Fundamentals of Human Nutrition/Glucose

12.1 Glucose

Please use this HELP:EDITING link for information about contributing and editing the book.

12.1.1 Glycolysis

Written by: André Cunha

Metabolism is broadly defined as the sum of biochemical processes in living organisms that either produce or consume energy (DeBerardinis, R.J et al. 2012). Core metabolism can be simplified to those pathways involving abundant nutrients like carbohydrates, fatty acids, and amino acids, essential for energy homeostasis and macromolecular synthesis in humans. Pathways of core metabolism can then be separated conveniently into three classes: those that synthesize simple molecules or polymerize them into more complex macromolecules (anabolism); those that degrade molecules to release energy (catabolism); and those that help to eliminate the toxic waste produced by the other classes (waste disposal). These pathways are profoundly important. Stated bluntly, they are the sole source of energy that allows life to resist the urge to degrade into entropy (DeBerardinis, R.J. 2012; Nelson, D & Cox, M. 2011).

Glucose occupies a central position in the metabolism of plants, animals and many microorganisms. It has relatively high potential energy and therefore it is a good fuel: the full oxidation of one molecule of glucose to carbon dioxide and water occurs in a range of standard free energy of - 2840 kJ/mol. By means of storage of glucose as a polymer of high molecular mass, such as starch and glycogen, the cell can store large quantities of hexose units while maintaining the cytosolic osmolarity relatively low. When the demand for energy increases, glucose can be converted from such intracellular storages into adenosine triphosphate (ATP) in an aerobic or anaerobic manner. (Nelson, D & Cox, M. 2011; Stryer, L. 2008).

Glucose is not only an excellent fuel, but it is also a precursor admirably versatile, able to meet a variety of metabolic intermediates in biosynthetic reactions (FIGURE 02). A bacterium like Escherichia coli can use the glucose as carbon skeletons for every amino acid, nucleotide, enzyme, fatty acid or other metabolic intermediate required for their growth (Nelson, D & Cox, M. 2011)

Glycolysis literally means "splitting sugars." In glycolysis, glucose (a six carbon sugar) is split into two molecules of a three-carbon sugar. Glycolysis yields two molecules of ATP (free energy containing molecule), two molecules of pyruvic acid and two "high energy" electron carrying molecules of NADH. Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called fermentation.

Glycolysis (from glycose, an older term[1] for glucose + -lysis degradation) is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy compounds ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).[2]

Glycolysis is a definite sequence of ten reactions involving ten intermediate compounds (one of the steps involves two intermediates). The intermediates provide entry points to glycolysis. For example, most monosaccharides, such as fructose, glucose, and galactose, can be converted to one of these intermediates. The intermediates may also be directly useful. For example, the intermediate dihydroxyacetone phosphate (DHAP) is a source of the glycerol that combines with fatty acids to form fat.

It occurs, with variations, in nearly all organisms, both aerobic and anaerobic. The wide occurrence of glycolysis indicates that it is one of the most ancient known metabolic pathways.[3] It occurs in the cytosol of the cell.

The most common type of glycolysis is the Embden–Meyerhof–Parnas (EMP pathway), which was first discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. Glycolysis also refers to other pathways, such as the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways. However, the discussion here will be limited to the Embden–Meyerhof–Parnas pathway.[4]

The entire glycolysis pathway can be separated into two phases (FIGURE 1)[5]:

The Preparatory Phase – in which ATP is consumed and is hence also known as the investment phase The PayOff Phase – in which ATP is produced.

In glycolysis, one molecule of glucose is degraded into a series of enzyme-catalyzed reactions, generating two molecules of the compound in three carbon atoms, pyruvate. During these sequential reactions part of glycolysis of glucose free energy is conserved as ATP and NADH (Nelson, D & Cox, M. 2011; Stryer, L. et al. 2008).

The glycolytic pathway (FIGURE 01) starts when glucose enters the cell through glucose transporters (GLUTs) and, once inside, it is phosphorylated to glucose-6-phosphate (G6P) by hexokinase 2 (HK2). Phosphoglucose isomerase catalyzes G6P to fructose-6-phosphate (F6P), which yields fructose-1,6-bisphosphate by phosphofructokinase 1 (PFK1), and then pyruvate and ATP by pyruvate kinase (PK) in the final step of glycolysis. Pyruvate is converted to acetyl-CoA, which enters the TCA cycle. Ultimately, glycolysis produces two ATP molecules and six NADH molecules per unit of glucose. In normal tissues, most of the pyruvate is directed into the mitochondrion to be converted into acetyl-CoA by the action of pyruvate dehydrogenase (PDH) or transaminated to form alanine (Marie, S. Shinjo, S. 2011; DeBerardinis, R.J. 2012).

FIGURE 01: The glycolytic pathway

FIGURE 02: The main routes of glucose utilization. Although it is not the only possible glucose ways, these four paths are most significant in terms of quality of glucose that flows through them in the majority of cells.

According to Marie and Shinjo (2011), for normal cells to produce two viable daughter cells at mitosis, all the cellular contents must be replicated, and energy is necessary for this to happen. Glucose participates in cellular energy production with two ATP synthesis through glycolysis and up to 36 ATPs through its complete catabolism by the TCA cycle and OXPHOS (oxidative phosphorylation). The large requirements for nucleotides, amino acids, and lipids for the daughter cells are provided by intermediate metabolites of these pathways.

In addition to glucose, glutamine is the other molecule catabolized in appreciable quantities for most mammalian cells in culture. Both molecules supply carbon, nitrogen, free energy, and reducing equivalents necessary to support cell growth and division. This means that glucose, in addition to being used for ATP synthesis, should also be diverted to macromolecular precursors such as acetyl-CoA for fatty acids, glycolytic intermediates for non-essential amino acids, and ribose for nucleotides to generate biomass (Marie, S. Shinjo, S. 2011; Nelson, D & Cox, M. 2011; Stryer, L. et al. 2008).

Glycolysis/Gluconeogenesis

Glycolysis (solid lines) is the main pathway of glucose oxidation and it occurs in the cytosol of all cells, including but not limited to red blood cells, skeletal muscle, adipose tissue, liver and brain. It is commonly split into two phases: the preparatory phase and the payoff phase. The goal of the preparatory phase is to phosphorylate glucose and convert it to glyceraldehyde 3-phosphate, while the payoff phase oxidizes glyceraldehyde 3-phopshate to pyruvate and forms ATP and NADH. For every molecule of glucose that undergoes glycolysis there is a net gain of 2 pyruvate, 2 ATP and 2 NADH.

Gluconeogenesis (dotted lines) is the metabolic pathway through which precursors such as lactate, pyruvate, glycerol and amino acids are converted to glucose. Gluconeogenesis requires three bypass steps due to the large negative ∆G˚ for these reactions (pyruvate kinase, phosphofructokinase-1 and hexokinase). The first bypass step is broken down into two enzymatic steps. The first of these occurs in the mitochondria of the cell and is catalyzed by pyruvate carboxylase, while the second is the synthesis of phosphoenolpyruvate (PEP) from oxaloacetate. The oxaloacetate it shuttled into the cytoplasm as malate, once in the cytosol it is oxidized back into oxaloacetate that is then converted to PEP. The second bypass step is the dephosphorylation of fructose 1,6-bisphosphate; it is highly regulated and is a highly exergonic, irreversible hydrolysis reaction. The third bypass step is the dephosphorylation of glucose 6-phosphate; it is present in liver, renal cells and intestinal epithelial cells, but not in muscle or other tissues.

References

Zeile, W. (2015). Lecture 22: Glycolysis [PowerPoint Slide]. Retrieved from: https://ufl.instructure.com/courses/313829/files/23725587?module_item_id=5774537

Zeile, W. (2015). Lecture 23: Gluconeogenesis [PowerPoint Slide]. Retrieved from: https://ufl.instructure.com/courses/313829/files/23725603?module_item_id=5774538

12.1.2 Gluconeogenesis

Introduction

As the name implies, this process calls for the synthesis of new glucose. However, the production of this glucose comes from non-carbohydrate sources such as pyruvate, lactate, glycerol and alanine. Gluconeogenesis for the most part occurs in the liver but the kidney and small intestine may also house pathways for this activity. It is a constant occurrence in the liver in order to maintain blood glucose levels and to meet the demands that organs such as the brain, heart and skeletal muscles have but very little gluconeogenesis takes place in these areas (Ophardt, 2003). Many believe that this procedure is just the reverse reactions of glycolysis and although most steps are similar to these, there are unique properties in particular reactions that only pertain to gluconeogenesis.

Why does This Take Place?

Our body is usually in one of two states, fed or fasted. When we have a constant or timely intake of food throughout the day, such forms get digested and the resulting glucose is then absorbed for direct use in maintaining our blood glucose level. Conversely, when we are not eating and glycogen levels are depleted, absent of filled stores in the liver unlike during the fed state, this is where gluconeogenesis is necessary. Overnight fasting, starvation, pregnancy, lactation, traumatic injury, and exercise are situations in which the demand for this process rises. Hypoglycaemia occurs when one has an abnormally low blood glucose concentration and this poses a particular problem for the body which can be even potentially life-threatening because the brain and red blood cells depend on glucose as a source of energy and the key metabolic pathway that guards against it is gluconeogenesis (Barrit & Wallace, 2002).

Challenges

In the process of glycolysis, ten enzymes are used in the synthesis of pyruvate from glucose. As mentioned before, gluconeogenesis can be seen to be the reverse reactions of glycolysis but there is a key difference. Only seven out of the ten enzymes are the same in both processes because three reactions have to be bypassed. This is due to the fact that in glycolysis, too large of a negative delta G value or free energy change occurs as a result of the reactions catalyzed by pyruvate kinase, phosphofructokinase-1 and hexokinase, which ultimately causes these three reactions to be irreversible. (Diwan, 2007). The new enzymes that are unique to the gluconeogenic pathway are pyruvate carboxylase and phosphoenolpyruvate, fructose-1,6-biphosphate and glucose-6-phosphate which catalyze the reactions for bypass of the pyruvate kinase reaction, phosphofructokinase-1 reaction and hexokinase reaction respectively.

References

Barritt, G.J., Wallace J.C. (2002) Gluconeogensis. Encyclopedia of Life Sciences. Retrieved from http://www.javeriana.edu.co/Facultades/Ciencias/neurobioquimica/libros/metabolismo/m etabolismo_archivos/Gluconeogenesis.pdf

Diwan, J.J. (2007). Gluconeogenesis; Regulation of Glycolysis & Gluconeogensis. Retrieved from https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/gluconeo.htm#intro

Ophardt, C.E. (2003). Glycogenesis, Glycogenolysis and Gluconeogenesis. Retrieved from http://chemistry.elmhurst.edu/vchembook/604glycogenesis.html

The storage of glucose in the body is called glycogen. Glycogen can be converted to glucose in a process called gluconeogenesis when blood glucose levels are low. The biosynthesis of new glucose through non-carbohydrate carbon substrates such as amino acids, pyruvate, and glycerol is called gluconeogenesis. This process occurs mainly in the liver, and to a short extent in the kidney and small intestines. In this process two moles of pyruvate are converted to two moles of biphosphoglycerate, which consumes six moles ATP in the form of energy (King, M 2015). Since so much energy is used this process it is considered very costly from the energy standpoint.

Most of the same enzymes used in glycolysis are used in gluconeogenesis in the synthesis of glucose from pyruvate. The main source of carbon atoms for synthesizing glucose during gluconeogenesis is lactate. Pyruvate becomes reduced to lactate with the aide of lactate dehydrogenase (LDH). For lactate to form, the lactate dehydrogenase needs the help of NADH, and then will yield NAD+ in the process. The NAD+ then becomes readily available so it can then be used by glyceraldehyde-3-phosphate dehydrogenase of the glycolysis reaction. After these two reactions take place they become coupled during anaerobic glycolysis. In the step that follows, the lactate that was produced by the lactate dehydrogenase reaction is released within the bloodstream, transported to the liver and then converted to glucose. The glucose loops back to the blood where it is then used for the muscles as an energy source. This process is known as the Cori cycle (King, M 2015). 

There are three reactions in glycolysis that are essentially irreversible, phosphofructokinase, hexokinase, and pyruvate kinase. In order for gluconeogenesis to occur appropriately those three reactions of glycolysis have to be skipped or bypassed. There is a forward reaction catalyzed by glycolysis enzymes paired with a bypass reaction catalyzed by gluconeogenesis enzymes (Diwan, J 2007). Hexokinase (Glycolysis): Glucose + ATP⇒glucose-6-phosphate + ADP Glucose-6-phosphotase (Gluconeogenesis): Glucose-6-phosphate + H2O⇒glucose + Pi Phosphofructokinase (Glycolysis): Fructose-6-phosphate+ATP⇒fructose-1, 6-bisphosphate+ADP Fructose-1, 6-bispospatase (Gluconeogenesis): Fructose-1, 6-bisphosphtase + H2O⇒fructose-6-phopspate+Pi Pyruvate Kinase (Glycolysis): Phosphoenolpyruvate + ADP ⇒pyruvate + ATP

                                                                             (Diwan, J 2007)

The pathways of both glycolysis and gluconeogenesis are strictly controlled in order to insure that they do not take place at the same time in the same cell. This is important because if both are working at the same time then there will be a loss of 2 ATPs and 2 GTPs and in the end there is no net gain of anything. Pathways that continuously result in zero net change are recognized as a futile cycle (Berg, JM, Tymoczko, JL & Stryer L, 2002).   Reference:

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 16, Glycolysis and Gluconeogenesis.Available from: http://www.ncbi.nlm.nih.gov/books/NBK21150/ Diwan, J. (2007). Gluconeogenesis; Regulation of Glycolysis and Gluconeogenesis. Retrieved from https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/gluconeo.htm King, M. (2015. October 17). Gluconeogenesis: Endogenous Glucose Synthesis. In The Medical Biochemistry Page. Retrieved from http://themedicalbiochemistrypage.org/gluconeogenesis.php#substrates

12.1.3 Lactic Acid

Lactic acid, an organic compound that can be produced naturally or synthetically is a white, water-soluble or clear liquid. In contrast to what the name advocates, it does not come from milk, but it is actually an acid ingredient in sour dairy, fermented fruits and vegetables, and sausages. Found in animals and humans, it is the most important source related to supply energy in muscle tissue and formed from the anaerobic breakdown of glycogen when oxygen supply is inadequate. Many misconceptions about lactic acid state that it is deemed to be one of the causes of both fatigue during exercise and the stiffness felt afterwards. However, it is actually a vital fuel used by the muscles during extensive periods of exercise. (What Is the Fermentation That Happens When Lactic Acid Builds Up and Your Muscles Get Sore? (n.d.). Retrieved December 3, 2015, from http://livehealthy.chron.com/fermentation-happens-lactic-acid-builds-up-muscles-sore-5577.html). Because lactate can be oxidized in another muscle it helps postpone the onset of fatigue and improvement in sport performance. Accumulation of this acid occurs only during a short sessions of exercise that tend to be at high intensity and because it is rapidly removed from the blood and muscle after exercise, it is very unlikely that lactate is the root of muscle soreness. On the other hand, during an exercise that focuses on endurance, the reduction of muscle glycogen causes fatigue. In this situation, lactate can operate as an energy source. By reasoning of its insulin independence, lactate has the capability of providing a readily available energy substrate. The spike in blood lactate levels throughout exercising is eased by acclimation and can be related to enhanced performance. (WebExercises | Does Lactic Acid Cause Muscle Soreness? (n.d.). Retrieved December 3, 2015, from https://www.webexercises.com/2007/08/does-lactic-acid-cause-muscle-soreness.html). Resulting in the idea that training and lactate supplementation has the ability to proliferate the clearance of lactate. Being seen in a wide range of food products, lactic acid is most commonly served as a pH regulator or a preservative and occasionally a flavoring agent. Other than food, lactic acid can be seen used in pharmacy, biomaterials, technically, and detergents. The production of lactic acid comes from sucrose or glucose, water, and lime or chalk, which then goes through lactic acid fermentation with the help of bacteria. Now being considered crude calcium lactate, the removal of gypsum turns the calcium lactate into crude lactic acid with the final process being purification and concentration creating the end product; lactic acid. In reference to the environment pH, lactic acid is one example of a weak acid, which can be present as the acid in its associated form as the ion salt in higher pH or its associated form at a low pH. Most of the lactic acid in the body will dissociate and be present as lactate. Being in the unionized form, the substrates are capable of being abe to pass through the lipid membranes, which is different from the ionized form, unable to pass through the membranes. (Lactate and exercise. (n.d.). Retrieved December 3, 2015, from http://www.lactic-acid.com/lactate_and_exercise.html)

References

What Is the Fermentation That Happens When Lactic Acid Builds Up and Your Muscles Get Sore? (n.d.). Retrieved December 3, 2015, from http://livehealthy.chron.com/fermentation-happens-lactic-acid-builds-up-muscles-sore-5577.html

WebExercises | Does Lactic Acid Cause Muscle Soreness? (n.d.). Retrieved December 3, 2015, from https://www.webexercises.com/2007/08/does-lactic-acid-cause-muscle-soreness.html

Lactate and exercise. (n.d.). Retrieved December 3, 2015, from http://www.lactic-acid.com/lactate_and_exercise.html