WHERE'D THE SUGAR GO?

part i

MARGIE SANCHEZ, SCIENCE CONTRIBUTOR

 

          Jacob is 9 years old and is a typical healthy active boy. He loves baseball, hockey, basketball and video games. Like many of us, Jacob loves eating a bowl of milk and cereal for breakfast. Sometimes he likes almond or soy milk on his cereal, but this morning Jacob is pouring regular 2% cow milk on the Cinnamon Toast Crunch in his bowl.

          Jacob is 9 years old and is a typical healthy active boy. He loves baseball, hockey, basketball and video games. Like many of us, Jacob loves eating a bowl of milk and cereal for breakfast. Sometimes he likes almond or soy milk on his cereal, but this morning Jacob is pouring regular 2% cow milk on the Cinnamon Toast Crunch in his bowl.

The cereal has whole grain and several other kinds of carbohydrates including the sugar called sucrose. Besides protein, Vitamin D, and calcium, cow’s milk contains a sugar called lactose. Digestion for some of the carbohydrates begins in Jacob’s mouth. His saliva contains amylase which is an enzyme that immediately begins to break down the simple starch in the cereal he’s eating. In contrast, the sucrose and lactose remain unchanged until they reach Jacob’s small intestine. Jacob’s physiology is well equipped to assimilate the nutrients and calories he consumes, like the carbohydrates and proteins in his bowl of milk and cereal. The biological pathways of the sugars that Jacob is consuming are particularly fascinating. Through a complex metabolic process of catabolism and anabolism, Jacob’s metabolism will break down, and then use the sugars in his milk and cereal via two major molecular, biochemical physiological pathways known as glycolysis and gluconeogenesis.

The cereal has whole grain and several other kinds of carbohydrates including the sugar called sucrose. Besides protein, Vitamin D, and calcium, cow’s milk contains a sugar called lactose. Digestion for some of the carbohydrates begins in Jacob’s mouth. His saliva contains amylase which is an enzyme that immediately begins to break down the simple starch in the cereal he’s eating. In contrast, the sucrose and lactose remain unchanged until they reach Jacob’s small intestine. Jacob’s physiology is well equipped to assimilate the nutrients and calories he consumes, like the carbohydrates and proteins in his bowl of milk and cereal. The biological pathways of the sugars that Jacob is consuming are particularly fascinating. Through a complex metabolic process of catabolism and anabolism, Jacob’s metabolism will break down, and then use the sugars in his milk and cereal via two major molecular, biochemical physiological pathways known as glycolysis and gluconeogenesis.

Part 1

The Biochemical Pathways of Glycolysis and Gluconeogenesis

     What are glycolysis and gluconeogenesis? In simple terms, the two are opposites of each other. Glycolysis is the process by which glucose is broken down, or catabolized. Gluconeogenesis is the process by which glucose is synthesized, or anabolized. Catabolism and anabolism are ongoing processes of metabolism, regulated constantly by Jacob’s bodily needs throughout the day. Glycolysis is a series of reactions by which glucose or other monosaccharides are catabolized, or broken down, into a simpler molecule of pyruvate. The glycolytic pathway may occur in the presence of oxygen, but oxygen is not necessary for glycolysis.  Pyruvate can generate two molecules of ATP (adenotriphosphate) per one molecule of monosaccharide. ATP, stored in the mitochondria of our cells, is the fuel that meets the cells’ need for energy. The glycolytic pathway occurs in Jacob’s muscles and other tissues. Gluconeogenesis results in the creation, rather than breakdown, of new glucose. To synthesize new glucose, Jacob’s body uses the process of gluconeogenesis to make glucose from precursors like amino acids, glycerol, or lactate. This process occurs mainly in Jacob’s liver via a pathway that is essentially the reverse of glycolysis. In contrast to glycolysis, gluconeogenesis requires oxygen.

Glycolysis

     Glycolysis, also known as the glycolytic pathway, is a 10 step reaction sequence that converts one molecule of glucose into two molecules of pyruvate, a 3-carbon compound.  Phase 1 of the glycolytic pathway facilitates preparation and cleavage of glucose:

·      The 6-carbon glucose molecule is phosphorylated (phosphate is added) twice by ATP

·      Glucose then splits twice to form…

·      Two molecules of glyceraldehyde-3-phosphate

This first phase requires an input of two ATP molecules per one molecule of glucose.

Phase 2 of the glycolytic pathway is the oxidation and ATP generation step:

·      The two molecules of glyceraldehyde-3-phosphate are oxidized to 3-phosphoglycerate

·      Some of the energy of this oxidation is conserved as…

·      Two molecules of ATP and two molecules of NADH are produced

NAD⁺, nicotinamide adenine dinucleotide, is a coenzyme that accepts two electrons and one proton to create a reduced form, NADH. Production of NADH in Phase 2 of the glycolytic pathway is critical because it is an important electron carrier in energy metabolism.

Phase 3 in the glycolytic pathway results in the formation of pyruvate and generation of ATP:

·      The two molecules of 3-phosphoglycerate are converted to…

·      Pyruvate with the accompanying synthesis of…

·      Two more ATP molecules

In the absence of oxygen, glycolysis will result in fermentation. In the presence of oxygen, glycolysis results in aerobic respiration. Glycolysis requires enzymes to catalyze the 10-step reaction. The purpose of the reaction is suggested in its name since the term glycolysis is derived from the Greek words glykos which means sweet and lysis which means loosening or splitting. The splitting occurs in Phase 1, the point where the molecule of glucose, a six carbon compound, is cleaved into two three-carbon molecules. One of these molecules, glyceraldehyde-3-phosphate, is the only molecule that undergoes oxidation in this pathway. Important features of the glycolytic pathway are the initial input of ATP, the sugar-splitting reaction for which the pathway is named, the oxidative event that generates NADH, and the two specific steps at which the reaction sequence is coupled to ATP generation.

      There are other substrates besides glucose that provide the starting point for glycolysis. Thus, by implication, there are alternative compounds that provide the molecules necessary for cellular energy metabolism. Glucose may be the major substrate for both fermentation and respiration for many organisms and tissues. However, for many organisms glucose is rather insignificant. Yet, one important principle remains: regardless of the form and chemical nature of the alternative substrate, it is often converted into an intermediate in the main pathway for glucose metabolism. Alternative substrates include sugars other than glucose. In fact, many other sugars besides glucose are available to cells depending on the food sources of the organisms, including humans. Jacob routinely consumes monosaccharides like the hexose known as fructose and the disaccharide milk sugar known as lactose which contains glucose and galactose. (Remember the lactose in the milk Jacob poured on his cereal? We will return to that point later in this article.) Besides glucose, fructose, and galactose, mannose is another common dietary hexose, or 6 carbon sugar. In general, disaccharides are hydrolyzed into their component monosaccharides, and each monosaccharide is converted to a glycolytic intermediate in one or a few steps. Glucose and fructose enter most directly after phosphorylation on carbon atom 6. Mannose is converted to mannose-6-phosphate, and then to fructose-6-phosphate, a glycolytic intermediate. Phosphorylated pentose (5 carbons) sugars may also be funneled into the glycolytic pathway, although they must first be converted to hexose phosphates.  Besides other sugars, there are also compounds like glycerol that can be used in the glycolytic pathway. Glycerol, a 3 carbon molecule resulting from a lipid (fat) breakdown, begins glycolysis after conversion to dihydroxyacetone phosphate.

     Although glucose is the immediate substrate for both fermentation and respiration for many cells and tissues, it is not largely present in cells as the free monosaccharide. Rather, it occurs mainly in the form of storage polysaccharides, most commonly as starch in plants and as glycogen in animals (including humans). It is advantageous to store glucose as starch and glycogen because both these glucose polymers are insoluble in water. Thus, they don’t overload the limited solute capacity of our cells.  These storage polysaccharides can be mobilized by a process called phosphorolytic cleavage. Phosphorolysis is similar to hydrolysis (breaking or splitting water) but uses inorganic phosphate instead of water break a chemical bond.

·      Inorganic phosphate breaks the α (1→4) bond between successive glucose units

·      Glucose monomers are liberated as glucose-1-phosphate

·      Glucose-1-phosphate is converted to glucose-6-phosphate

·      Glucose-6-phosphate can now be catabolized by the glycolytic pathway

Glucose stored in polymerized form like glycogen enters the glycolytic pathway as glucose-6-phosphate WITHOUT the input of ATP required for the initial phosphorylation of the free sugar. Therefore, the overall yield for glucose is greater by one molecule of ATP when it is catabolized from a polysaccharide than when it is catabolized from the free sugar as the starting substrate. However, energy is required in the polymerization process of the polysaccharide to activate the glucose units that are added to the growing chain of glycogen during its polysaccharide synthesis.

Thus, the typical cell has the capability to convert most naturally occurring sugars as well as other compounds to one of the glycolytic intermediates for further catabolism under aerobic (the presence of oxygen) or anaerobic (no oxygen) conditions.

Gluconeogenesis

    As we have already covered, cells can metabolize glucose and other carbohydrates to address their energy needs. In addition, cells can synthesize sugars and polysaccharides needed for other purposes. The process of glucose synthesis is known as gluconeogenesis, which means the “genesis” or formation of new glucose. Specifically, gluconeogenesis is defined as the process by which cells synthesize glucose from the 3 and 4-carbon precursors that are usually non-carbohydrates by nature. The most common starting material is pyruvate, an end product of glycolysis, or lactate into which pyruvate is converted under anaerobic (without the presence of oxygen) conditions.

    Remember, gluconeogenesis is the opposite of glycolysis so the two pathways have much in common. In fact, the two pathways share seven of the same reactions. In other words, seven of the reactions in the gluconeogenic pathway occur as a reversal of the corresponding reactions in glycolysis. In each case, the same enzyme is used in both directions. BUT, not all of the steps in gluconeogenesis are a simple reversal of the glycolytic reactions. Three of the reactions-the first, third, and tenth- are reacted by other means in the direction of gluconeogenesis. These are the three most exergonic (energy-releasing, negative free energy change) reactions of glycolysis, making them very thermodynamically difficult to reverse. In fact, biosynthetic pathways are seldom a simple reversal of the corresponding catabolic pathways.  This basic principle is best demonstrated by the energy requirements in each direction.

     If a particular metabolic reaction is thermodynamically favorable in a specific direction, it has to be sufficiently exergonic in that direction. This principle certainly runs true in the glycolytic pathway. The overall sequence from glucose to pyruvate has a ∆G’ value of about -20 kcal/mol under typical intracellular conditions in the human body. So, ∆G’ for the reverse process would be approximately +20 kcal/mol, rendering glucose synthesis by the direct reversal of glycolysis highly endergonic (energy consuming) and, therefore, thermodynamically impossible.  BUT…

     Gluconeogenesis IS possible because the three most important exergonic reactions in the glycolytic pathway do not actually just run in reverse in the gluconeogenic direction. Instead, the gluconeogenic pathway has bypass reactions at each of those three sites. These alternative reactions effectively bypass the three glycolytic reactions that would be the most difficult to reverse. Indeed, these three reactions are always illustrated as unidirectional (one arrow pointing right) in the chemical equations for glycolysis. For each of these instances, the bypass reactions of gluconeogenesis circumvent the irreversibility of the glycolytic step.

   In Gly (glycolysis) -1 and Gly-3, ATP synthesis requirements in the reverse direction are bypassed by a simple hydrolytic reaction which overcomes the thermodynamic hurdle. For example, in the case of the interconversion of glucose and glucose-6-phosphate the reaction is exergonic in the glycolytic direction because of the input of an ATP molecule. AND in the gluconeogenic direction exergonicity is maintained through simple hydrolysis of the phosphoester bond.

   The third site of irreversibility in the glycolytic pathway is at the reaction Gly-10. But this is bypassed in gluconeogenesis with a two-reaction sequence. Both of these reactions are driven by the hydrolysis of a phosphoanhydride, from ATP in one case and from the related compound GTP (guanosine triphosphate). The first of the two steps is the addition of CO₂ (carbon dioxide) to pyruvate, a carboxylation reaction. This step results in the formation of a four-carbon compound called oxaloacetate. In the second step, the carboxyl group is removed in a decarboxylation reaction to form PEP (phosphoenolpyruvate). In this case, the phosphate group and the energy are both provided by GTP, the energetically equivalent of ATP.

     The impact of these bypass reactions are best demonstrated when the pathways for glycolysis and gluconeogenesis are directly compared. Glycolysis utilizes two molecules of ATP but generate four molecules of ATP, a net yield of two ATP molecules for every molecule of glucose catabolized. On the other hand, gluconeogenesis requires four molecules of ATP and two of GTP per glucose synthesized. The difference of four ATP molecules per glucose accounts for enough energy to ensure that gluconeogenesis runs at least as exergonically in the direction of glucose synthesis as glycolysis does in the direction of glucose breakdown.

   To summarize, the pathways for glycolysis and gluconeogenesis have nine intermediate and seven enzyme-catalyzed reactions in common. The three irreversible reactions in the glycolytic pathway are circumvented in gluconeogenesis through the facilitation of four bypass reactions. As a catabolic pathway, glycolysis is definitely exergonic, yielding two ATP molecules for every glucose molecule. In contrast, gluconeogenesis is an anabolic pathway and requires the coupled hydrolysis of six phosphoanhydride bonds – four from ATP, two from GTP- in order to drive it to form glucose. Glycolysis occurs in muscle and other tissues, while gluconeogenesis occurs mostly in the liver.

   So what does glycolysis and gluconeogenesis mean in terms of every day events in the human body? Let’s follow the two sugars Jacob is eating in his bowl of milk and cereal.