CHO Metabolism

CHO Metabolism

Dr. Wajdy J. Majid Assit.Prof. in clinical biochemistry College of Medicine University of Thi-Qar

Digestion of dietary CHO

The principal source of carbohydrate in the diet include the polysaccharide starch and glycogen, which are based on glucose units linked by α-glucosidic links.The first step in metabolism of digestible CHO is the conversion of higher polymers to simpler , soluble forms that can be transported across the intestinal wall and delivered to the tissue.The breakdown of polymeric sugar begin in the mouth , saliva has a slightly acidic PH of 6.8 and contain lingual amylase that begins the digestion of CHO the action of salivary amylase is limited to the area of the mouth and esophagus , it is inactivated by stronger acid PH of the stomach once the food has arrived in the stomach , acid hydrolysis contributes to its degradation , specific gastric protease and lipase aid this process for protein and fat , respectively the mixture of gastric secretion, saliva and food known collectively as chyme which move to the small intestine.

The main polymeric CHO digesting enzyme of S.I is α-amylase this enzyme secreted by pancreas and has the same activity as salivary amylase producing disaccharide and trisaccharide , the later are converted to monosaccharide by intestinal disaccharidase including maltase that hydrolyse di and trisaccharide , and more specific disaccharidase sucrase, lactase and trehalase, the net result is conversion of digestible CHO to constituent monosaccharides. The resultant glucose and other simple CHO are absorbed by S.I. and transport to the liver and other tissue .they are converted to fatty acid , amino acid and glycogen, or else oxidized by various catabolic pathways of cells . the oxidation of glucose called glycolysis, glucose is oxidized either to pyruvate or lactate .

Site of Glycolysis

Enzymes of glycolysis are present in the cytosol of most of the cells present in the body .
Source of Glucose Dietary glucose formed from the digestion of dietary carbohydrates enter liver through portal venous system after its absorption from the intestine. Liver distributes glucose to all other organs (cells) of the body.

Entry of the Glucose in to the Cells Entry of the Glucose in to the Cells Glucose enters cells by facilitated transport. 1. Liver Glucose enters liver cells by facilitated diffusion. It is an insulin-independent transport mechanism for the transport of glucose across liver cells. 2- Extra hepatic tissues Glucose enters adipocytes, erythrocytes, brain and skeletal muscle by facilitated transport involving carrier molecule. The transport of glucose across the membranes of adipose tissue and skeletal muscle by carrier is dependent on insulin.

Reactions of Glycolysis

Initial reaction of glycolysis is catalyzed by hexokinase. It is widely distributed. It phosphorylates glucose at 6 carbon in presence of Mg2+ and ATP. Hexokinase is an allosteric enzyme. The reaction catalyzed by this enzyme is irreversible under normal physiological conditions . One high energy bond of ATP is used in this reaction to generate glucose-6-phosphate. Liver contains glucokinase, which phosporylates only glucose. It is an inducible enzyme. Its Km for glucose is high compared to Km of hexokinase. Hence, it phosphorylates glucose only when blood glucose concentration is high. Liver hexokinase phosphorylates glucose even blood glucose is low.



Energetics of Glycolysis
Degradation of glucose to two molecules of pyruvate or lactate by sequence of enzyme catalyzed reactions constitutes the process of glycolysis. It is a catabolic pathway. If glucose is degraded to pyruvate then it is called as aerobic glycolysis. Usually it occurs in presence of oxygen. If glucose is degraded to lactate then it is anaerobic glycolysis , usually it occurs in the absence of oxygen.Generation and consumption of ATP in anaerobic and aerobic glycolysis is given below : In aerobic glycolysis: 1. Number of ATPs generated by phosphoglycerate kinase 2 2. Number of ATPs generated by Pyruvate kinase 23. Number of ATPs generated by respiratory chain oxidation of 2 NADH produced in reaction 6 4. Number of ATPs consumed in reaction 1 and 3 –2 Net result = 8

In anaerobic glycolysis : 2 NADH produced in reaction 6 are used to convert pyruvate to lactate. Hence, ATP is not generated. Therefore, the net ATP production in anaerobic glycolysis is only 2 (8 – 6 = 2). Thus, oxidation of glucose to pyruvate (aerobic glycolysis) generates 8 ATP molecules whereas oxidation of glucose to lactate (anaerobic glycolysis) generates 2 ATP molecules.


Usually metabolic pathways are regulated by altering activities of few enzymes of that pathway. Glycolysis is under allosteric and hormonal control. Hexokinase , phosphofructo kinase and pyruvate kinase are regulatory enzymes of glycolysis. Their activities are allosterically controlled. Further glucokinase, phosphofructokinase-1 and pyruvate kinase are under hormonal control also. -Allosteric regulation of glycolysis Phosphofructokinase-1 is the major regulatory enzymes of glycolysis. It is an allosteric enzyme and catalyzes rate limiting reaction of glycolysis. It is inhibited by ATP and citrate. AMP and fructose-6-phosphate are activators of this enzyme . Pyruvate kinase is the second regulatory enzyme. It is inhibited by ATP and phosphoenolpyrvate.
Regulation of glycolysis :

Glucose 6-phosphate inhibits activity of hexokinase. So, when ATP (energy) concentration is high glycolysis is inhibited and decrease in ATP level increases rate of glycolysis.

The citric acid cycle ( catabolism of acetyl-CoA) : The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a series of reactions in the mitochondria that oxidize acetyl CoA to formation of ATP . The TCA cycle is the final common pathway for aerobic oxidation of CHO , lipid and protein because glucose , fatty acid and most amino acid are metabolized to acetyl CoA or intermediate of the cycle.

Regulation of Citric Acid CycleEnzymes of citric acid cycle are under allosteric control. Citrate synthase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are involved in the regulation of citric acid cycle and their activities are allosterically regulated. Citrate synthase activity is inhibited by ATP and long chain acyl-CoA. Isocitrate dehydrogenase is inhibited by ATP and NADH and activated by ADP. Succinyl-CoA and NADH are allosteric inhibitors of third regulatory enzyme α-ketoglutarate dehydrogenase. So the rate of citric acid cycle increases in the absence of ATP and decreases in the presence of ATP and NADH. The energy demand of cell determines the rate of citric acid cycle

Glycogen metabolism :Glycogen is a highly branched polysaccharide composed of D-glucose unit joined to each other by glycosidic bond, the major linkage are α-1,4 glycosidic bond , at interval of about ten units , there are branches in the chain involving α-1,6 linkage , each branch then continue with α-1,4 linkage . The glycogen is the store of excess glucose to supply the tissue with an oxidizable energy source are found principally in the liver as glycogen . The second major source of stored glucose is the glycogen of skeletal muscles. Muscle glycogen is not generally available to other tissue because muscle lack the enzyme glucose-6-phosphatase. The major site of daily glucose consumption ( 75% ) is the brain via aerobic pathway , most of remainder of it is utilized by erythrocyte , skeletal muscle and heart muscle.

The body obtained glucose either directly from diet or from amino acid and lactate via gluconeogensis . Glucose obtained from these two primary sources either remains soluble in the body fluids or stored as glycogen . Glycogen is considered the principal storage form of glucose and found mainly in the liver and muscle , with kidney and intestine adding minor storage sites. Stores of glycogen in the liver are consider the main buffer of blood glucose levels.

GLYCOGENESIS Glycogenesis is the synthesis of glycogen from glucose .Site : glycogen it chiefly occurs in liver and skeletal muscle. In the muscle, about 245 gms of glycogen and in the liver about 72 gms of glycogen is stored under well fed condition. Even though-energy, rich fat is abundant in the body skeletal muscle prefers to store glucose (energy) as glycogen because :1. Fat can not be oxidized under anaerobic condition . 2. Acetyl-CoA of fat oxidation can not be converted to glucose.3. Skeletal muscle is unable to mobilize fat rapidly.The rate of( glycogensis) may be increased by insulin which is secreted by β-cells of the pancreas in response to systemic hyperglycemia and stored as glycogen in the liver and also the excess glucose enter the muscle under influence of insulin and stored as glycogen.


Glycogenolysis : Degredation of stored glycogen ( glycogenolysis) occur by the action of glycogen phosphorylase , the phosphorylase is remove single glucose residue from α(1,4)-linkage within the glycogen molecules. The product of this reaction is glucose -1-phosphate , which converted to G6P by phosphoglucomutase the conversion of G6P to glucose which occur in the liver , kidney and intestine by the action of enzyme glucose-6-phosphatase which does not occur in skeletal muscle as these cells because lack this enzyme . Therefore any glucose released from glycogen stores of muscle will be oxidized in the glycolytic pathway. In the liver the action of G-6-phosphatase allow glycogenolysis to generate free glucose for maintaining blood glucose levels.

Glycogen phosphorylase can not remove glucose residue from the branch points (α-1,6 linkage) in glycogen . the activity of phosphorylase cease 4 glucose residue from the branch point . The removal of these branch point glucose residue require the action of debranching enzyme ( also called glucan transferase) which contain 2 activities :glucotransferase and glucosidase , the transferase activity remove the terminal 3- glucose residue of one branch and attaches them to a free C-4 end of a second branch. The glucose in α-(1,6)-linkage at the branch is then removed by action of glucosidase.

Hormonal Regulation of Glycogen Metabolism

Epinephrine and glucagon increases cAMP mediated phosphorylation, which in turn converts inactive glycogen phosphorylase B to active phosphorylase A , as a result glycogenolysis is enhanced. At the same time cAMP mediated phosphorylation converts active glycogen synthase A to inactive glycogen synthase B which results in decreased glycogenesis. Insulin: decreases glycogenolysis by decreasing cAMP mediated phosphorylation. At the same time insulin favours dephosphorylation of glycogen synthase B , which results in formation of more glycogen synthase A and increased glycogenesis.

Medical Importance of Hormonal Regulation of Glycogen Metabolism In between meals hypoglycemia induces glucagon production , Glucagon causes breakdown of glycogen in liver to maintain supply of glucose to brain and cardiac muscle. Epinephrine causes breakdown of glycogen in skeletal muscle to maintain fuel supply for muscle contraction. After a meal, hyperglycemia induces insulin secretion. Insulin causes inactivation of enzymes of glycogenolysis and activation of glycogen forming enzymes. As a result glycogenesis occurs in liver and muscle.


Glycogen Storage Diseases : These are group of inherited (genetic) diseases of glycogen metabolism. In these diseases, there is an abnormal accumulation of large amount of glycogen or its metabolites in the tissues due to deficiency or absence of enzymes of glycogen metabolism. Some of them are not serious mild disorders but few of them are fatal.

a ) Von Geirke’s disease (Type-1 glycogen storage disease) : It is due to the deficiency of glucose-6-phosphatase in liver, kidney and intestine. The incidence of this disease is 1 in 2,00,000. it is lead to accumulation of glycogen in liver and kidney and enlargement of liver occurs. Hypoglycemia is common symptom other symptoms are hyperuricemia, hyperlipemia and ketosis.b) Pompe’s disease (Type-II) It is due to deficiency of lysosomal α-glucosidase. Accumulation of glycogen occurs in all tissues. Accumulation of glycogen in heart leads to cardiomegaly. It is a fatal disorder and death occurs before second year of life due to cardio respiratory failure.

C) Cori’s disease (Type-III) : It is due to deficiency of debranching enzyme. Limit dextrin a metabolite of glycogenolysis accumulates in liver. Hence, this condition is also called as limit dextrnosis.d ) Anderson’s disease (Type-IV): It is a fatal disease. It is due to absence of branching enzyme. Amylopectin an intermediate of glycogenesis accumulatres in liver, spleen and heart. Hence, this condition is called as amylopectinosis.e) Mc Ardle’s syndrome (Type-V): It is due to the absence of muscle phosphorylase. Glycogen accumulates in muscle and lactic acid production in muscle is not increased after exercise. Affected person suffer from painful muscle cramps and diminished tolerance to exercise.f) Her’s disease (Type-VI): It is due to the absence of liver phosphorylase. Glycogenolysis is defective and glycogen accumulates in liver.

Gluconeogensis : It is the biosynthesis of new glucose ( not glucose from glycogen) , the production of glucose from other metabolite is necessary for use as fuel source by the brain , testis, erythrocyte and kidney medulla since glucose is the sole energy source for these organs , it is the process that converts non-carbohydrate substance to glucose. Glucose is synthesized from pyruvate, which is derived from glucogenic amino acids, intermediates of TCA cycle and glycerol. Since pyruvate can be formed from lactate by the reversal of lactate dehydrogenase reaction synthesis of glucose occurs from lactate also. Gluconeogenesis is an energy-consuming process . - Gluconeogenesis occurs mainly in the liver and kidney. -Enzymes of gluconeogenesis are present in mitochondria and cytosol .


Substrate for gluconeogensis : lactate : is a predominant source for glucose synthesis by gluconeogensis . during anaerobic glycolysis in skeletal muscle , pyruvate is reduced to lactate by lactate dehydrogenase (LDH) this reduction serves two critical function during anaerobic glycolysis first , in the direction of lactate formation the LDH reaction require NADH and yield NAD which then available for use by glyceraldehydes -3- phosphate dehydrogenase reaction of glycolysis .secondly , the lactate produced by the LDH reaction is released to blood stream and transport to the liver where it is converted to glucose , the glucose then returned to the blood for use by muscle as an energy source and to replenish glycogen stores. This cycle is termed the Cori cycle.



pyruvate : pyruvate generated in muscle and other peripheral tissue can transaminated to alanine which is returned to the liver for gluconeogensis. Within the liver alanine is converted back to pyruvate and used as substrate for gluconeogensis (if that is hepatic requirement ), or oxidized in TCA cycle. Amino acids : All 20 of the amino acid except leucine and lysine can degraded to TCA cycle intermediate. This allow the amino acid to be converted to those in oxaloacetate and then into pyruvate , the pyruvate can be utilized by gluconeogensis . When glycogen store are depleted in muscle during exertion and in liver during fasting , so catabolism of muscle protein to amino acid contribute the major source of carbon for maintenance of blood glucose levels .

glycerol : In the liver, glycerol is converted to dihydroxyacetone phosphate which enters pathway of gluconeogenesis , the glycerol backbone of lipid can be used for gluconeogensis, this require phosphorylation to glycerol 3-phosphate by glycerol kinase and dehydrogenation to (DHAP) by glyceraldehydes-3-phosphate dehydrogenase (G3PDH) . so the triacylglycerol which stored in adipose tissue can be used its glycerol as substrate for gluconeogensis.propionate :Oxidation of fatty acid with an odd number of carbon atoms and oxidation of some amino acid generate as the terminal oxidation product (propionyl-CoA) ,propionyl –CoA is converted to the TCA intermediate , (succinyl-CoA) this conversion is carried out by the ATP-requiring enzyme, propionyl-CoA carboxylase.


Medical and Biological Importance of gluconeogenesis : Gluconeogenesis meets the glucose requirement of body when carbohydrate is in short supply i.e., during fasting and starvation. 2.Tissues like brain, skeletal muscle, erythrocytes and testis are completely depend on glucose for energy and hence decrease in glucose supply cause brain dysfunction. Body glycogen can meet glucose requirement for only 24 hours so, beyond that period gluconeogenesis ensures glucose supply to these organs. 3. Gluconeogenesis clears metabolic products of other tissues from blood , for example, lactate produced by erythrocytes, skeletal muscle, glycerol produced by breakdown of adipose tissue TG and a . a. produced by muscle protein breakdown. 4. Gluconeogenesis converts excess of dietary glucogenic amino acids into glucose. 5. Lactic acidosis occurs in fructose-1, 6-bis phosphatase deficiency. 6. Gluconeogenesis is impaired in alcoholics .

Regulation of Gluconeogenesis Enzymes of gluconeogenesis are subjected to allosteric regulation and hormone regulation. Pyruvte carboxylase and fructose-1, 6-bisphosphatase regulates gluconeogenesis. Allosteric regulation Pyruvate carboxylase is an allosteric enzyme. Acetyl-CoA is its activator. When glucose is in short supply fatty acid oxidation generates acetyl-CoA this in turn activates gluconeogenesis. Fructose-1, 6-bisphosphatase is another allosteric enzyme. AMP is its allosteric inhibitor. So when there is energy crisis gluconeogenesis is inhibited. Hormonal regulation Insulin decreases the synthesis of key enzymes of gluconeogenesis thus inhibit gluconeogenesis.


Glucose-alanine Cycle : In the skeletal muscle pyruvate is converted to alanine by transamination. Through the circulation alanine reaches liver. In the liver pyruvate regenerated from alanine by transamination is used for glucose synthesis. This process is called as glucose-alanine cycle. This cycle operates during starvation when muscle proteins are degraded. This cycle is meant for the transport of amino group nitrogen from muscle to liver .

Regulation of blood glucose level : Because of the demands of the brain for oxidizable glucose, that the human body regulate the level of glucose circulating in the blood , this level maintained in the range of 5 mm . Nearly all CHO ingested in the diet are converted to glucose following transport to the liver , catabolism of dietary or cellular protein can be utilized for glucose synthesis via gluconeogensis, additionally other tissue besides the liver that incompletely oxidize glucose ( predominantly skeletal muscle and erythrocyte) provide lactate that can be converted to glucose via gluconeogensis. Maintenance of blood glucose homeostasis is very important for survival of human organism. The hormones concerned with glucose homeostasis are :


Insulin : Is the most important hormone controlling the plasma glucose concentration , it is secreted by β-cell of pancreas , these cells produce proinsulin, which consists of the 51-amino-acid polypeptide insulin and a linking peptide ( C-peptide) , then released into plasma mainly in response to rising plasma glucose level. Insulin bind to specific receptors on the surface of insulin-sensitive cells of adipose tissue and muscles, the most important effect is stimulation of glucose entry into these cells with resultant decrease in plasma level , also insulin promote glycogen synthesis in the liver and in the muscle, also it stimulate fat synthesis in adipose tissue and protein synthesis in the muscle, but it inhibit gluconeogensis , lipolysis and proteolysis.The normal response to hyperglycemia there for depend on :1- adequate insulin secretions.2- Adequate insulin receptors.3- Normal intracellular response to receptors binding of insulin ( post receptors events) .

Glucagon : Glucagon is a single-chain polypeptide , synthesized by α-cell of pancreas and it secretion stimulated by hypoglycemia and fasting , glucagons stimulate hepatic glycogenolysis by activating glycogen phosphorylase and stimulate gluconeogensis.Growth hormone : Released from anterior pituitary gland and act to increase blood glucose by inhibiting uptake of glucose by extrahepatic tissue specially in muscle, and increase lipolysis , but it increase synthesis of muscle protein , it is secretion stimulated by hypoglycemia, stress and sleep.

Glucocorticoid : It act to increase blood glucose level by inhibiting glucose uptake in the muscles , cortisol the major glucocorticoid released from adrenal cortex, is secreted in response to the increase in circulating ACTH , glucocorticoid increase gluconeogensis , increase protein breakdown in the muscles and increase lipolysis in adipose tissue. It is secretion stimulated by hypoglycemia and stress. Adrenalin : It is secreted from adrenal medulla it stimulate production of glucose by activating glycogenolysis in the liver and muscle by activating phosporylase enzyme in response to stressful stimuli , adrenalin also stimulate lipolysis.


Pentose phosphate pathway : The pentose phosphate pathway is primarily an anabolic pathway that utilize the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents. Primary function of this pathway are : 1-To generate reducing equivalents in the form of NADPH for reductive biosynthesis reaction within the cells. 2-To provide the cell with ribose-5-phosphate (R5P) for the synthesis of nucleotides and nucleic acids. 3-It can operate to metabolize dietary pentose sugars derived from the digestion of nucleic acids as well as to rearrange the carbon skeletons of dietary CHO into glycolytic and gluconeogenic intermediates. Site : Enzymes of this pathway are present in cytosol of liver, adipose tissue, erythrocytes, adrenal cortex, thyroid, testis, ovaries and lactating mammary gland. In the skeletal muscle the pathway is less active. The reaction of F.A and steroid biosynthesis utilize large amount of NADPH , erythrocyte utilize the reaction of PPP to generate large amount of NADPH .


Reactions of hexose monophosphate shunt : The reaction of PPP take place in the cytoplasm , the PPP has both oxidative and non oxidative arm. The oxidation steps , utilizing glucose-6-phosphate (G6P) as the substrate occur at the beginning of pathway and the reaction that generate NADPH. the reaction catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase generate one mole of NADPH each for every mole of glucose-6-phosphate that enter the PPP. The non oxidative reactions of PPP are primarily designed to generate R-5-P. Also the important reaction of PPP are to convert dietary 5 carbon sugars into both (fructose-6-phosphate) and (glceraldehyde-3-phosphate) which can then be utilized by pathway of glycolysis.


Medical Importance of Pentose Phosphate Pathway: Glucose-6-Phosphate Dehydrogenase Deficiency ( G6PDD ) : The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione. In some individual carry defective gene which less active glucose-6-phosphate dehydrogenase and becomes inactive in presence of certain drugs. So, the affected individuals are normal until they are exposed to those drugs. Glucose-6-phosphate dehydrogenese deficiency occurs when drugs like aspirin, antibiotics : ciprofloxacin , nitofurantoin , sulphonamide also anti-malarial drug and sulfonamide are administered to these individuals. Since NADPH production is blocked in these individuals due to the deficiency of G6PD the susceptibility of RBC to hemolysis is increased. Therefore, the affected individuals develop hemolytic anemia on exposure to these drugs. Consumption of fava beans also causes G6PD deficiency in the susceptible individuals. Favism is the name given to this type of G6PD deficiency.


The inability to maintain reduced glutathione in the RBC lead to increase accumulation of peroxides H2O2 , predominantly H2O2 that result in a weakening of the cell wall and concomitant haemolysis , accumulation of H2O2 also lead to increase rate of oxidation of hemoglobin to methemoglobin that also lead to weakening of the cell wall. Glutathione remove peroxide via the action of glutathione peroxidase, the PPP in erythrocyte is the only pathway for these cells to produce NADPH , so any defect in production of NADPH lead to effect on RBC survival . So deficiency of G6PD may cause haemolytic anemia this enzyme catalyze the first step in P.P.P pathway and need for formation of NADPH which is essential for maintenance of intact red cell mem. and for protection RBC against oxidative stress. The G6PD is X-linked recessive disorder affecting mainly the male.

The role of liver in the buffering blood glucose :The blood glucose level in atypical person after an overnight fast is 80 mg/dl (4.4 mmol/l) , the blood glucose level during the day normally range from about 80 mg/dl before meal to about 120 mg/dl after meal , the blood glucose level is controlled primarily by the action of the liver , which can take up or release large amounts of glucose in response to hormonal signals and the level of glucose itself. After the CHO containing meal , the liver can store some of excess glucose as glycogen , the rate of glycogen synthesis (glycogensis) from (G6P) may be increased by insulin which is secreted by the β-cells of pancreas in response to systemic hyperglycemia . The liver can convert some of excess glucose to fatty acid which are ultimately transported as triglyceride in VLDL and store in adipose tissue Under aerobic condition the liver can synthesize glucose (by gluconeogensis) using the metabolic products from other tissue , such as glycerol , lactate or carbon chains resulting from deamination of most amino acid (mainly alanine)

The liver contain enzyme( G6Pase) which can release free glucose from (G6P) , the (G6P) result from glycogen breakdown (glycogenolysis) or by gluconeogensis the releasing of free glucose from (G6P) help to maintain extracellular fasting level. Hepatic glycogenolysis is stimulated by hormone glucagone secreted by α-cells of pancreas , and by catecholamine such as adrenalin or nor adrenalin . During fasting the liver can convert fatty acid , released from adipose tissue as a consequence of low insulin activity to ketones , these can be used by other tissue , including the brain as an energy source when glucose is in short supply.

Other organs The renal cortex is the only other tissue capable of gluconeogenesis, and of converting G6P to glucose. The gluconeogenic capacity of the kidney is particularly important in hydrogen ion homeostasis and during prolonged fasting. Other tissues, such as muscle, can store glycogen but, because they do not contain glucose-6-phosphatase, they cannot release glucose from cells and so can only use it locally; this glycogen plays no part in maintaining the plasma glucose concentration.