CARBOHYDRATEMETABOLISM Dr. hassan el-banna
INTRODUCTIONMetabolism • Metabolism is a term which includes all the reactions which occur to the food stuffs inside the body.
Metabolic pathways fall into three categories: 1-Anabolic pathways (synthetic): Such reactions require energy which is provided by breaking down ATP to ADP+P. e.g. synthesis of protein, glycogen, fat (triglyceride).
2-Catabolic pathways (degradative): • Involve oxidative processes that release free energy. • The complex molecules are broken down into their simple building blocks e.g. proteins are degraded to amino acids, polysaccharides to monosaccharides and triglyceride to free fatty acids and glycerol. • The resulting building blocks are further degraded to acetyl CoA.
Acetyl CoA is oxidized in the citric acid cycle which is the final common pathway in oxidation to give CO2+H2O and energy. 3-Amphibolic pathways: (both anabolic and catabolic) e.g. citric acid cycle has more than one function and occur as link between the anabolic and catabolic pathways.
CARBOHYDRATE METABOLISM Digestion and absorption Dietary carbohydrates : The major dietary carbohydrates are starch, sucrose and lactose. Small amounts of free glucose and fructose are also present in the diet, in addition to glycogen and indigestible polysaccharides such as cellulose.
1- Digestion of dietary carbohydrates in the mouth: Salivary α-amylase cleaves starch and glycogen by breaking in random the α-1, 4 linkages between glucose residues within the chains leading to α-dextrins. 2- Digestion of carbohydrates in the intestine: A- Digestion by pancreatic enzymes: Pancreatic α-amylase, like the salivary enzyme, cleaves α 1-4 linkages giving rise to maltose, maltotriose and small oligosaccharides containing α-1-4 and α-1-6 linkages.
B- Digestion by intestinal enzymes: Brush borders intestinal enzymes are: 1- Oligo 1,6 glucosidase releases glucose residues from branched oligosaccharides. 2- disaccharidasess - Sucrase converts sucrose to glucose and fructose. - Lactase converts lactose to glucose and galactose. - Maltase converts maltose to two glucose molecules
Fates of the absorbed glucose: Glucose is absorbed through portal blood to the liver. Fructose and galactose are converted to glucose in the liver. The only sugar utilized by the body is glucose. The majority of it is taken by the liver to be stored as glycogen or oxidized by glycolysis for acetyl CoA and lipid synthesis. A minimal amount passes through systemic circulation to maintain blood sugar level in fasting conditions (the fasting blood glucose level 60 – 110 mg/dl).
Glucose is used in: • Oxidation: The pathways for oxidation of glucose are classified into two main groups: A. The major pathways which are mainly for energy production: 1. Glycolysis. 2. Citric acid cycle (CAC). B. The minor pathways for oxidation which are not for energy production: 1. Hexosemonophosphate shunt (HMS). 2. Uronic acid pathway.
II. Conversion to biologically active substances as: 1. Galactose: which is essential for formation of lactose, glycolipids, mucopolysaccharides, ...etc. 2. Fructose: needed for nutrition of sperms. 3. Amino sugars. 4. Non-essential amino acids. 5. Fatty acids. 6. Ribose-5-P. 7. Glucuronic acid.
III. Storage of glucose: 1.As glycogen in the liver and muscles mainly. 2. As triglycerides (TG), mainly in adipose tissues. IV. Excretion of glucose in urine: When blood glucose level exceeds a certain limit (renal sugar threshold), it will pass to urine. This will occur when blood glucose level is above 180 mg/dl and this is known as glucosuria.
Glycolysis (Embden Meyerhof Pathway) Definition: It is degradation of glucose to generate ATP and to provide intermediates for other synthetic and metabolic pathways. Aerobically it ends with pyruvate while anaerobically lactate is the end product. Site of glycolysis: It occurs in the cell cytosolof all tissues.
Steps of glycolysis: • Conversion of glucose to G-6-P, is irreversible (non-equilibrium reaction). It is catalyzed by glucokinase (in liver and pancreas) or by hexokinase (in all other tissues). • Aldolase A: (F1,6 diphosphatealdolase) Occurs in most tissues.
The glyceraldehyde -3-P dehydrogenase enzyme is NAD dependent enzyme, having SH group at its active site of the enzyme. *PhosphoglycerateKinase and pyruvatekinase are examples of phosphorylation at substrate level. Two molecules of ATP are liberated in each step since every molecule of glucose gives rise to two trioses.
Importance of glycolysis: I-Energy production : 1. Under aerobic conditions: Glucose 2 Pyruvate + 8 ATP. • For tissues that have mitochondria, glycolysis is considered a preparatory step for complete oxidation via citric acid cycle since pyruvate is transported into the mitochondria to provide oxaloacetate or active acetate( Acetyl CoA )where it is oxidized by CAC for more energy.
The hydrogens of NADH produced by glycolysis are transported to the mitochondria mainly to be oxidized by electron transport chain (ETC). 2. Under anaerobic conditions: Glucose 2 lactate + 2 ATP Pyruvateis reduced to lactate in a reversible reaction catalyzed by lactate dehydrogenase (LDH).
This occurs in muscles during severe exercise (hypoxic conditions) and in tissues that lack mitochondria as RBCs and lens.
II-Synthetic functions: 1- Dihydroxy acetone phosphate can give glycerol-3-phosphate for synthesis of TAG and phospholipids. 2- Pyruvate acetyl coA FA and sterols and ketone bodies 3- Synthesis of amino acids from intermediates of glycolysis ALT Pyruvatealanine
III. Importance of glycolysis in red cells: a) Energy production: it is the main pathway that supplies the red cells with ATP. b) Reduction of methemoglobin: glycolysis provides NADH for reduction of met Hb by NADH-cytb5-reductase enzyme.
Regulation of glycolysis The key irreversible regulatory Enzymes are: - Hexokinase(glucokinase), - Phosphofructokinase (PFK-1) - Pyruvatekinase. PFK-1 is the main pacemaker, - Since it catalyzes the first irreversible reaction unique to the glycolytic pathway.
Hormonal regulation of glycolysis: a) Glucagon: is secreted in hypoglycemia or in carbohydrate deficiency. It affects liver cells mainly as follows: It acts as inhibitors for glycolytic key enzymes (glucokinase,PFK-1, pyruvatekinase). b) Insulin: It is secreted in hyperglycemia and after carbohydrates feeding, it causes: Stimulation of glycolytic key enzymes.
Cori Cycle • Lactate is formed during anaerobic oxidation of glucose in muscles and in RBCs , then diffuses to the blood then to the liver . • In the liver lactate is converted to glucose by gluconeogenesis . • Glucose goes back to the red cells , muscles or any tissue and is reutilized for production of energy .
Glucose G-6-P Glycogen Glycolysis Pyruvate NADH+H LDH NAD Lactate Glucose Glycogen G-6-P GluconeogenesisGlycolysis Pyruvate NADH+H LDH NAD Lactate G Blood Lactate Liver Red cells or muscles
Inhibitors of glycolysis: 1- 2- deoxyglucose inhibits hexokinase. 2- Mercury and iodoacetate inhibit glyceraldehyde-3-P dehydrogenase. 3- Fluoride inhibits enolase by removal of Mg2+ as Mg fluoride.
* Pyruvate is transported to the mitochondria via a special pyruvate transporter where it can be transformed into: 1.oxaloacetic acid.(CO2 fixation):
S S This reaction needs five cofactors which are: thiamine pyrophosphate (TPP or active vitamin B1 ), lipoic acid ( L ), coenzyme A (CoASH), FAD and NAD+.
*It provides extra 6 molecules of ATP formed by oxidation of NADH by the ETC.
In animals: The formation of acetyl CoA from pyruvate is a key irreversible step in metabolism because they are unable to convert acetyl CoA into glucose.
Citric acid cycle (CAC)or Tricarboxylic Acid Cycle or Krebs Cycle Definition: It is a series of reactions in mitochondria, that brings about the catabolism of acetyl residues, liberating hydrogen equivalents which upon oxidation, leads to the release of energy. Site and steps: CAC occurs only in cells containing mitochondria.
1- CAC comprises condensation of acetyl CoA with oxaloacetate to form citric acid. This reaction is catalyzed by citrate synthase (condensing enzyme). 2-Citrate is then isomerized into isocitrate. This is done by a dehydration step followed by re hydration step. The enzyme catalyzing both steps is called aconitase. 3-Isocitrate is oxidized and decarboxylated to -ketoglutarate. This reaction is catalyzed by isocitratedehydrogenase. The intermediate of this reaction is oxalosuccinate.
4- SuccinylCoA is formed by oxidative decarboxylation of α-ketoglutarate.This reaction is catalyzed by α-ketoglutaratedehydrogenase complex 5- The succinylCoA has an energy rich bond. The cleavage of the thioester bond of succinylCoA is coupled to the phosphorylation of guanosinediphosphate (GDP) leading to the formation of succinate and GTP. This reversible reaction is catalyzed by thiokinase enzyme . The -phosphate group of GTP is readily transferred to adenosine diphosphate (ADP) to form ATP .
6- Oxaloacetate is regenerated by oxidation of succinate. This is done through three steps: a)Succinate is oxidized to fumarate by succinatedehydrogenase using FAD as a hydrogen acceptor. b)Fumarate is hydrated by fumarase enzyme to malate. c) Finally, malate is oxidized to oxaloacetate by malatedehydrogenase using NAD+ as hydrogen carrier.
ETC ETC Importance of CAC: I- Energy production: Every one mole of acetyl- CoA produces 12 moles of ATP as follows: 3 NADH, H+ 9ATP FADH2 2ATP Substrate level ADP + Pi ATP Total 12 ATP
II-It is amphibolic pathway i.e. both catabolic and anabolic: a) Catabolic functions: It is the final common metabolic pathway for oxidation of carbohydrates, fats and proteins.
b) Anabolic functions: The most important anabolic functions are: 1- Citrate in the cytosol by ATP-citartelyase gives acetyl CoA which is used for synthesis of fatty acids and cholesterol. CoA Citrate Lyase Citrate Oxaloacetate + acetyl CoASH ATP ADP+Pi
2- By transamination -ketoglutarateis converted to glutamate and oxaloacetate is converted to aspartate. 3 - Oxaloacetate in the cytosol is converted to PEP which is converted to glucose (gluconeogenesis). 4- SuccinylCoA is used for heme synthesis, oxidation of ketone bodies (ketolysis) and detoxification. 5- Malate gives pyruvate by malic enzyme in the cytosol.
Malicenzyme Citrate Pyruvate + CO2 NADP+ NADPH + H+ 6- CO2 produced is used in many important reactions including different CO2-fixation reactions, purines and pyrimidines and urea synthesis and synthesis of H2CO3/ BHCO3 buffer system.
Total ATP produced from complete oxidation of one molecule of glucose during glycolysis, oxidative decarboxylation and CAC : Glycolysis G 2PA 8 ATP Oxidative decarboxylation 2PA 2 acetyl CoA 6 ATP (2 NADH + H+ ×3) Oxidation of 2 acetyl CoA in CAC (12 ATP ×2) 24 ATP 38 ATP
Inhibitors of CAC: 1- Fluorocitrate inhibits aconitase. 2- Mercury and arsenite inhibit pyruvateand α–ketoglutaratedehydrogenasecomplex. 3- Malonic acid inhibits succinic acid dehydrogenase.
Definition: Synthesis of glucose (and/or glycogen) from non-carbohydrate precursors such as, lactate, glucogenic amino acids, glycerol and propionate. Some tissues such as the brain, RBCs, kidney medulla, lens and cornea of the eyes, testis, and exercising muscle require a continuous supply of glucose as a metabolic fuel. Liver glycogen can meet these needs for only 10-18 hours in the absence of dietary intake of CHO. During prolonged fasting, hepatic glycogen stores are depleted and glucose is formed from, non- carbohydrate precursors.
Site: The major site of gluconeogenesis is the liver (90%); it can also occur in the cortex of the kidney (10%). It occurs mainly in the cytoplasm and partly in the mitochondria. Steps: It is nearly the reversal of glycolysis except for the three irreversible kinases as follows:-
I- Dicarboxylic acid shuttle: It is a mechanism by which phosphoenolpyruvateis formed from pyruvate by the way of oxaloacetate (dicarboxylic acid). Pyruvate is carboxylated to oxaloacetateby pyruvatecarboxylase. This enzyme is liver and kidney mitochondrial enzyme but not present in muscle. Oxaloacetate formed is transported across mitochondrial membrane in the form of malate. Malate is reoxidized to oxaloacetate by a NAD-linked malatedehydrogenase in the cytosol.