What is Glycolysis? Where Glycolysis Takes Place, Definition, and Steps of the Glycolysis Pathway.

• Learn about the pathways of glucose breakdown and oxidation
• Find out how glucose metabolism contributes to cell energy production
• Learn how glucose metabolism is associated with the aging process
• Discover how glucose metabolism can be supported

What Is Glycolysis?

Glycolysis is the metabolic pathway that breaks down the carbohydrate glucose to produce cell energy in the form of ATP. Glycolysis generates ATP directly, as a product of the pathway’s chemical reactions, and indirectly, using energy generated by electrons extracted from the chemical bonds of glucose. These electrons are carried by the molecule NAD to the mitochondrial electron transport chain, where they are used to power the production of ATP through oxidative phosphorylation (OXPHOS).

In this article, we will look into these processes in more detail. But before we do so, it’s important to review some concepts that will help us understand what glycolysis is and the part it plays in cellular metabolism.

Glycolysis Is A Pathway of Cellular Metabolism

Metabolism is the set of chemical reactions that sustain life. Among them are those that allow us to obtain energy from food and to use that energy to synthesize molecules needed for cellular activity. Metabolism is divided into two types of processes: 1) catabolism, the set of degradative pathways that break down large molecules into smaller molecules, releasing the energy stored in the chemical bonds; and 2) anabolism, the set of constructive biosynthesis pathways that build large molecules from smaller molecules and store cell energy in their chemical bonds. This means that the breakdown of food to obtain energy takes place through catabolic reactions, whereas the synthesis of the complex cellular molecules from simpler units takes place through anabolic reactions [1,2]. 

The energy contained in food is not readily available to be used by cells and must be converted through metabolic reactions into a form of energy cells can use. This form is a molecule called adenosine triphosphate (ATP), “the energy currency of the cell” common to all biological systems. ATP is used for all sorts of biological functions in all types of cells and tissues, including, for example, powering muscle contraction or sustaining neuronal activity. 

Metabolism is the sum of interconnected energy-requiring and energy-consuming processes that sustain life.

The set of metabolic reactions through which energy is extracted from nutrients to generate ATP is called cellular respiration. In the process of respiration, oxygen (O₂) is consumed and carbon dioxide (CO₂), water (H₂O), and heat are produced. Energy is extracted from nutrients through a type of chemical reaction called redox reactions (from reduction—the gain of electrons + oxidation—the loss of electrons). In redox reactions, electrons (the electrically charged particles that orbit the nuclei of atoms) are transferred from one molecule (which is oxidized) to another (which is reduced). Therefore, we say that, in cellular respiration, nutrients are oxidized to generate cell energy as ATP. 

Glucose, obtained from the breakdown of carbohydrates, and fatty acids, obtained from the breakdown of triglycerides (fats), are the main fuels cells use to generate ATP; amino acids obtained from protein breakdown are also used, but to a lesser extent. 

Cellular respiration occurs in three major stages. In the first, fuel molecules are oxidized to yield two-carbon fragments in the form of the acetyl group of acetyl-coenzyme A (acetyl-CoA). In the second stage, the acetyl groups are oxidized in mitochondria in the citric acid cycle, with electrons being transferred to the electron carriers nicotinamide adenine dinucleotide (NAD), which is vitamin B3-dependent, and flavin adenine dinucleotide (FAD), which is vitamin B2-dependent. In the third stage of respiration, electrons are transferred to oxygen via the electron transport chain (ETC) in mitochondria, with their energy being used to power the production of ATP by a process called oxidative phosphorylation (OXPHOS) [1,2]. 

Glycolysis is part of the first stage of cellular respiration: it is the pathway that oxidizes glucose. Glycolysis generates the molecule pyruvate, which is then converted into acetyl-CoA to be used in the second stage of cellular respiration.

Cellular respiration is like a very slow combustion reaction: it burns fuels to generate energy, consuming oxygen and releasing carbon dioxide and heat in the process.

Figure 1: Metabolic extraction and storage of energy from food. Source: OpenStax, Anatomy and Physiology; 24.1 Overview of Metabolic Reactions. License CC BY 4.0.

What Is Glucose? Why Is Glycolysis Important?

Glucose is a carbohydrate. Carbohydrates are a type of biomolecules made up of carbon, hydrogen, and oxygen that include sugars, starches, and cellulose. More specifically, glucose is a monosaccharide, the simplest form of sugar and the most basic type of carbohydrate [1,2].

We get dietary glucose from the breakdown of complex carbohydrates, such as starch, made up of several to many monosaccharide units joined together (i.e., they’re polysaccharides). We also get glucose from the breakdown of simpler dietary sugars such as disaccharides (made up of two monosaccharide units), which include sucrose or lactose, for example. Other monosaccharides, such as fructose or galactose, can also be used for energy production, but they must first be converted into other molecules that can be used in glucose metabolic pathways [1,2].

Glycolysis is important because it is the metabolic pathway through which glucose generates cellular energy. Glucose is the most important source of energy for all living organisms. In the human body, glucose is the preferred fuel for the vast majority of cells: it’s the only fuel red blood cells can use, the preferred fuel used by the brain under non-starvation conditions, and the main fuel used by muscles during strenuous exercise. 

Glucose is so important that our body has several mechanisms to ensure that blood sugar levels are kept relatively constant so the brain is always adequately supplied with glucose. After a meal, the rise in blood glucose increases the release of the pancreatic hormone insulin, which in turn stimulates glucose uptake by tissues, mainly the liver and skeletal muscle, and glucose storage in the form of glycogen. Between meals, a decrease in blood glucose levels increases the release of the pancreatic hormone glucagon, which in turn stimulates the breakdown of glycogen stores into glucose and its release into the blood. Glycogen stores are also mobilized when glucose is being used to support physical activity.

Glucose, which is a carbohydrate, is the most important source of energy for most cells. Its blood levels are kept relatively constant to ensure a steady supply to the brain.

Glucose Is Oxidized in Glycolysis to Produce ATP

Glycolysis takes place in the fluid matrix of cells (the cytosol) in a sequence of ten reactions divided into two stages. In the first stage, glucose (which has six carbons) is split into two three-carbon fragments in a process that actually consumes ATP to prepare glucose for degradation. In the second stage, each three-carbon fragment is oxidized to a molecule called pyruvate in a process that produces ATP. 

Electrons extracted in the oxidation reactions are transferred to NAD⁺, a redox molecule that carries electrons to the mitochondrial electron transport chain (ETC) to produce more ATP through oxidative phosphorylation (OXPHOS). 

Figure 2: Glycolysis. Source: OpenStax, Anatomy and Physiology; 24.2 Carbohydrate Metabolism.
License CC BY 4.0.

Glycolysis is the metabolic pathway that breaks down glucose to produce ATP.

The First Stage Of Glycolysis Uses ATP

Glucose can easily enter and exit cells through membrane transporters. However, a simple structural modification is enough to keep it inside cells to be metabolized: the addition of a chemical structure called phosphoryl group (a phosphorus atom with three oxygen atoms attached) in a reaction known as phosphorylation. Glycolysis starts by doing precisely that: it phosphorylates glucose and traps it inside cells. 

Glucose phosphorylation is carried out by an enzyme called hexokinase which takes a phosphoryl group from ATP and transfers it to glucose, producing glucose 6-phosphate (see step 1 in figure 3) [1,2]. Hexokinase belongs to a family of enzymes called kinases (which is the classification of enzymes that phosphorylate a substrate using a phosphoryl group from ATP or vice versa). All kinase enzymes require magnesium for their activity. Therefore, magnesium plays an important part in this reaction. In fact, magnesium has a key role in glycolysis in general because it’s a cofactor for all kinases that participate in this pathway [3]. One of the main reasons that Qualia Life (previously called Eternus) contains Magnesium is to support glycolysis reactions.

In the next steps of glycolysis, glucose 6-phosphate is converted into fructose 6-phosphate (step 2, figure 3), which in turn is phosphorylated again to yield fructose 1,6-bisphosphate (step 3, figure 3). This second phosphorylation is carried out by another kinase (phosphofructokinase) using another molecule of ATP and magnesium as cofactor. This six-carbon molecule is then cleaved into two three-carbon molecules (step 4, figure 3), which are different but interconvertible (step 5, figure 3); the glyceraldehyde-3-phosphate form is used in the second stage of glycolysis [1,2].

Figure 3: The first stage of glycolysis. Source: OpenStax, Biology; 7.2 Glycolysis.
License CC BY 4.0.

The first stage of glycolysis uses ATP to prepare glucose for degradation; it is an investment of ATP that will pay off.

The Second Stage Of Glycolysis Produces ATP

The first reaction of the second stage transforms glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (step 6, figure 4) [1,2]. This reaction includes two coupled processes: an oxidation and an addition of phosphate. The oxidation reaction extracts two electrons from the glyceraldehyde 3-phosphate, which are transferred to the redox molecule NAD⁺ (derived from vitamin B3), reducing it to the NADH form. In the next step, 3-phosphoglycerate is produced by another kinase (phosphoglycerate kinase, with magnesium as cofactor) with the concomitant production of ATP (step 7, figure 4). Qualia Life supports these reactions by supplying Magnesium [3] and Vitamin B3 in the form of Niacinamide and Nicotinic Acid [4]. 

In the next two steps, 3-phosphoglycerate is rearranged (step 8, figure 4) and then dehydrated (step 9, figure 3) to form phosphoenolpyruvate. In the final step of glycolysis, phosphoenolpyruvate is converted into pyruvate and another ATP molecule is produced pyruvate kinase using magnesium as cofactor (step 10, figure 4). 

In the second stage, two molecules of ATP are generated from each three-carbon unit, meaning that each glucose molecule yields four ATP molecules. Given that the first stage of glycolysis uses two molecules of ATP to prepare glucose for breakdown, the net outcome of glycolysis is the production of two ATP molecules per glucose molecule [1,2].

This mechanism of ATP production is called substrate-level phosphorylation. It uses the chemical energy released by the conversion of a higher energy substrate into a lower energy product to power the transfer of a phosphoryl group to produce the high energy molecule ATP. Substrate-level phosphorylation is a faster, but less efficient source of ATP. It also comes with a cost of using one NAD⁺, which becomes NADH. As we’ll see in part 4 of this series, most ATP is generated by oxidative phosphorylation, and the NAD⁺ will be recovered during that final stage of cellular respiration. 

Figure 4: The second half of glycolysis. Source: OpenStax, Biology; 7.2 Glycolysis.
License CC BY 4.0.

The second stage of glycolysis generates pyruvate, NADH and ATP; the net outcome of glycolysis is the production of two ATP molecules per glucose molecule but at a cost of one NAD⁺.

Pyruvate Produced in Glycolysis Yields Acetyl-CoA In Mitochondria

Glycolysis yields only a fraction of the ATP that can be produced from the complete oxidation of glucose. That’s because the pyruvate molecules produced in glycolysis can still be further oxidized. It’s in the following pathways of cell energy generation, the citric acid cycle and OXPHOS, that the vast majority of ATP production takes place. 

Whereas glycolysis takes place in the cytosol, the citric acid cycle and OXPHOS take place in mitochondria [1,2]. Therefore, pyruvate, the final product of glycolysis, is transported into mitochondria where it is converted into two-carbon fragments—acetyl units—and carbon dioxide (CO₂). In this reaction, carried out by a cluster of enzymes called pyruvate dehydrogenase complex, electrons extracted from pyruvate are transferred to NAD⁺, reducing it to NADH. Acetyl units are transferred to coenzyme A (CoA, derived from pantothenic acid i.e., vitamin B5) to form acetyl-CoA, the molecule that feeds two-carbon units to the citric acid cycle, where they will be further oxidized. The electrons extracted from each acetyl unit will then be used to generate ATP through OXPHOS.

Figure 5: Pyruvate oxidation. Source: OpenStax, Anatomy and Physiology; 24.2 Carbohydrate Metabolism.
License CC BY 4.0.

The mitochondrial conversion of pyruvate into acetyl-CoA is the link between glycolysis and the citric acid cycle. This is an important reaction that requires several cofactors: CoA (derived from pantothenic acid), NAD⁺ (synthesized from compounds with vitamin B3 activity or by using L-tryptophan as a substrate and Vitamin B6 as a cofactor), FAD⁺ (flavin adenine dinucleotide, derived from vitamin B2 i.e., riboflavin), the coenzyme thiamine pyrophosphate (derived from vitamin B1 i.e., thiamine), and lipoic acid  [1,2]. 

Qualia Life supports these reactions by supplying Niacinamide and Nicotinic acid (Vitamin B3) [4], Riboflavin (Vitamin B2) [5], Pantothenic Acid (Vitamin B5) [6], Thiamine HCl (Vitamin B1) [7], Pyridoxal-5’-Phosphate (Vitamin B6) [8], Lipoic Acid [9], L-Tryptophan [8], and Magnesium [3].

Pyruvate is converted into acetyl-CoA in mitochondria; this step links glycolysis to the citric acid cycle and oxidative phosphorylation. It comes with a cost of one NAD⁺ unit (which will later be recovered).

NADH Produced in Glycolysis Generates ATP In Mitochondria

NAD⁺ is a redox molecule that carries electrons to the mitochondrial electron transport chain (ETC) to produce ATP through OXPHOS. In the oxidation of each molecule of glucose to acetyl-CoA, four NAD⁺ molecules are used, each receiving two electrons and becoming the NADH form. Because NAD⁺ is a central element in ATP production, it is important that cells maintain a pool of NAD⁺ available to receive electrons [1,2]. 

Qualia Life contains a set of ingredients that support the upregulation of the NAD⁺ pool in cells. These include Grape Proanthocyanidins (in Grape extract, BioVin^®) [10], Resveratrol (in Grape extract, BioVin^®) [11–13], Coenzyme Q10 [14], and Lipoic acid [15,16].

NADH carries electrons to the mitochondrial electron transport chain to produce ATP through oxidative phosphorylation. NAD⁺ is regenerated in the process.

How Glucose Metabolism Impacts Aging

Glucose and other monosaccharides have the ability to react with amino groups of proteins, lipids, and nucleic acids to produce a structural modification called non-enzymatic glycation. These modified molecules are called advanced glycation end-products (AGEs) and they lose their function—they are damaged molecules. 

AGEs are usually degraded by cellular quality control mechanisms, but they can accumulate in tissues. AGE production increases when there is prolonged exposure to high blood glucose levels, for example. AGE degradation decreases with aging due to the progressive loss of metabolic efficiency and cellular defense mechanisms [17].

AGE accumulation is a major player in aging and in the development of age-related dysfunctions. For example, protein glycation can contribute to the stiffening of blood vessels and to the neurodegenerative aggregation of proteins in the brain. Furthermore, besides being damaged molecules, AGEs can activate signaling pathways that contribute to tissue dysfunction by increasing oxidative stress and the production of other damaging molecules [17].

Therefore, the efficiency of carbohydrate metabolism is important not only for the production of cell energy, but also for the minimization of cellular damage associated with glycation. If sugars aren’t used in cell energy pathways, they can react with proteins, fats, and other molecules and contribute to unhealthy aging. For these reasons, it is important to support glucose metabolic pathways to help our body protect itself against AGE accumulation.

Age-related loss of efficiency of glucose metabolism and cellular defense mechanisms can lead to the accumulation of damaging advanced glycation end-products (AGEs).

Why Supporting Glucose Metabolism/Glycolysis Is Important

Supporting glucose metabolism contributes to the maintenance of a healthy glycolytic flow. This is crucial, first and foremost, because glucose is the most important source of energy for our cells and tissues. Healthy carbohydrate metabolism is important for an efficient production of ATP to power biological processes. 

An efficient glucose metabolism is also fundamental for the maintenance of healthy blood sugar levels. Among other benefits (such as healthy insulin signaling, for example), this helps decrease the likelihood of detrimental glycation reactions of proteins and fats.

Glucose metabolism can be supported by providing precursors for the cofactors that participate in glycolysis and acetyl-CoA production. As we’ve seen, Qualia Life provides those ingredients. Qualia Life also provides ingredients that support glucose regulatory enzymes, such as Rosmarinus officinalis Leaf Extract (50% ursolic acid) [18]. Furthermore, Qualia LIfe also contains ingredients that support cellular quality control pathways that function to protect against  AGEs. These include Sirtmax^® Kaempferia parviflora Root Extract [19] and Rosmarinus officinalis Leaf Extract (50% ursolic acid) [20–22].

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