Biological Chemistry I
Unit 7 Overview: Glycolysis and Gluconeogenesis
7.1 Overview of glucose metabolism
7.2 Glycolysis: steps, regulation, and energy yield
7.3 Gluconeogenesis: pathway and regulation
7.4 Integration of glycolysis and gluconeogenesis in metabolism

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7.2 Glycolysis: steps, regulation, and energy yield

Last Updated on August 7, 2024

Glycolysis is the first step in breaking down glucose for energy. It's a series of 10 reactions that happen in your cells, turning one glucose into two pyruvate molecules. This process is super important for keeping you alive and kicking.

The cool thing about glycolysis is it doesn't need oxygen. It happens in two phases: prep and payoff. You start with glucose, add some phosphates, split it in half, and end up with pyruvate. Along the way, you make some ATP and NADH for energy.

Overview of Glycolysis

Glucose Breakdown Process

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  • Glycolysis is a metabolic pathway that breaks down glucose into two pyruvate molecules
  • Occurs in the cytosol of cells and does not require oxygen (anaerobic process)
  • Consists of a series of 10 enzyme-catalyzed reactions divided into two phases: the preparatory phase and the payoff phase
  • Glucose is first phosphorylated to form glucose-6-phosphate, an important regulatory step that traps glucose inside the cell

Key Intermediates

  • Glucose-6-phosphate is an intermediate formed by the phosphorylation of glucose in the first step of glycolysis
  • Fructose-1,6-bisphosphate is a key intermediate formed by the phosphorylation of fructose-6-phosphate by phosphofructokinase
  • Splitting of fructose-1,6-bisphosphate into two three-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate) marks the end of the preparatory phase
  • Pyruvate is the final product of glycolysis, formed by the dephosphorylation of phosphoenolpyruvate in the last step of the payoff phase

Enzymes and Regulation

Key Regulatory Enzymes

  • Hexokinase catalyzes the first step of glycolysis, the phosphorylation of glucose to glucose-6-phosphate
  • Phosphofructokinase catalyzes the third step, the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, and is a key regulatory enzyme in glycolysis
  • Pyruvate kinase catalyzes the final step, the dephosphorylation of phosphoenolpyruvate to pyruvate, and is also a key regulatory enzyme

Allosteric Regulation

  • Allosteric regulation involves the binding of effectors (activators or inhibitors) to enzymes at sites other than the active site, modulating their activity
  • Phosphofructokinase is allosterically inhibited by high levels of ATP and citrate, indicating an abundance of energy and biosynthetic precursors
  • Phosphofructokinase is allosterically activated by AMP and fructose-2,6-bisphosphate, signaling a need for increased energy production
  • Pyruvate kinase is allosterically inhibited by ATP and alanine, while activated by fructose-1,6-bisphosphate, coupling its activity to the energy state of the cell and the flux through glycolysis

Energy Production

ATP and NADH Generation

  • ATP (adenosine triphosphate) is the primary energy currency of the cell, and glycolysis generates a net gain of 2 ATP per glucose molecule
  • NADH (reduced nicotinamide adenine dinucleotide) is an electron carrier that is generated during glycolysis and can be used to produce ATP in the electron transport chain (oxidative phosphorylation)
  • Glycolysis produces a net gain of 2 NADH per glucose molecule, which can yield additional ATP in the mitochondria (approximately 3-5 ATP per NADH)

Substrate-Level Phosphorylation

  • Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy substrate to ADP, forming ATP
  • In glycolysis, substrate-level phosphorylation occurs in two steps: the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate and the conversion of phosphoenolpyruvate to pyruvate
  • Each of these steps generates 2 ATP, resulting in a total of 4 ATP produced by substrate-level phosphorylation (2 ATP per triose phosphate, with 2 triose phosphates formed per glucose)

Net Energy Yield

  • The net energy yield of glycolysis is 2 ATP and 2 NADH per glucose molecule
  • This accounts for the 2 ATP consumed in the preparatory phase (priming steps) and the 4 ATP produced by substrate-level phosphorylation in the payoff phase
  • The 2 NADH generated can be further oxidized in the mitochondria to yield additional ATP (approximately 6-10 ATP), increasing the total energy yield of glycolysis to 8-12 ATP per glucose molecule under aerobic conditions

Key Terms to Review (19)

Aerobic respiration: Aerobic respiration is a biological process where organisms convert glucose and oxygen into energy, carbon dioxide, and water. This process is vital for producing ATP, the energy currency of cells, and takes place in several stages, including glycolysis, the citric acid cycle, and oxidative phosphorylation, all of which are tightly regulated to ensure efficiency and balance within cellular metabolism.
Cytosol: Cytosol is the fluid portion of the cytoplasm that surrounds organelles within a cell, consisting mainly of water, dissolved ions, small molecules, and large water-soluble molecules such as proteins. This semi-fluid environment is crucial for cellular processes, as it serves as a medium for biochemical reactions, including metabolic pathways like glycolysis and the citric acid cycle.
Energy payoff phase: The energy payoff phase is a critical part of glycolysis where ATP is produced, and energy-rich molecules like NADH are generated. During this phase, the cell converts glucose into pyruvate while capturing energy in the form of ATP and NADH, ultimately contributing to the overall energy yield of the metabolic pathway.
Energy investment phase: The energy investment phase is the initial stage of glycolysis, where glucose is phosphorylated and converted into fructose-1,6-bisphosphate. During this phase, ATP is consumed to add phosphate groups to glucose, which helps to destabilize the molecule and prepares it for subsequent breakdown, ultimately facilitating the extraction of energy in later steps of glycolysis.
Fermentation: Fermentation is a metabolic process that converts sugars into acids, gases, or alcohol in the absence of oxygen. It occurs in various organisms and is a crucial way for cells to generate energy when oxygen levels are low, especially following glycolysis, which breaks down glucose to produce pyruvate. This process allows cells to continue producing ATP through substrate-level phosphorylation, playing a significant role in energy yield under anaerobic conditions.
Net ATP production: Net ATP production refers to the total amount of adenosine triphosphate (ATP) generated through metabolic processes, after accounting for ATP consumed during those processes. In the context of glycolysis, it highlights the balance between energy investment and energy yield, emphasizing how glucose is converted into pyruvate while producing a net gain of ATP and NADH.
Pyruvate kinase reaction: The pyruvate kinase reaction is a crucial step in glycolysis that catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, producing ATP from ADP in the process. This step is considered one of the key regulatory points of glycolysis and is essential for energy production in cellular metabolism.
NADH generation: NADH generation refers to the biochemical process through which the coenzyme nicotinamide adenine dinucleotide (NAD+) is reduced to its active form, NADH, during metabolic pathways such as glycolysis and the citric acid cycle. This process is crucial for cellular respiration, as NADH serves as a key electron carrier that transfers electrons to the electron transport chain, ultimately leading to ATP production.
Phosphofructokinase: Phosphofructokinase (PFK) is a key regulatory enzyme in glycolysis that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, using ATP as a phosphate donor. It plays a critical role in controlling the rate of glycolysis and, consequently, cellular energy production. By being a major control point in this metabolic pathway, PFK's activity is influenced by various metabolites and energy levels in the cell, which integrates it into broader metabolic regulation.
Hexokinase reaction: The hexokinase reaction is the first step of glycolysis where glucose is phosphorylated to form glucose-6-phosphate, using ATP as the phosphate donor. This reaction is crucial as it helps trap glucose within the cell and prepares it for further breakdown to generate energy. The regulation of this step is vital since it acts as a control point in glycolysis, influencing the overall pathway based on the cell's energy needs.
Glyceraldehyde-3-phosphate: Glyceraldehyde-3-phosphate (G3P) is a three-carbon sugar phosphate that plays a crucial role in cellular metabolism, particularly in glycolysis. It is produced during the breakdown of glucose and is pivotal in both energy production and the synthesis of carbohydrates. G3P acts as a key intermediate, linking several metabolic pathways and contributing to the overall energy yield of the cell.
Dihydroxyacetone phosphate: Dihydroxyacetone phosphate (DHAP) is a three-carbon sugar phosphate that plays a critical role in glycolysis, serving as an intermediate in the metabolic pathway that converts glucose into pyruvate. It is formed from fructose-1,6-bisphosphate and can be isomerized to glyceraldehyde-3-phosphate, linking it to energy production and regulation processes.
Fructose-1,6-bisphosphate: Fructose-1,6-bisphosphate is a six-carbon sugar phosphate that plays a crucial role as an intermediate in the glycolytic pathway. It is formed from fructose-6-phosphate through the action of the enzyme phosphofructokinase-1 (PFK-1) and is a key regulatory point in glycolysis, influencing the flow of glucose metabolism and energy production.
Pyruvate: Pyruvate is a three-carbon molecule that plays a key role in cellular metabolism as an end product of glycolysis and a precursor for gluconeogenesis. It acts as a critical junction point, linking anaerobic and aerobic pathways, while also being involved in the production of acetyl-CoA for the Krebs cycle and serving as a substrate for various biosynthetic reactions.
Glucose-6-phosphate: Glucose-6-phosphate is a crucial intermediate in the metabolic pathways of glycolysis and gluconeogenesis, formed when glucose is phosphorylated by the enzyme hexokinase or glucokinase. This compound plays a vital role in regulating glucose levels in the body, acting as a key junction between energy production and storage, which ties into both the breakdown of glucose for energy and the synthesis of glucose from non-carbohydrate sources.
Cytoplasm: Cytoplasm is the jelly-like substance that fills the interior of a cell, excluding the nucleus. It plays a vital role in cellular processes by hosting organelles, enzymes, and the cytoskeleton, providing a medium for metabolic reactions and energy production to occur. This environment is crucial for maintaining cellular structure and facilitating the movement of materials necessary for life.
Feedback inhibition: Feedback inhibition is a regulatory mechanism in metabolic pathways where the end product of a reaction inhibits an enzyme involved in its synthesis, thereby preventing the overproduction of that product. This process ensures metabolic balance and efficient use of resources within a cell, linking it to various aspects of metabolism, enzyme function, and cellular signaling.
Allosteric Regulation: Allosteric regulation refers to the process by which the activity of an enzyme is modified through the binding of an effector molecule at a site other than the active site, leading to a change in its conformation. This regulatory mechanism plays a vital role in metabolic pathways, allowing cells to adaptively modulate enzyme function and coordinate biochemical processes.
Glycolysis: Glycolysis is the metabolic pathway that converts glucose into pyruvate, producing energy in the form of ATP and NADH. This process is fundamental for cellular respiration and plays a crucial role in how organisms derive energy from carbohydrates.