Chapter Twenty ThreeCarbohydrate Metabolism

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Outline► 23.1 Digestion of Carbohydrates► 23.2 Glucose Metabolism: An Overview► 23.3 Glycolysis► 23.4 Entry of Other Sugars into Glycolysis► 23.5 The Fate of Pyruvate► 23.6 Energy Output in Complete Catabolism of Glucose► 23.7 Regulation of Glucose Metabolism and Energy

Production► 23.8 Metabolism in Fasting and Starvation► 23.9 Metabolism in Diabetes Mellitus► 23.10 Glycogen Metabolism: Glycogenesis and

Glycogenolysis► 23.11 Gluconeogenesis: Glucose from Noncarbohydrates

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23.1 Digestion of Carbohydrates

► The first stage in catabolism is digestion, the breakdown of food into small molecules.

► Digestion entails the physical grinding, softening, and mixing of food, as well as the enzyme-catalyzed hydrolysis of carbohydrates, proteins, and fats.

► Digestion begins in the mouth, -amylase in saliva catalyzes hydrolysis of the glycosidic bonds in carbohydrates. Salivary -amylase continues to act on polysaccharides in the stomach until, after an hour or so, it is inactivated by stomach acid. No further carbohydrate digestion takes place in the stomach.

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► -Amylase is secreted by the pancreas and enters the small intestine, where conversion of polysaccharides to maltose continues.

► Enzymes from the mucous lining of the small intestine hydrolyze maltose, sucrose and lactose to glucose, fructose, and galactose, which are then transported across the intestinal wall into the bloodstream.

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23.2 Glucose Metabolism: An Overview

► When glucose enters a cell from the bloodstream, it is immediately converted to glucose 6-phosphate.

► Once this phosphate is formed, glucose is trapped within the cell because phosphorylated molecules cannot cross the cell membrane.

► Like the first step in many metabolic pathways, the formation of glucose-6-phosphate is highly exergonic and not reversible in the glycolytic pathway, thereby committing the initial substrate to subsequent reactions.

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► Glucose-6-phosphate can enter the pentose phosphate pathway. This multistep pathway yields two products of importance to our metabolism.

► One is a supply of the coenzyme NADPH, a reducing agent that is essential for various biochemical reactions.

► The other is ribose 5-phosphate, which is necessary for the synthesis of nucleic acids (DNA and RNA).

► Glucose-6-phosphate enters the pentose phosphate pathway when a cell’s need for NADPH or ribose-5-phosphate exceeds its need for ATP.

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► When cells are already well supplied with glucose, the excess glucose is converted to other forms for storage: to glycogen, the glucose storage polymer, by the glycogenesis pathway, or to fatty acids by entrance of acetyl-SCoA into the pathways of lipid metabolism rather than the citric acid cycle.

► When energy is needed, glucose 6-phosphate undergoes glycolysis to pyruvate and then to acetyl-S-coenzyme A, which enters the citric acid cycle.

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Glucose 6-phosphate can be converted to pentose products, stored as glycogen, or broken down to acetyl-SCoA for production of energy, proteins, or fats.

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23.3 Glycolysis

► Glycolysis is a series of 10 enzyme-catalyzed reactions that break down glucose molecules.

► The net result of glycolysis is the production of two pyruvate molecules, two ATPs, and two NADH/H+s.

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► Steps 1-5 of glycolysis break one glucose molecule down into two

D-glyceraldehyde 3-phosphate fragments.

► An investment of 2 ATP molecules is required.

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►Steps 6-10 occur twice for each glucose that enters in at step 1.

►Steps 6-10 produce:

2 pyruvates,

4 ATPs

2 NADH/H+ per glucose.

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23.4 Entry of Other Sugars into Glycolysis

► The major monosaccharides from digestion other than glucose also eventually join the glycolysis pathway.

► Fructose, from fruits or hydrolysis of the disaccharide sucrose, is converted to glycolysis intermediates in two ways: - In muscle, it is phosphorylated to fructose 6-

phosphate.- In the liver, it is converted to glyceraldehyde 3-

phosphate.

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Mannose is a product of the hydrolysis of plant polysaccharides other than starch.

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► Mannose is converted by hexokinase to a 6-phosphate, which then undergoes a multistep, enzyme-catalyzed rearrangement and enters glycolysis as fructose 6-phosphate.

► Galactose from hydrolysis of the disaccharide lactose is converted to glucose 6- phosphate by a five-step pathway.

► A hereditary defect affecting any enzyme in this pathway can be a cause of galactosemia.

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23.5 The Fate of Pyruvate

► The conversion of glucose to pyruvate is a central metabolic pathway in most living systems.

► The further reactions of pyruvate depend on metabolic conditions and on the nature of the organism.

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► Aerobic: In the presence of oxygen.► Under normal oxygen-rich (aerobic) conditions,

pyruvate is converted to acetyl-SCoA.► Pyruvate diffuses across the outer mitochondrial

membrane, then is carried by a transporter protein across the inner mitochondrial membrane.

► Once inside, pyruvate dehydrogenase complex catalyzes the conversion of pyruvate to acetyl-SCoA.

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► Anaerobic: In the absence of oxygen.► If electron transport slows because of insufficient

oxygen, NADH concentration increases, NAD+ is in short supply, and glycolysis cannot continue.

► An alternative way to reoxidize NADH is essential because glycolysis, the only available source of fresh ATP, must continue. The reduction of pyruvate to lactate solves the problem.

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► NADH serves as the reducing agent and is reoxidized to NAD+ which is then available in the cytosol for glycolysis. Lactate formation serves no purpose other than NAD+ production, and the lactate is reoxidized to pyruvate when oxygen is available.

► Microorganisms often must survive in the absence of oxygen and have evolved numerous anaerobic strategies for energy production, generally known as fermentation.

► When pyruvate undergoes fermentation by yeast, it is converted into ethanol plus carbon dioxide. This process, known as alcoholic fermentation, is used to produce beer, wine, and other alcoholic beverages and also to make bread.

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23.6 Energy Output in Complete Catabolism of Glucose

► The total energy output from oxidation of glucose is the combined result of - (a) glycolysis- (b) conversion of pyruvate to acetyl-SCoA- (c) conversion of two acetyl groups to four

molecules of CO2 in the citric acid cycle- (d) the passage of reduced coenzymes from each

of these pathways through electron transport and the production of ATP by oxidative phosphorylation.

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► The sum of the net equations for each pathway that precedes oxidative phosphorylation is shown below.

► Since each glucose yields 2 pyruvates and 2 acetyl-SCoAs, the net equations for pyruvate oxidation and the citric acid cycle are multiplied by 2.

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► The total number of ATPs per glucose molecule is the 4 ATPs from glucose catabolism plus the number of ATPs produced for each reduced coenzyme that enters electron transport.

► For a long time, based on the belief that 3 ATPs are generated per NADH and 2 ATPs per FADH2 the maximum yield was taken as 38 ATPs.

► 10 NADH(3ATP/NADH) + 2 FADH2(2ATP/FADH2) + 4 ATP = 38 ATP

► The 38 ATPs per glucose molecule are viewed as a maximum yield of ATP, most likely possible in bacteria and other prokaryotes. In humans and other mammals, the maximum is most likely 30–32 ATPs per glucose molecule.

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23.7 Regulation of Glucose Metabolism and Energy Production

► Normal blood glucose concentration a few hours after a meal ranges roughly from 65 to 110 mg/dL.

► Hypoglycemia: Lower-than normal blood glucose concentration.

► Hyperglycemia: Higher-than normal blood glucose concentration.

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► Low blood glucose (hypoglycemia) causes weakness, sweating, and rapid heartbeat, and in severe cases, low glucose in brain cells causes mental confusion, convulsions, coma, and eventually death. The brain can use only glucose as a source of energy. At a blood glucose level of 30 mg/dL, consciousness is impaired or lost, and prolonged hypoglycemia can cause permanent dementia.

► High blood glucose (hyperglycemia) causes increased urine flow as the normal osmolarity balance of fluids within the kidney is disturbed. Prolonged hyperglycemia can cause low blood pressure, coma, and death.

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► Two hormones from the pancreas have the major responsibility for blood glucose regulation.

► The first, insulin, is released when blood glucose concentration rises.

► The second hormone, glucagon, is released when blood glucose concentration drops.

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23.8 Metabolism in Fasting and Starvation

► The metabolic changes in the absence of food begin with a gradual decline in blood glucose concentration accompanied by an increased release of glucose from glycogen.

► All cells contain glycogen, but most is stored in liver cells (about 90 g in a 70-kg man) and muscle cells (about 350 g in a 70-kg man). Free glucose and glycogen represent less than 1% of our energy reserves and are used up in 15–20 hours of normal activity (3 hours in a marathon race).

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►During the first few days of starvation, protein is used up at a rate as high as 75 g/day.

►Lipid catabolism is mobilized, and acetyl-SCoA molecules derived from breakdown of lipids accumulate.

►Acetyl-SCoA begins to be removed by a new series of metabolic reactions that transform it into ketone bodies.

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As starvation continues, the brain and other tissues are able to switch over to producing up to 50% of their ATP from catabolizing ketone bodies instead of glucose. By the 40th day of starvation, metabolism has stabilized at the use of about 25 g of protein and 180 g of fat each day. So long as adequate water is available, an average person can survive in this state for several months; those with more fat can survive longer.

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23.9 Metabolism in Diabetes Mellitus

► Diabetes mellitus: A chronic condition due to either insufficient insulin or failure of insulin to activate crossing of cell membranes, by glucose.

► The symptoms by which diabetes is usually detected are excessive thirst accompanied by frequent urination, abnormally high glucose concentrations in urine and blood, and wasting of the body despite a good diet. These symptoms result when available glucose does not enter cells where it is needed.

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► Type II diabetes is thought to result when cell membrane receptors fail to recognize insulin. Drugs that increase either insulin or insulin receptor levels are an effective treatment because more of the undamaged receptors are put to work.

► Type I diabetes is classified as an autoimmune disease, a condition in which the body misidentifies some part of itself as an invader. Gradually, the immune system wrongly identifies pancreatic beta cells as foreign matter, develops antibodies to them, and destroys them. To treat Type I diabetes, the missing insulin must be supplied by injection.

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► Diabetic individuals are subject to several serious conditions that result from elevated blood glucose levels. Excess glucose is reduced to sorbitol.

► Sorbitol is not transported out of the cell. Its rising concentration increases the osmolarity of fluid in the eye, causing increased pressure, cataracts, and blindness. Elevated sorbitol is also associated with blood vessel lesions and gangrene in the legs.

► Ketoacidosis results from the buildup of acidic ketones in the late stages of uncontrolled diabetes. It can lead to coma and diminished brain function.

► Hypoglycemia by contrast, may be due to an overdose of insulin or failure to eat. Diabetic hypoglycemia can cause nerve damage or death.

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23.10 Glycogen Metabolism: Glycogenesis and Glycogenolysis

► Glycogen synthesis, known as glycogenesis, occurs when glucose concentrations are high.

► Glucose 6-phosphate is first isomerized to glucose 1-phosphate.

► The glucose residue is then attached to uridine diphosphate (UDP):

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The resulting glucose-UDP transfers glucose to a growing glycogen chain in an exergonic reaction.

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►Glycogenolysis: The biochemical pathway for breakdown of glycogen to free glucose.

►Glycogenolysis occurs in muscle cells when there is an immediate need for energy.

►Glycogenolysis occurs iin the liver when blood glucose is low.

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23.11 Gluconeogenesis: Glucose from Noncarbohydrates

► Gluconeogenesis, which occurs mainly in the liver, is the pathway for making glucose from noncarbohydrate molecules—lactate, amino acids, and glycerol.

► This pathway becomes critical during fasting and the early stages of starvation. Failure of gluconeogenesis is usually fatal.

► During exercise lactate produced in muscles under anaerobic conditions during exercise is sent to the liver, where it is converted back to glucose.

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Gluconeogenesis requires energy, so shifting this pathway to the liver frees the muscles from the burden of having to produce even more energy.

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► Steps 1, 3, and 10 in glycolysis are too exergonic to be directly reversed. Gluconeogenesis uses reactions catalyzed by different enzymes that reverse these steps. The 7 other steps of glycolysis are reversible because they operate at near-equilibrium conditions.

► Gluconeogenesis begins with conversion of pyruvate to phosphoenolpyruvate, the reverse of the highly exergonic step 10 of glycolysis. Two steps are required, utilizing two enzymes and the energy provided by two triphosphates, ATP and GTP.

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Chapter Summary► Carbohydrate digestion, the hydrolysis of

disaccharides and polysaccharides, begins in the mouth and continues in the stomach and small intestine. The products that enter the bloodstream from the small intestine are monosaccharides—mainly glucose, fructose, and galactose.

► Glucose is converted to glucose 6-phosphate and undergoes glycolysis to pyruvate, which is fed into the citric acid cycle via acetyl-SCoA. One alternative pathway for glucose is glycogenesis, the synthesis of glycogen. Another is the pentose phosphate pathway, which provides the five-carbon sugars and NADPH needed for biosynthesis.

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Chapter Summary Cont.► Glycolysis is a 10-step pathway that produces two

molecules of pyruvate, two reduced coenzymes (NADH), and two ATPs for each molecule of glucose metabolized.

► When oxygen is in good supply, pyruvate is transported into mitochondria and converted to acetyl-SCoA for energy generation via the citric acid cycle and oxidative phosphorylation.

► When there is insufficient oxygen, pyruvate is reduced to L-lactate, with the production of the oxidized coenzyme NAD+ that is essential to the continuation of glycolysis.

► In the presence of yeast, pyruvate undergoes anaerobic fermentation to yield ethyl alcohol.

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Chapter Summary Cont. ► Insulin, produced when blood glucose concentration

rises, accelerates glycolysis and glycogen synthesis. Glucagon, produced when blood glucose concentration drops, accelerates production of glucose in the liver from stored glycogen and from other precursors via the gluconeogenesis pathway.

► Adaptation to starvation begins with the effects of glucagon and energy production from protein, and then proceeds to reliance on ketone bodies from fatty tissue for energy generation.

► Diabetes mellitus may be insulin-dependent or non-insulin-dependent. Among the serious outcomes of uncontrolled diabetes are cataracts, blood vessel lesions, ketoacidosis, and hypoglycemia.

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Chapter Summary Cont.► Glycogenesis, the synthesis of the polysaccharide

glycogen, puts excess glucose into storage, mainly in muscles and the liver.

► Glycogenolysis is the release of stored glucose from glycogen. Glycogenolysis occurs in muscles when there is an immediate need for energy. It occurs in the liver when blood glucose concentration is low.

► Gluconeogenesis maintains glucose levels by synthesizing it from lactate, from certain amino acids derived from proteins, and from glycerol derived from fatty tissue; it is critical during fasting and starvation.

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End of Chapter 23