Chapter 24: Introduction to Glycolysis (EMP) Glycolysis, via the pathway named after its co-discoverers, is the primary route for glucose catabolism in mammalian cells leading to the cytoplasmic production of pyruvate, and its subsequent oxidation in the mitochondrial tricarboxylic acid (TCA) cycle through production of either acetyl-CoA, or oxaloacetate. Glycolysis is also a major pathway for the catabolism of fructose and galactose derived from dietary sucrose and lactose, respectively. Of crucial biomedical importance is the ability of glycolysis to provide ATP for cells in the absence of oxygen (O2), thus allowing skeletal muscle, for example, to continue contracting when aerobic oxidation becomes insufficient. Anaerobic glycolysis also allows poorly perfused tissues to survive, and also those that lack mitochondria (e.g., mature erythrocytes). Conversely, heart muscle, which is adapted for sustained aerobic performance, has limited anaerobic glycolytic potential, and therefore does not perform well under conditions of ischemia. A small number of inherited hemolytic anemias occur among domestic animals, in which enzymes of glycolysis (e.g., pyruvate kinase or phosphofructokinase (PFK)), have reduced activity. Glycolysis is a highly regulated process, with just enough glucose being metabolized at any one time to meet the cell's need for ATP. Metabolic intermediates between glucose and pyruvate are phosphorylated compounds, which promote their retention within the cytoplasm. Four molecules of ATP are generated from ADP in anaerobic glycolysis: two in the step catalyzed by phosphoglycerate kinase, and two in the step catalyzed by pyruvate kinase. However, two ATP molecules are consumed during earlier steps of this pathway: the first by the addition of a phosphate residue to glucose in the reaction catalyzed by hexokinase, and the second by the addition of a second phosphate to fructose 6-phosphate in the reaction catalyzed by PFK. Thus, there is a net gain of two ATP molecules in anaerobic glycolysis (when starting with glucose). Since the breakdown of glycogen through glucose 1- phosphate does not require ATP, the cytoplasmic phase of glycogenolysis can provide a net gain of 3 ATP molecules. When oxygen becomes limited to cells, glucose cannot be oxidized completely to CO2 and H2O. Cells will thus "ferment" each glucose molecule to two moles of lactic acid -- again, with the net production of only two molecules of ATP from each glucose molecule. In fast-growing cancer cells, for example, glycolysis frequently proceeds at a much higher rate than can be accommodated by the mitochondrial TCA cycle. Many tumors are also poorly vascularized, thus reducing O2 availability. Thus, more pyruvate is produced than can be metabolized inside mitochondria. This, in turn, results in excessive production of lactic acid, which produces an acid environment in and around the tumor, a situation that may have implications for certain types of cancer therapy. Lactic acidosis results from other causes as well, including exercise, pyruvate dehydrogenase deficiency, or, for example, grain overload in ruminant animals. The overall equation for anaerobic glycolysis (from glucose to lactate- ) is: Glucose + 2 ADP + 2 Pi —> 2 Lactate- + 2 ATP + 2 H2O + 2 H+ The term lactic acid (CH3-CHOH-COOH) is often used interchangeably with lactate (CH3- CHOH-COO- ), which is an anion. Since lactic acid, like many organic acids, is largely dissociated in body fluids (CH3-CHOH-COOH —> CH3-CHOH-COO- + H+ ), use of the term "lactate" is more appropriate. It should be noted that when lactate accumulates in plasma, it displaces other important anions (e.g., HCO3 - and Cl- ) from extracellular fluids, thus having important implications in acid/base chemistry. When protons (H+ ) from lactic acid accumulate in muscle tissue, fatigue ensues because the Vmax of PFK is lowered, the release of Ca++ from the sarcoplasmic reticulum is compromised, actomyosin ATPase activity is reduced, and the conformation of muscle contractile proteins is affected, thus causing pain. Much of the lactate in blood normally passes into liver cells (where it can be converted to pyruvate, and then oxidized or used for glucose formation), or into cardiac muscle cells which can also convert it to pyruvate, and then oxidize it in mitochondria for energy purposes. Anaerobic glycolysis is nearly universal among all cell types, although the end products may vary. That is, lactate (of mammals) may be replaced by a variety of different substances such as propionate in bacteria, or ethanol in yeast. As pyruvate is converted to lactate (or ethanol), the NADH produced in the initial stages of glycolysis is reoxidized to NAD+ , thus allowing anaerobic glycolysis to proceed (note: conversion of glyceraldehyde 3-phosphate to 1,3- bisphosphoglycerate requires NAD+ ). This anaerobic fermentation of carbohydrate in yeast forms the basis of the beer and wine industry. Although anaerobic glycolysis produces only about 5% of the ATP provided during the catabolism of glucose, there are a number of reasons why it is necessary:

1. There are several instances where animals need quick energy. In moving from rest to full flight, for example, aerobic oxidation would require a rapid increase in the O2 supply, which could only be achieved by increasing the blood supply (which usually takes a number of seconds). Thus, an animal who initiates a sprint from the resting position relies heavily on anaerobic glycolysis.

2. A rapid increase in the O2 supply to tissues requires a well-developed vascular network. In some instances, it may prove inefficient to supply a large body mass (i.e., big muscles), with a well-developed blood supply. This is certainly the case for the pectoral muscles of game birds (e.g., pheasants), which are frequently used for escape purposes. In others, the blood supply may be limited because of pathology (e.g., tumors), or physiology (the kidney medulla). In these examples, anaerobic glycolysis may be the major, or only, source of energy.

3. The two major groups of skeletal muscle fibers are red, slow-twitch oxidative fibers (type I), and white, fast-twitch glycolytic fibers (type IIB). The type I fibers have high aerobic capacity, and therefore are reasonably fatigue resistant; whereas the type IIB fibers are largely anaerobic. Many fish possess mainly type IIB fibers, with only a thin section along the lateral line being of type I. The lateral line fibers are used during normal periods of swimming, while the large white muscle mass is used for short bursts of rapid activity. Bluefish, however, contain many type I fibers which provide them with far more aerobic capacity. When resting skeletal muscle is compared to more highly perfused, oxygen-dependent areas of the body (e.g., liver, kidneys, brain, and heart), a key distinction becomes apparent: The liver, kidneys, brain, and heart normally account for only about 7% of the body mass, yet receive almost 70% of the cardiac output (CO), and consume 58% of the O2 utilized in the resting state. Skeletal muscle accounts for nearly 50% of the normal body mass, yet receives only 16% of the CO at rest, and consumes only 20% of the O2 utilized in the resting state. It is no wonder that anaerobic glycolysis is so important in skeletal muscle, since O2 is being utilized by more "vital" organs in the resting state, even though these organs occupy a rather small fraction of the total body mass. If exercise were to commence quickly from the resting state, anaerobic glycolysis would be mandatory.

4. Aerobic oxidation of carbohydrates, fats, and amino acids is carried out in mitochondria, rather bulky cell organelles. In some cases, it may be desirable to reduce the number of mitochondria (because of their bulk) and, in these instances, the cell would be more dependent on anaerobic glycolysis. For example, the eye (namely the cornea and lens) needs to transmit light signals with high efficiency. Optically dense structures such as mitochondria and capillaries would reduce this efficiency (and, if they were present in large amounts, animals might literally "see" those extra mitochondria, as well as the blood flowing by in capillaries). Therefore, most of the glucose (over 80%) used by the cornea and lens is normally metabolized anaerobically.

Mature red blood cells have no mitochondria, so all of their energy needs are supplied by anaerobic glycolysis. The space is needed for other molecules, in this case hemoglobin, which occupies about 33% of the cell interior. Also, red blood cells are located in a medium (blood plasma), that always has glucose available. On the other hand, heart muscle is an example of a tissue that has retained its aerobic capacity (many mitochondria), but lacks the ability to exhibit powerful contractile forces (like type IIB anaerobic skeletal muscle fibers that have many more actin and myosin filaments (and fewer mitochondria) per unit area).

SUMMARY

Glycolysis is the primary pathway for glucose catabolism in mammalian cells, leading to the production of pyruvate. It is also important for the catabolism of fructose and galactose. Glycolysis can provide ATP for cells in the absence of oxygen, allowing tissues to survive in low oxygen conditions. It is a highly regulated process, with just enough glucose being metabolized to meet the cell's ATP needs. Anaerobic glycolysis produces two molecules of ATP from each glucose molecule, but also consumes two ATP molecules in earlier steps. Lactic acid is produced when oxygen is limited, and excessive production of lactic acid can create an acidic environment in tumors. Anaerobic glycolysis is necessary in certain situations where quick energy is needed, such as during sprinting or in tissues with limited blood supply. It is also important in skeletal muscle, which receives less oxygen compared to other organs. Some tissues, like the cornea and lens, rely heavily on anaerobic glycolysis due to their need for high efficiency in transmitting light signals. Mature red blood cells rely solely on anaerobic glycolysis for energy, while heart muscle retains its aerobic capacity but lacks powerful contractile forces.

OUTLINE

I. Glycolysis as the primary route for glucose catabolism in mammalian cells

• Leads to cytoplasmic production of pyruvate

• Subsequent oxidation in the mitochondrial TCA cycle

II. Importance of glycolysis in providing ATP in the absence of oxygen

• Allows skeletal muscle to continue contracting

• Allows poorly perfused tissues to survive

• Enables mature erythrocytes to function without mitochondria

III. Inherited hemolytic anemias and reduced activity of glycolytic enzymes

IV. Regulation of glycolysis

• Metabolizes enough glucose to meet ATP needs

• Phosphorylated intermediates promote retention within the cytoplasm

V. ATP production and consumption in anaerobic glycolysis

• Net gain of 2 ATP molecules

• Breakdown of glycogen through glucose 1-phosphate provides a net gain of 3 ATP molecules

VI. Fermentation of glucose to lactic acid in the absence of oxygen

• Common in fast-growing cancer cells and poorly vascularized tumors

• Lactic acidosis can result from exercise or other causes

VII. Overall equation for anaerobic glycolysis

VIII. Use of the term "lactate" instead of "lactic acid" in body fluids

IX. Implications of lactate accumulation in plasma and muscle tissue

X. Anaerobic glycolysis in different organisms and end products

XI. Reasons for the necessity of anaerobic glycolysis

• Quick energy needs in animals

• Limited blood supply to certain tissues

• Different types of skeletal muscle fibers

• Reduction of mitochondria in certain cells

• Energy supply in mature red blood cells and tissues with aerobic capacity

XII. Examples of tissues with high reliance on anaerobic glycolysis: cornea, lens, and mature red blood cells

• Cornea and lens metabolize over 80% of glucose anaerobically

• Mature red blood cells lack mitochondria and rely solely on anaerobic glycolysis

• Heart muscle retains aerobic capacity but lacks powerful contractile forces

QUESTIONS

Qcard 1:

Question: What is the primary route for glucose catabolism in mammalian cells?

Answer: Glycolysis

Qcard 2:

Question: What is the net gain of ATP molecules in anaerobic glycolysis?

Answer: 2 ATP molecules

Qcard 3:

Question: What is the end product of anaerobic glycolysis?

Answer: Lactate

Qcard 4:

Question: Why is anaerobic glycolysis necessary?

Answer: It provides quick energy in situations where aerobic oxidation is not sufficient.

Qcard 5:

Question: What is the major source of energy in white, fast-twitch glycolytic muscle fibers?

Answer: Anaerobic glycolysis

Qcard 6:

Question: What is the primary source of energy for mature red blood cells?

Answer: Anaerobic glycolysis

Mind Map: Introduction to Glycolysis (EMP)

Central Idea: Glycolysis is the primary pathway for glucose catabolism in mammalian cells, providing ATP and pyruvate for energy production.

Main Branches:

1. Importance of Glycolysis

   • ATP production in the absence of oxygen

   • Survival of poorly perfused tissues and cells lacking mitochondria

   • Implications for certain types of cancer therapy

2. Regulation of Glycolysis

   • Metabolic intermediates and their retention in the cytoplasm

   • Net gain and consumption of ATP molecules

3. Anaerobic Glycolysis

   • Conversion of glucose to lactate in the absence of oxygen

   • Reoxidation of NADH to NAD+ for glycolysis to proceed

   • Implications for acid/base chemistry and muscle fatigue

4. Variations in End Products

   • Lactate in mammals, propionate in bacteria, ethanol in yeast

   • Anaerobic fermentation in beer and wine production

5. Reasons for the Necessity of Anaerobic Glycolysis

   • Quick energy supply in situations requiring rapid movement

   • Limited blood supply to certain tissues or organs

   • Different muscle fiber types and their energy requirements

   • Reduction of mitochondria in specialized cells or tissues

6. Unique Cases

   • Energy supply in mature red blood cells without mitochondria

   • Retained aerobic capacity in heart muscle with limited contractile forces

Study Plan: Chapter 24: Introduction to Glycolysis (EMP)

Day 1: Introduction and Regulation of Glycolysis

• Read and understand the introduction to glycolysis, its significance in glucose catabolism, and its role in the production of pyruvate.

• Study the regulation of glycolysis, including the control of glucose metabolism and the role of phosphorylated compounds in promoting retention within the cytoplasm.

• Review the ATP generation and consumption in anaerobic glycolysis, focusing on the net gain of ATP molecules.

• Take notes on the biomedical importance of glycolysis, such as its ability to provide ATP in the absence of oxygen and its implications in different tissues and diseases.

Day 2: Anaerobic Glycolysis and Lactic Acid Production

• Study the process of anaerobic glycolysis, including the conversion of glucose to lactate, the reoxidation of NADH to NAD+, and the end products of glycolysis in different organisms.

• Understand the reasons why anaerobic glycolysis is necessary, such as providing quick energy in situations where aerobic oxidation is insufficient or inefficient.

• Learn about lactic acidosis and its implications in acid/base chemistry, muscle fatigue, and certain types of cancer therapy.

• Take note of the overall equation for anaerobic glycolysis and the interchangeability of the terms lactic acid and lactate.

Day 3: Energy Needs and Muscle Fiber Types

• Explore the energy needs of different tissues and organs, focusing on the importance of anaerobic glycolysis in skeletal muscle when oxygen is being utilized by more vital organs.

• Understand the distinction between red, slow-twitch oxidative muscle fibers and white, fast-twitch glycolytic muscle fibers, and their reliance on aerobic and anaerobic metabolism.

• Study the role of mitochondria in aerobic oxidation and the instances where cells may reduce the number of mitochondria, leading to increased dependence on anaerobic glycolysis.

• Take note of the unique energy metabolism of mature red blood cells, which rely solely on anaerobic glycolysis due to the absence of mitochondria.

Day 4: Glycolysis in Different Organisms and Industries

• Learn about the variations in end products of glycolysis in different organisms, such as the production of propionate in bacteria and ethanol in yeast.

• Understand the role of anaerobic fermentation of carbohydrates