# 9.1: Glycolysis - Reaction and Regulation (2023)

1. Last updated
2. Save as PDF
• Page ID
166215

Glycolysisis the first step in the breakdown of glucose to extract energy for cellular metabolism. Nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen and is thereforeanaerobic. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which the transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist in the facilitated diffusion of glucose.

## First Half of Glycolysis (Energy-Requiring Steps)

Step 1. The first step in glycolysis (Figure 9.1.1) is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can no longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.

Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, fructose-6-phosphate. Anisomeraseis an enzyme that catalyzes the conversion of a molecule into one of its isomers. (This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules.).

Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when the concentration of ADP is high; it is less active when ADP levels are low and the concentration of ATP is high. Thus, if there is “sufficient” ATP in the system, the pathway slows down. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.

Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. Thus, the pathway will continue with two molecules of a single isomer. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule.

(Video) Metabolism | Regulation of Glycolysis

## Second Half of Glycolysis (Energy-Releasing Steps)

So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules. Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules.

Step 6. The sixth step in glycolysis (Figure 9.1.2) oxidizes the sugar (glyceraldehyde-3-phosphate), extracting high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.

Here again is a potential limiting factor for this pathway. The continuation of the reaction depends upon the availability of the oxidized form of the electron carrier, NAD+. Thus, NADH must be continuously oxidized back into NAD+in order to keep this step going. If NAD+is not available, the second half of glycolysis slows down or stops. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP. In an environment without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH to NAD+.

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. (This is an example of substrate-level phosphorylation.) A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.

Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing this step is a mutase (isomerase).

Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).

Step 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate). Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and reverse reactions (these may have been described initially by the reverse reaction that takes place in vitro, under non-physiological conditions).

(Video) Regulation of Glycolysis : allosteric and transcriptional control

The net reaction in the transformation of glucose into pyruvate is:

Thus,two molecules ofare generated in the conversion of glucose into two molecules of pyruvate.

Note that the energy released in the anaerobic conversion of glucose into two molecules of pyruvate is -21 kcal mol-1(- 88 kJ mol-1).

## The Fates of Pyruvate

Pyruvic acid can be made from glucose through glycolysis, converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA. It can also be used to construct the amino acid alanine, and it can be converted into ethanol.

Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle) when oxygen is present (aerobic respiration); when oxygen is lacking, it ferments to produce lactic acid. Pyruvate is an important chemical compound in biochemistry. It is the output of the anaerobic metabolism of glucose known as glycolysis. One molecule of glucose breaks down into two molecules of pyruvate, which are then used to provide further energy in one of two ways. Pyruvate is converted into acetyl- coenzyme A, which is the main input for a series of reactions known as the Krebs cycle.

The net reaction of converting pyruvate into acetyl CoA and CO2is:

(Video) Regulation of glycolysis and gluconeogenesis | Biomolecules | MCAT | Khan Academy

Pyruvate is also converted to oxaloacetate by an anaplerotic reaction, which replenishes Krebs cycle intermediates; also, oxaloacetate is used for gluconeogenesis. These reactions are named after Hans Adolf Krebs, the biochemist awarded the 1953 Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes. The cycle is also known as the citric acid cycle or tri-carboxylic acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.

If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by fermentation to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation. Alternatively it is converted to acetaldehyde and then to ethanol in alcoholic fermentation.

Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine, and to ethanol. Therefore, it unites several key metabolic processes.

## Regulation

Figure 9.1.3:Glycolysis Regulation

Control of glycolysis is unusual for a metabolic pathway, in that regulation occurs at three enzymatic points:

(Video) Cellular Respiration (UPDATED)

Glycolysis is regulated in a reciprocal fashion compared to its corresponding anabolic pathway, gluconeogenesis. Reciprocal regulation occurs when the same molecule or treatment (phosphorylation, for example) has opposite effects on catabolic and anabolic pathways. Reciprocal regulation is important when anabolic and corresponding catabolic pathways are occurring in the same cellular location.

As an example, consider regulation of PFK. It is activated by several molecules, most importantly fructose-2,6- bisphosphate (F2,6BP). This molecule has an inhibitory effect on the corresponding gluconeogenesis enzyme, fructose-1,6-bisphosphatase (F1,6BPase).

You might wonder why pyruvate kinase, the last enzyme in the pathway, is regulated. The answer is simple. Pyruvate kinase catalyzes the most energetically rich reaction of glycolysis. The reaction is favored so strongly in the forward direction that cells must do a ‘two-step’ around it in the reverse direction when making glucose. In other words, it takes two enzymes, two reactions, and two triphosphates to go from pyruvate back to PEP in gluconeogenesis. When cells are needing to make glucose, they can’t be sidetracked by having the PEP they have made in gluconeogenesis be converted directly back to pyruvate by pyruvate kinase. Consequently, pyruvate kinase is inhibited during gluconeogenesis, lest a “futile cycle" occur.

Another interesting control mechanism called feedforward activation involves pyruvate kinase. Pyruvate kinase is activated allosterically by F1,6BP. This molecule is a product of the PFK reaction and a substrate for the aldolase reaction. It should be noted that the aldolase reaction is energetically unfavorable (high +ΔΔG°’), thus allowing F1,6BP to accumulate. When this happens, some of the excess F1,6BP activates pyruvate kinase, which jump-starts the conversion of PEP to pyruvate. The resulting drop in PEP levels has the effect of “pulling" on the reactions preceding pyruvate kinase. As a consequence, the concentrations of G3P and DHAP fall, helping to move the aldolase reaction forward.

## Outcomes of Glycolysis

Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further (via the citric acid cycle or Krebs cycle), it will harvest only two ATP molecules from one molecule of glucose.

Mature mammalian red blood cells do not have mitochondria and are not capable of aerobic respiration, the process in which organisms convert energy in the presence of oxygen. Instead, glycolysis is their sole source of ATP. Therefore, if glycolysis is interrupted, the red blood cells lose their ability to maintain their sodium-potassium pumps, which require ATP to function, and eventually, they die. For example, since the second half of glycolysis (which produces the energy molecules) slows or stops in the absence of NAD+, when NAD+ is unavailable, red blood cells will be unable to produce a sufficient amount of ATP in order to survive.

(Video) Glycolysis Pathway | Enzymes, Regulation and Products

Additionally, the last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will continue to proceed, but only two ATP molecules will be made in the second half (instead of the usual four ATP molecules). Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.

## FAQs

### Is step 9 of glycolysis reversible? ›

In the case of glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are 1st, 3rd, and last steps of glycolysis are irreversible.

What is the 9th reaction of glycolysis? ›

Step 9 of glycolysis:

Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).

What is the regulation of glycolysis reaction? ›

Glycolysis is regulated in a reciprocal fashion compared to its corresponding anabolic pathway, gluconeogenesis. Reciprocal regulation occurs when the same molecule or treatment (phosphorylation, for example) has opposite effects on catabolic and anabolic pathways.

What are the 3 regulatory steps of glycolysis? ›

There are three major enzymatic control points within the glycolytic pathway. These include the hexokinase, the phosphofructokinase, and the pyruvate kinase transitions.

What molecule is released in step 9 of glycolysis? ›

3-phosphoglycerate is converted into its isomer, 2-phosphoglycerate. Step 9. 2-phosphoglycerate loses a molecule of water, becoming phosphoenolpyruvate ( PEPstart text, P, E, P, end text). PEPstart text, P, E, P, end text is an unstable molecule, poised to lose its phosphate group in the final step of glycolysis.

Are there 3 reactions in glycolysis that are irreversible? ›

3 irreversible steps in glycolysis: hexokinase; phosphofructokinase; pyruvate kinase.

Is glycolysis a 9 step process? ›

During glycolysis, a single molecule of glucose is used to produce a net two molecules of pyruvate, two molecules of ATP, and two molecules of NADH. The pyruvate may then be used in aerobic respiration or, in the absence of oxygen, anaerobic respiration. The 10 steps of glycolysis can be divided into two parts.

What are the products of glycolysis 9.2 quizlet? ›

What are the products of glycolysis? 2 molecules of pyruvic acid, 2 molecules of NADH, and a net gain of 2 ATP molecules.

What is the last reaction of glycolysis? ›

The overall reaction for glycolysis is: glucose (6C) + 2 NAD+ 2 ADP +2 inorganic phosphates (Pi) yields 2 pyruvate (3C) + 2 NADH + 2 H+ + 2 net ATP.

What is the regulatory factor of glycolysis? ›

The key regulatory enzyme of glycolysis is phosphofructokinase. It is inhibited by ATP and citrate and activated by AMP (and ADP), Pi, and fructose 2,6-bisphosphate.

### What is glycolysis pathway and its regulation? ›

Glycolysis refers to a metabolic pathway by which organisms extract energy in the form of ATP during the conversion of glucose into pyruvate and lactate. Glycolysis produces ATP required for energy-requiring reactions and processes, for example, ion transport, protein synthesis and reactions catalysed by kinases.

What enzymes regulate glycolysis? ›

The three key enzymes of glycolysis are hexokinase, phosphofructokinase, and pyruvate kinase.

What is the 3 reaction of glycolysis? ›

Reaction 3: is another kinase reaction. Phosphorylation of the hydroxyl group on C1 forming fructose-1,6- bisphosphate. Enzyme: phosphofructokinase. This allosteric enzyme regulates the pace of glycolysis.

How many regulatory steps are there in glycolysis? ›

Glycolysis is the catabolic process in which glucose is converted into pyruvate via ten enzymatic steps. There are three regulatory steps, each of which is highly regulated.

What are the three 3 reactants needed to begin glycolysis? ›

Glycolysis is the first stage of cellular respiration, and the reactants are one molecule of glucose and two molecules of ATP (adenosine triphosphate). The ATP molecules provide energy so that the reaction can occur.

How many ATP does glycolysis produce? ›

During glycolysis, glucose ultimately breaks down into pyruvate and energy; a total of 2 ATP is derived in the process (Glucose + 2 NAD+ + 2 ADP + 2 Pi --> 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O).

Which steps of glycolysis produce ATP? ›

In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. (This is an example of substrate-level phosphorylation. )

Why are reactions 1 3 and 10 irreversible in glycolysis? ›

Answer and Explanation: Some steps in glycolysis are irreversible because they are needed to control the glycolytic pathway and ensure the production of ATP. For example, the first step in glycolysis, when glucose is converted to glucose-6-phosphate is an irreversible step.

Is step 7 in glycolysis irreversible? ›

GLYCOLYSIS REVIEW & OVERVIEW

Two phases of glycolysis. There are ten steps (7 reversible; 3 irreversible).

What are the 3 rate limiting steps for reversal of glycolysis? ›

Instead, the control of the flow of glycolysis depends primarily on the activity of three key rate-limiting enzymes, hexokinases (HKs), phosphofructokinase-1 (PFK-1), and pyruvate kinases (PKs) (46, 47).

### What is glycolysis summary? ›

Glycolysis is the process in which glucose is broken down to produce energy. It produces two molecules of pyruvate, ATP, NADH and water. The process takes place in the cytoplasm of a cell and does not require oxygen. It occurs in both aerobic and anaerobic organisms.

Are all steps in glycolysis regulated? ›

Glycolysis and gluconeogenesis can be regulated by the enzymes and the molecules that help the enzymes in catalyzing the reactions. Glycolysis can be regulated by enzymes such as hexokinase, phosphofructokinase and pyruvate kinase. Gluconeogenesis can be regulated by fructose 1,6-bisphosphatase.

What process is glycolysis an _____? ›

Glycolysis is the process in which one glucose molecule is broken down to form two molecules of pyruvic acid (also called pyruvate). The glycolysis process is a multi-step metabolic pathway that occurs in the cytoplasm of animal cells, plant cells, and the cells of microorganisms.

What does glycolysis produce _____ _______ and ______ as end products? ›

The final end products of cellular respiration are ATP and H2O. Glycolysis produces two pyruvate molecules, four ATPs (a net of two ATP), two NADH, and two H2O.

What are the products of glycolysis answers? ›

Answer and Explanation: The products of glycolysis are Pyruvate, ATP, NADH (answer choice B). Glycolysis takes one molecule of glucose and breaks it down to produce 2 net molecules of ATP, 2 pyruvate molecules and 2 NADH molecules.

What are the 4 products of glycolysis? ›

Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH.

What is the most important reaction in glycolysis? ›

The most important regulatory step of glycolysis is the phosphofructokinase reaction. Phosphofructokinase is regulated by the energy charge of the cell—that is, the fraction of the adenosine nucleotides of the cell that contain high‐energy bonds.

What type of reaction is glycolysis? ›

Glycolysis is a linear metabolic pathway of enzyme-catalyzed reactions that converts glucose into two molecules of pyruvate in the presence of oxygen or two molecules of lactate in the absence of oxygen.

What are the names of the regulatory steps of glycolysis? ›

Regulated Enzymes in Glycolysis

The three regulatory enzymes are hexokinase (or glucokinase in the liver), phosphofructokinase, and pyruvate kinase. The flux through the glycolytic pathway is adjusted in response to conditions both inside and outside the cell.

Why is glycolysis regulation necessary? ›

Glycolysis provides an important source of energy for most cells as well as a source of substrate for a number of other metabolic pathways. Its regulation is complex, involving allosteric control, phosphorylation control and transcriptional control of glycolytic enzymes.

### What parts of glycolysis are regulated? ›

There are three points of regulation within glycolysis and these points are all enzymes that catalyze irreversible steps in the glycolytic pathway. These enzymes are phosphofructokinase, hexokinase and pyruvate kinase.

What is the first reaction of glycolysis? ›

Reaction 1: In the first reaction of glycolysis, the enzyme hexokinase rapidly phosphorylates glucose entering the cell, forming glucose-6-phosphate (G-6-P).

What is the main product of glycolysis? ›

The final product of glycolysis is pyruvate in aerobic settings and lactate in anaerobic conditions. Pyruvate enters the Krebs cycle for further energy production.

Which stages of glycolysis are reversible? ›

How many steps in glycolysis are irreversible?
...
• Glycolysis involves a total of 10 steps out of which 7 are reversible and 3 are irreversible.
• Steps 1, 3, and 10 are irreversible.
• In step 1, Glucose is converted to glucose 6-phosphate in the presence of the enzyme hexokinase.

Which parts of glycolysis are reversible? ›

All the reactions in glycolysis are reversible except
• A. 3-Phosphoglycerate to 1,3-Biphosphoglycerate.
• B. Glucose 6-phosphate to fructose -6-phosphate.
• C. 1,3-Biphosphoglycerate to glyceraldehyde 3-P.
• D. Fructose -6-phosphate to fructose-1,6-diphosphate.

What is the reverse step of glycolysis? ›

Gluconeogenesis means new synthesis of glucose. It is the reverse of glycolysis.

Which reaction of glycolysis is reversible? ›

The reactions of glycolysis convert glucose 6-phosphate to pyruvate. The entire process is cytosolic. Glucose 6-phosphate is reversibly isomerized to form fructose 6-phosphate.

How many ATP are produced in glycolysis? ›

During glycolysis, glucose ultimately breaks down into pyruvate and energy; a total of 2 ATP is derived in the process (Glucose + 2 NAD+ + 2 ADP + 2 Pi --> 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O).

What steps of glycolysis require energy? ›

There are two phases of glycolysis. Priming phase because it requires an input of energy in form of 2 ATPs per glucose molecule and the pay off phase because energy is released in the form of 4 ATPs.

What is stage 3 of glycolysis called? ›

GLYCOLYSIS. Stage 1 (Priming stage) Stage 2 (Splitting stage) Stage 3 (Oxidoreduction-phosphorylation stage)

### What allows glycolysis to start again? ›

Lactic acid fermentation produces lactic acid (lactate) and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made.

What is the importance of glycolysis? ›

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 is the only fuel red blood cells can use.

Does glycolysis require ATP? ›

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP.

Does glycolysis require oxygen? ›

Glycolysis, which is the first step in all types of cellular respiration is anaerobic and does not require oxygen. If oxygen is present, the pathway will continue on to the Krebs cycle and oxidative phosphorylation.

What does glycolysis produce? ›

1: Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules: Glycolysis, or the aerobic catabolic breakdown of glucose, produces energy in the form of ATP, NADH, and pyruvate, which itself enters the citric acid cycle to produce more energy.

Which type of reaction is glycolysis? ›

Glycolysis is an anaerobic reaction, and in low oxygen conditions is the cell's sole source of ATP. You can read more about anaerobic respiration here.

Which reactions occur twice in glycolysis? ›

The second half of glycolysis is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule.

Which reaction comes after glycolysis? ›

In the presence of oxygen, the next stage after glycolysis is oxidative phosphorylation, which feeds pyruvate to the Krebs Cycle and feeds the hydrogen released from glycolysis to the electron transport chain to produce more ATP (up to 38 molecules of ATP are produced in this process).

## Videos

1. Regulation of Glycolysis | Allosteric vs Hormonal Regulation of Glycolysis | Biochemistry
(PC Biochemistry)
2. Regulation of Glycolysis in Liver Cells
(Andrey K)
3. Reciprocal Regulation of Gluconeogenesis and Glycolysis
(Andrey K)
4. Regulation of glycolysis
(Periodica)
5. 6: Glycolysis: Regulation | Carbohydrates Metabolism-6 | Biochemistry | N'JOY Biochemistry
(N'JOY Biochemistry)
6. Regulation of GLYCOLYSIS (Hexokinase, PFK and Pyruvate Kinase)
(Minal Sharma)
Top Articles
Latest Posts
Article information

Author: Pres. Lawanda Wiegand

Last Updated: 04/20/2023

Views: 5650

Rating: 4 / 5 (51 voted)

Author information

Name: Pres. Lawanda Wiegand

Birthday: 1993-01-10

Address: Suite 391 6963 Ullrich Shore, Bellefort, WI 01350-7893

Phone: +6806610432415

Job: Dynamic Manufacturing Assistant

Hobby: amateur radio, Taekwondo, Wood carving, Parkour, Skateboarding, Running, Rafting

Introduction: My name is Pres. Lawanda Wiegand, I am a inquisitive, helpful, glamorous, cheerful, open, clever, innocent person who loves writing and wants to share my knowledge and understanding with you.