Storage Mechanisms and Control of Carbohydrate Metabolism

Learning Objectives 1. How Is Glycogen Produced and Degraded? 2. How Does Gluconeogenesis Produce Glucose from Pyruvate? 3. How Is Carbohydrate Metabolism Controlled? 4. Why Is Glucose Sometimes Diverted through the Pentose Phosphate Pathway?

Why do animals store any energy as glycogen Why do animals store any energy as glycogen? Why not convert all excess fuel into fatty acids? Why not store energy as free glucose?

Adult Human 70 kg Triacylglyceride: 100.000 kcal Protein (muscle): 25.000 kcal Glycogen: 600 kcal Glucose: 40 kcal Triacylglyceride: approx 11 kg of body weight glycogen storage instead of fat: increase in weight: 55 kg!!

Glycogen Breakdown ”glycogenolysis” Glycogen is cleaved by glycogen phosphorylase by adding phosphate to give a-D-glucose-1-phosphate (phosphorolysis) No ATP is involved in this phosphorolysis Occur in the liver maintains blood glucose

Enzyme-catalyzed isomerization converts the 1-phosphate to the 6-phosphate Note: more ATP is produced from glucose of glycogen glycolysis

Glycogen transferase enzyme transfers three glucose residues from (limit branch) to another branch, where they are removed by glycogen phosphorylase Glycogen debranching enzyme then hydrolyzes the a(1,6) glycosidic bond of the last glucose residue remaining at the point of branching.

Glycogenesis glycogenin Glucose 1-phosphate reacts with uridine triphosphate to give UDPG and pyrophosphate Glucose- 1-phosphate + UTP UDP G PP i UDP-glucose pyrophosphorylase O - P H C 2 N Uridine diphosphate glucose (UDPG )

Coupling of UDPG formation with hydrolysis of pyrophosphate drives formation of UDPG to completion

Exchange of phosphate from ATP regenerates UTP Uridine diphosphate glucose (UDPG) then adds its glucose unit to the growing glycogen chain Exchange of phosphate from ATP regenerates UTP HO(Glucose) n OH + Glycogen UDP G glycogen synthase HO-Glucose- O(Glucose) new glucose unit added α (1-4 bond)

Glycogenesis Branching enzyme transfers about seven glucose residue-long segment from growing branch to a new branch point via α(1-6) glycosidic bond

Control of Glycogen Metab Glycogen phosphorylase - a major control point (Dephosphorylated form( (Phosphorylated form)

Coordinate Control of Glycogen Metabolism Inactive forms are shown in red, and active ones in green.

Control of Glycogen Metab The activity of glycogen synthase is subject to the same type of covalent modification as glycogen phosphorylase the response, however, is opposite hormonal signals (glucagon or epinephrine) stimulate its phosphorylation once phosphorylated, glycogen synthase becomes inactive at the same time the hormonal signal is activating glycogen phosphorylase glycogen synthase can be phosphorylated by several other enzymes including glycogen synthase kinase dephosphorylation is by phosphoprotein phosphatase

Glycogen Loading ?? http://runnersconnect.net/running-nutrition-articles/carbohydrate-loading-marathon/

Glycogen storage diseases. Type I Von Gierke’s disease Deficiency of glucose-6-phosphatase Liver cells and renal tubule cells loaded with glycogen. Hypoglycemia, lactic acidemia, ketosis, hyperlipemia.

Summary Glycogen is the storage form of glucose in animals, including humans. Glycogen releases glucose when energy demands are high Glucose polymerizes to form glycogen when the organism has no immediate need for the energy derived from glucose breakdown Glycogen metabolism is subject to several different control mechanisms, including covalent modification and allosteric effects

Gluconeogenesis

Gluconeogenesis The synthesis of glucose from none carbohydrate sources like lactate, glycerol and amino acids. gluconeogenesis is not the exact reversal of glycolysis; that is, pyruvate to glucose does not occur by reversing the steps of glucose to pyruvate It is impossible to reverse any kinase reaction under physiological conditions. gluconeogenesis occur in the cytosol & mitochondria gluconeogenesis takes place in the liver 90% and in kidneys 10%

Gluconeogenesis there are three irreversible steps in glycolysis --- phosphoenolpyruvate to pyruvate + ATP --- fructose-6-phosphate to fructose-1,6- bisphosphate --- glucose to glucose-6-phosphate the net result of gluconeogenesis is reversal of these three steps, but by different reactions and using different enzymes (bypassing)

+ 2 ATP - 6 ATP

Gluconeogenesis Step 1: carboxylation of pyruvate (1st bypass) requires biotin pyruvate carboxylase is subject to allosteric control; it is activated by acetyl-CoA

Biotin Biotin is a carrier of CO2 (carboxylation)

Gluconeogenesis decarboxylation of oxaloacetate is coupled with phosphorylation by GTP to give PEP the net reaction of carboxylation/decarboxylation is net reaction is close to equilibrium: DG0’ = 2.1 kJ•mol-1

Gluconeogenesis Second different reaction (2nd bypass) in gluconeogenesis G° = -16.7•kJ mol-1 fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and F2,6P and activated by ATP

Gluconeogenesis Third different reaction (3rd bypass) in gluconeogenesis G°’ = -13.8 kJ•mol-1

The Cori Cycle The Cori cycle under vigorous anaerobic exercise, glycolysis in muscle tissue converts glucose to pyruvate; NAD+ is regenerated by reduction of pyruvate to lactate lactate from muscle is transported to the liver where it is reoxidized to pyruvate and converted to glucose thus, the liver shares the stress of vigorous exercise

The Cori Cycle

Control of carbohydrate metabolism How ?

Control of carbohydrate metabolism Allosteric: fructose-2,6-bisphosphate (F2,6P) high concentration of F2,6P stimulates glycolysis; a low concentration stimulates gluconeogenesis concentration of F2,6P in a cell depends on the balance between its synthesis (catalyzed by phosphofructokinase-2) and its breakdown (catalyzed by fructose bisphosphatase-2) AMP inhibits FBPase and stimulates PFK each enzyme is controlled by phosphorylation/ dephosphorylation

Fructose-2,6-bisphosphate Fructose-2,6-bisphosphate is an allosteric activator of phosphofructokinase (a glycolytic enzyme) and an allosteric inhibitor of fructose bisphosphate phosphatase (an enzyme in the pathway of gluconeogenesis). p. 520

Reciprocal Regulation of Gluconeogenesis and Glycolysis in the Liver

Control of carbohydrate metabolism

Control of carbohydrate metabolism Substrate cycling opposing reactions can be catalyzed by different enzymes and each opposing enzyme or set of enzymes can be regulated independently

Major Control Points in Carbohydrate Metabolism Three steps in glycolysis are major control points in glucose metabolism Hexokinase Inhibited by high levels of glucose 6-phosphate Phosphofructokinase, When glycolysis is inhibited through glucose 6-phosphate builds up, shutting down hexokinase Pyruvate kinase (PK) is an allosteric enzyme Inhibited by ATP and alanine Activated by fructose-1,6-bisphosphate PK has 3 different isoenzymes M predominates in muscle, L in liver, and A in other tissues Native PK is a tetramer Liver isoenzymes are subject to covalent modification

Control of Pyruvate Kinase

Summary A number of control mechanisms operate in carbohydrate metabolism. These include allosteric effectors, covalent modification, substrate cycles, and genetic control In the mechanism of substrate cycling, the synthesis and the breakdown of a given compound are catalyzed by two different enzymes

Pentose Phosphate Pathway As the name implies, five-carbon sugars, including ribose, are produced from glucose The oxidizing agent is NADP+; it is reduced to NADPH, which is a reducing agent in biosyntheses e.g. lipid PPP is composed from two reactions: Oxidative reactions: begins with two oxidation steps (using NADP+) to give ribulose-5-phosphate Non-oxidative reactions: a series of carbon-shuffling steps during which three-, four-, five-, six-, and seven-carbon monosaccharide phosphates are produced ATP production is not an important concern

PPP oxidative reactions H O 2 P 3 - Glucose-6-phosphate 6-Phosphogluconate NADP + NADPH Ribulose-5-phosphate

Non-oxidative reactions H 2 O H RNA C O C H O H O H O H H O H H O H H O H C H 2 O H O H H O H C H 2 O P 3 - C H 2 O P 3 - C O H O H Ribose-5-phosphate Sedoheptulose- 7-phosphate H O H C H 2 O P 3 - C H 2 O Ribulose-5- phosphate C O H O H C H O H O H H O H C H 2 O P 3 - C H 2 O P 3 - Xylulose-5-phosphate Glyceraldehyde- 3-phosphate

To glycolysis

Pentose Phosphate Pathway

Pentose Phosphate Pathway the carbon-shuffling reactions are catalyzed by ---transketolase for the transfer of two-carbon units requires thiamine pyrophosphate as a coenzyme ---transaldolase for the transfer of three-carbon units Control of the pentose phosphate pathway glucose-6-phosphate (G6P) can be channeled into either glycolysis or the pentose phosphate pathway if ATP needed, G6P is channeled into glycolysis if NADPH or ribose-5-phosphate are needed, G6P is channeled into the pentose phosphate pathway

G-6-PD More than 400 variants of G-6-PD have been characterized, which show less activity than normal. G-6-PD is the most common human enzyme deficiency in the world. It affect an estimated 400 million people. Hemolysis, abdominal pain, dizziness, headache, dyspnea, palpitation, neonatal jaundice

Precipitating Factors Infection & other ac. Illness(diabetic ketoacidosis) Drugs: Antimalarials, Antipyretics or Antibiotics Fava beans “favism” Neonatal jaundice : due to decrease hepatic catabolism or increase production of bilirubin.

Pentose Phosphate Pathway Summary of oxidative reactions Glucose-6-phosphate + 2 NADP Ribulose-5-phosphate + CO2 + 2 NADPH Summary of non-oxidative reactions Reactant Enzyme Products C 5 + Transketolase 7 3 6 4 Transaldolase 2 Net:

Relationship between PPP and Glycolysis

End Chapter 18