1 Metabolism of Carbohydrates The Energy Metabolism of Glucose Entry of other Carbohydrates into Glycolysis Pyruvate Metabolism Biosynthesis of Carbohydrates Regulation of Carbohydrate Metabolism The Energy Metabolism of Glucose Entry of other Carbohydrates into Glycolysis Pyruvate Metabolism Biosynthesis of Carbohydrates Regulation of Carbohydrate Metabolism

2 Metabolism of carbohydrates All organisms obtain energy from the oxidation of glucose and other carbohydrates. In some cells and organisms, glucose is the major or sole source of energy:brainerythrocytes many bacteria

3 Carbohydrate metabolism Glycolysis The main pathway for glucose oxidation. It forms pyruvate anaerobically. Phosphogluconate pathway An auxiliary route for glucose oxidation in animals. It produces ribose-5-phosphate.Gluconeogenesis Pathway for the synthesis of glucose from pyruvate.

4 Energy metabolism of glucose Glycogen (animals) Starch (plants) GlucosePyruvateAcetyl CoA Lactate Ethanol Ribose-5-phosphate + NADPH + H + Disaccharides ATP + NADH + H + catabolism anabolism phosphogluconate pathway glycolysis gluconeo- genesis aerobic anaerobic,muscles anaerobic,yeast

5 Glycolysis First stage of carbohydrate catabolism. Simple sugars are broken down to pyruvate. Anaerobic process - no oxygen needed. All life uses this process.Requires glucose, 2 ADP, 2 ATP, 2 NAD +, 2 PO 4 = 10 different enzymes

6 First five reactions of glycolysis glucose glucose-6-P fructose-6-P fructose-1,6-bisP glyceraldehyde-3-Pdihydroxyacetone-P ATP ADP ATP ADP 6 carbon stage Requires energy

7 Reactions of glycolysis 1,3-bisphosphoglycerate 3-phosphoglycerate 2-bisphosphoglycerate phosphoenolpyruvate pyruvate glyceraldehyde-3-P ATP ADP ATP ADP NAD + NADH + H + H2OH2O PiPi 3 carbon stage Double this since two pyruvate are made. 3 carbon stage Double this since two pyruvate are made.

8 Overall glycolysis glucose 2 ADP + 2 PO 4 = + 2 NAD + 2 pyruvate + 2 NADH + 2 H 2 O + 2 ATP Net energy produced is 2 ATP In addition, the two pyruvate can go on to the citric acid cycle to produce more energy. 10 enzymes

9 Entry of other carbohydrates into glycolysis Dietary carbohydrates Polysaccharides Starches and glycogen are hydrolyzed to glucose by amylase in the mouth.Disaccharides Maltose, sucrose and lactose. Each is hydrolyzed to a different pair of monosaccharides.

10 Entry of other carbohydrates into glycolysis Disaccharides maltose + H 2 O 2 glucose sucrose + H 2 O fructose + glucose lactose + H 2 O glucose + galactose maltase invertase (bacteria) sucrase (animals) lactase

11 Entry of other carbohydrates into glycolysis Fructose Enters glycolysis by two different pathways depending on the tissue. Skeletal muscles The glycolytic enzyme, hexokinase accepts fructose as a substrate but with only 5% of the affinity of glucose. It only requires one phosphoryl transfer step to enter glycolysis.

12 Entry of other carbohydrates into glycolysis Fructose Liver Cells They have another enzyme, fructokinase. It has a stronger affinity for fructose. It catalyzes phosphoryl group transfer from ATP to produce fructose-1- phosphate. An aldolase-type cleavage and additional phosphorylation must also occur.

13 Entry of other carbohydrates into glycolysis Galactose Five reactions are required to transform it into glucose-6-phosphate. Galactose Galactose-1-phosphate UDP-galactoseUDP-glucose Glucose-1-phosphate Glucose-6-phosphate galactokinase galactose-1- phosphate uridyl transferase UDP-galactose -4-epimerase UDP-glucose pyrophosphorylase Phospho- glucomutase

14 Pyruvate metabolism Glycolysis ends with the production of two pyruvate per molecule of glucose. Several things can happen to the pyruvate based on the organism and cellular conditions. Fermentation Fermentation - subsequent processing under anaerobic conditions.

15 Fermentation An anaerobic process beyond glycolysis. In our body it is used to make NAD + when there is not enough oxygen. NAD + must be regenerated from NADH or glycolysis will stop. We’ll look at two types of fermentation: Lactate and Ethanol.

16 Lactate fermentation Lactate Produced by muscles when the body can’t supply enough oxygen. pyruvatelactate pyruvate lactate Anaerobic conversion of pyruvate to lactate permits regeneration of NAD +. Body can then make more ATP - at a cost. Creates an oxygen debt. Body must take in extra O 2 to oxidize lactate. NADH + H + NAD +

17 Alcohol fermentation Used by anaerobic bacteria to obtain additional energy from glucose. pyruvate acetaldehyde + CO 2 ethanol pyruvatedecarboxylase alcoholdehydrogenase NADH + H + NAD +

18 Biosynthesis of carbohydrates Gluconeogenesis Synthesis of glucose from noncarbohydrate precursors. The liver is the major site for glucose synthesis in higher animals. Pyruvate, lactate, glycerol and some amino acids act as precursors. In microorganisms, it is synthesized from acetate and propionate. Plants produce it photosynthetically.

19 Biosynthesis of carbohydrates Skeletal muscles Glycogen glucose-6-P exercise rest glucose-6-P pyruvate lactate Skeletal muscles Glycogen glucose-6-P exercise rest glucose-6-P pyruvate lactate Liver Glycogen glucose-6-P pyruvate lactate Liver Glycogen glucose-6-P pyruvate lactate glucose Muscles lack enzyme needed to convert pyruvate to glucose-6-P. Must be sent to liver. Blood

20 Gluconeogenesis Stage I mitochondria

21 Gluconeogenesis Stage III Stage II

22 Gluconeogenesis The process is sometimes called ‘reverse glycolysis’ but that is a misnomer. Only seven of the ten steps in glycolysis are reversible. The three steps to be bypassed are: 1. glucose + ATP glucose-6-phosphate + ADP 3. fructose-6-phosphate + ATP fructose-1,6 -bisphosphate + ADP 10. PEP + ADP pyruvate + ATP

23 Gluconeogenesis Bypass I. Bypass I. Pyruvate Phosphoenolpyruvate This reaction has the highest energy barrier of any reaction in the pathway. In higher animals, it begins with pyruvate in the mitochondrial matrix. Pyruvate is carboxylated to oxaloacetate by pyruvate carboxylase. Only mitochondria have the proper enzyme and it requires biotin as a cofactor.

24 Gluconeogenesis Bypass I. Bypass I. Overall reaction pyruvate + phosphoenolpyruvate + ATP + GTP ADP + GDP + P i Gluconeogenesis from lactate is also an important anabolic process. It requires initial conversion to pyruvate as shown earlier and requires the same amount of ATP and GTP.

25 Gluconeogenesis Bypass II Fructose-1,6-bisphosphate Fructose-6-phosphatePhosphofructokinase Major regulatory enzyme in glycolysis. Catalyzes the irreversible phosphoryl transfer from ATP to fructose-6-phosphate. In gluconeogenesis, the phosphoryl group is removed by hydrolysis, catalyzed by fructose-1,6-bisphosphatase.

26 Gluconeogenesis Fructose-6-phosphate Fructose-1,6-bisphosphate ATP H2OH2O PiPi phospho- fructokinase fructose-1,6- bisphosphatase gluconeogenesis glycolysis

27 Gluconeogenesis Bypass III. Glucose-6-phosphate Glucose + P i The final step is the removal of the phosphoryl group from glucose-6-phosphate. The enzyme, glucose-6-phosphatase catalyzes this hydrolysis. glucose-6-phosphate + H 2 O glucose + P i glucose-6- phosphatase

28 Synthesis of disaccharides and polysaccharides Activation of glucose and galactose. Not all glucose is immediately required for energy or other metabolic uses. Higher animals store excess as glycogen which is mobilized when needed for energy or other uses. In plants, glucose is the building block for sucrose, starch and cellulose. Nucleotide diphosphate sugars are used for synthesis.

29 Activation of glucose and galactose Nucleotide diphosphate sugars (NDP sugars) Used primarily to mark sugars to be set aside for bisynthetic purposes. Synthesis of NDP-glucose. NTP + glucose-1-phosphate NDP-glucose + PP i NTP = nucleotide triphosphate, ATP, UTP or GTP Enzyme =UDP-glucose pyrophosphorylase ADP-glucose pyrophosphorylase GDP-glucose pyrophosphorylase

30 Synthesis of UDP-galactose Two possible routes Isomerization of UDP-glucose UDP-glucose UDP-galactose Enzyme = UDP-glucose-4-epimerase Exchange of UDP galactose-1-phosphate + UDP-glucose UDP-galactose + glucose-1-P Enzyme = glactose-1-phosphate uridyl transferase

31 Synthesis of glycogen Glucose, activated and tagged by attachment of UDP is added to the nonreducing ends of an existing glycogen. Glycogen synthase catalyzes the formation of a new  (1 4) glycosidic linkage. UDP-glucose + (glucose) n + H 2 O (glucose) n+1 + UDP

32 Synthesis of starch Similar to glycogen formation except glucose is activated by ADP, not UDP. Starch synthase catalyzes the addition of glucose to an existing starch molecule by formation of  (1 4) glycosidic linkage. ADP-glucose + (glucose) n (glucose) n+1 + ADP

33 Synthesis of lactose This disaccharide is actively synthesized in the mammary glands of mammals. It is produced by combining activated galactose with glucose using lactose synthase. A  (1 4) linkage results. UDP-galactose + glucose UDP + lactose

34 Synthesis of sucrose Sucrose is present in most fruits and vegetables. It is produced by a two step process. UDP-glucose + fructose-6-phosphate sucrose-6-phosphate + UDP sucrose + P i sucrose-6- phosphate synthase phosphatase H2OH2O

35 Synthesis of cellulose Cellulose- major structural polysaccharide in cell walls of plants and some bacteria. It’s synthetic route is similar to starch except a  (1 4) linkage is produced. UDP-glucose or GDP-glucose + (glucose) n UDP or GDP + (glucose) n+1

36 Regulation of glycolysis As with all metabolic pathways, glycolysis is under constant control by the body. The process is regulated by three enzymes: hexokinase inhibited by glucose 6-phosphate phosphofructokinase inhibited by ATP and citrate pyruvate kinase inhibited by ATP

37 Regulation of glycolysis glucose glucose 6-phosphate fructose 6-phosphate fructose 1,6-bisphosphate phosphoenolpyruvate pyruvate feedback inhibition hexokinase phosphofructokinase pyruvate kinase