceptor, resulting in \(H_2O\) and \(CO_2\)production).

151 Overview of Glycolysis

Overall, anaerobic (complete) glycolysis is an exergonic fermentation. Two NADH molecules are made per glucose, and two will reduce pyruvate at the end of the fermentation pathway. Thus, by the end of complete glycolysis, there is no consumption of \(O_2\) and no net oxidation of nutrient (i.e., glucose).

A familiar anaerobic glycolytic pathway is the production of alcohol by yeast in the absence of oxygen. Another is the production of lactic acid by skeletal muscle during strenuous exercise, which leads to the muscle fatigue you might have experienced after an especially vigorous workout. Muscle fatigue is due to a buildup of lactic acid in the muscle cells, which under these conditions can’t oxidize pyruvate and instead reduces it to lactate. Other cell types produce different fermentative end products, while still capturing free energy in two ATPs per starting glucose.

We will also consider gluconeogenesis, a pathway that essentially reverses the glycolysis and results in glucose synthesis. Gluconeogenesis may occur under normal conditions as well as during high-protein/low-carb diets and during fasting or starvation. Next, we learn that cellular respiration—the aerobic oxidation of pyruvate after incomplete glycolysis—takes place in the mitochondria of eukaryotic cells. We’ll see the role of the Krebs cycle (also called the TCA, or Tri-Carboxylic Acid cycle) in the complete oxidation of pyruvate and why the oxidation takes a cycle. We will take a moment to look at the experiments of Hans Krebs that revealed this cycle that starts a respiratory pathway that oxidizes glucose to\(CO_2\) and \(H_2O\), leaving no carbohydrates behind. As we look at the reactions of glycolysis and the Krebs cycle, watch for redox reactions in both pathways.