Biochemistry of carbohydrates

Transcription

1 Biochemistry of carbohydrates الفريق الطبي األكاديمي Done By: - Hanan Jamal لكية الطب البرشي البلقاء التطبيقية / املركز /

2 In the last lecture we talked about Pyruvate, pyruvate is a central intermediate; it is not the end of reactions in metabolism. Some amino acids would be transaminated to give pyruvate. If you have shortage for example of some amino acid, pyruvate can be used to synthesize amino acid, pyruvate could synthesize the cell pyruvate to synthesize alanine for example. *How the cell will synthesize alanine from pyruvate? -by transamination. *How pyruvate could produce energy from amino acid? -Pyruvate will be oxidized to acytel CoA and acytel CoA will be oxidized in citric acid cycle. What is the importance of pyruvate for fermentation? -The importance of pyruvate in fermentation, it is a substrate for fermentation. In animals it will be converted to lactate-bylactate dehydrogenase induced and in bacteria, it will be interoxidative by pyruvate decarboxylase to acetaldehyde, then by alcohol dehydrogenase to ethanol. We do not have the enzyme pyruvate decarboxylase that convert pyruvate to acetaldehyde, but we have the enzyme alcohol dehydrogenase, even though we do not synthesize any ethanol, so what is the significant of having alcohol dehydrogenase? -for detoxification of alcohol. Page 1

3 Biochemistry of carbohydrates Glycolysis Fates of Pyruvate. Pyruvate Not an End Point. Important for Amino Acids, Energy, Fermentation. Pyruvate comes from glycolysis; this pyruvate will be decarboxylated to acetaldehyde, and then it will be converted to ethanol. So, as you can see from this picture that pyruvate is central intermediate that goes to many directions in metabolism. Page 2

4 Metabolism of other sugars and the sugar that we are talking about Galactose. *What is composed of Galactose? - Six-carbon sugar (it is aldose). *What is the source of galactose? -Milk. *What is the disaccharide that produce galactose? -Lactose (is a disaccharide, which is composed of galactose and glucose and on hydrolysis by the enzyme called lactase). *What will happen to Glucose? -it will oxidized in glycolysis to pyruvate, could use some ATP and NADH. *What will happen to galactose? -it will be converted to glucose and inter the glycolysis. In the picture above, the galactose will be phosphorylated to Galactose-1-phosphate by one enzyme (hexokinase) which is the same enzyme that phosphorylate glucose, hexokinase is not much specific, and so it could phosphorylate many sources including glucose and galactose. Galactose-1-phosphate will react with a sugar called UDP- Glucose, UDP is a carrier for glucose, and it will activate glucose in order to react with Galactose-1-phosphate. *What is the UDP? -Uridine diphosphate, the structure of the Uridine is composed of Uracil, pentose sugar and phosphate. Then glucose will be attached to it to form UDP-Glucose, UDP-Glucose will act with Galactose-1-phosphate. Page 3

5 In the presence of this enzyme (not necessary to memorize the name of this enzyme), you will have the Glucose-1-P + UDP- Galactose. UDP-Galactose by epimerase will be converted to UDP-Glucose react with Galactose-1-P and will the cycle repeats. Glucose-1-P is converted to Glucose-6-P by mutase isomerase enzyme and Glucose-6-P is an intermediate in glycolysis. Glycolysis Metabolism of Other Sugars - Galactose Reduced Quantities in Adults Leads to Lactose Intolerance Lactase Lactose Galactose Glucose This is the lactose and by an enzyme, called lactase it will converted to Galactose + Glucose. Lactose disaccharide Lactase enzyme Page 4

6 Some people have intolerance to lactose they cannot digest lactose into galactose and glucose because of lactase deficiency, you may see some babies also have colic with the fed milk and there is not because they have some lactase deficiency. Glycolysis Metabolism of Other Sugars - Fructose Fructokinase F1P Fructose Fructoaldolase DHAP + Glyceraldehyde Fructose-1-Phosphate (F1P) Glyceraldehyde-3-Phosphate To Glycolysis You could eat it in your diet or you could take it from sucrose. Sucrose is a disaccharide, which is composed of glucose and fructose and by the enzyme sucrase; it will be hydrolyzed into fructose and glucose. However, a lot of food is rich in fructose and by mistake; many diabetic people substitute glucose from fructose. They eat diets that are rich in fructose to bypass glucose, which is wrong. Page 5

7 Fructose will could give a lot of pyruvate and Acytel CoA because of bypassing regulatory steps in glycolysis, so always, one single fructose will be converted to pyruvate, and acytel COA will be converted to fat, so eating a lot of fructose will cause fating and this is very dangerous to diabetic people. Regulatory Enzyme Bypassed Fructose Glycolysis Metabolism of Other Sugars - Fructose Regulatory Enzyme Bypassed F1P DHAP + GA3P Excess Pyruvate? This is the fructose and by fructokinase it will be converted to fructose-1-p The aldolase will break fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde will phosphorylate to glyceraldehyde-3-phosphate. Both of these (DHAP +GA3P), will enter glycolysis. Page 6

8 So as you see in the picture, fructose metabolism bypassed the hexokinase and PFK1, the extra regulatory stops glycolysis and reduce the amount could be use. However, because of this bypass a lot of pyruvate a lot of acytel COA that will be used for fatty acid biosynthesis and stored in adipose tissue as triglyceride. *What is the Gluconeogenesis? - Synthesis of glucose from non-sugar molecule and the most important non-sugar molecules we can use for synthesis are pyruvate and alanine. *From where we get pyruvate? -from glycolysis or amino acids. *From where we get alanine? -from our proteins. A starved person for 2-3 days will start eating himself. Proteins will start to be degraded into amino acids and those amino acids will be used to synthesis glucose, because brain cannot survive without glucose. Therefore, the body firstly use the stored glycogen for glucose feeding to the brain, then the fatty acids produce ketone bodies that could be used to feed brain and finally its use protein, Page 7

9 start eating itself to produce amino acids that could be used as a substrate molecule in gluconeogenesis. Therefore, gluconeogenesis is very important for regulation of blood glucose. How? -liver is the major organ in gluconeogenesis, muscle do not do any gluconeogenesis, muscle are glycogenic organs or tissues that means it doing oxidized glucose. Gluconeogenesis means that the tissue or organ is synthesizing glucose. Liver buffers our blood glucose, this means that if you have high glucose in blood, urine must not have more than 100 mg. Under normal conditions if you eat rich meal of carbohydrates, your blood and liver will rise to more than 150, your liver starts taking those extra glucose molecules and put them inside the liver. *What is the first step the liver after taking the glucose from the blood circulation? -liver traps it and prevents glucose from coming back to the blood circulation. Phosphorylation will make glucose inside the liver and prevent it from coming back to blood circulation, so liver is very important to buffer our blood glucose. Glucose is poising to our blood, we have to get rid of excessive amounts of it otherwise it will damage many vital organs. Reactions are taking place in cytoplasm, mitochondrion and endoplasmic reticulum. Page 8

10 Gluconeogenesis Synthesis of Glucose Four Reactions in Gluconeogenesis Replace Three Reactions in Glycolysis All Other Reactions are Simply the Reverse of Glycolysis Reactions Two Pyruvates Needed for Each Glucose Now remember that glycolysis is composed of 10 reactions. Gluconeogenesis is composed of 10 reactions, seven of them are reversible reactions of glycolysis, and another four enzymes are specific for gluconeogenesis. Therefore, gluconeogenesis uses the seven reversible reactions of glycolysis to synthesize glucose, but it cannot use the irreversible reaction for glycolysis, so it must use other enzymes to bypass those irreversible reactions of glycolysis. Some those enzymes are found in mitochondria, and most are found in cytoplasmic and one enzyme is in endoplasmic reticulum. Liver, kidney, and tests are the major organs that could do gluconeogenesis, all cells in your body do glycolysis but only liver, kidney and tests are do gluconeogenesis. Page 9

11 Biochemistry of carbohydrates G6Pase Gluconeogenesis Synthesis of Glucose (Hexokinase) F1,6BPase )PFK) PEPCK Pyruvate Carboxylase (Pyruvate Kinase) How could your liver synthesize glucose from pyruvate? Pyruvate is in the liver, and when the synthesis of glucose starts, pyruvate starts to be converted to phosphoenolpyruvate (PEP) and then converted to 2-bisphosphoglycerate 3-phosphoglycerate Glyceraldehydes-3-P 1,3-bisphosphoglyceric acid then to one dihydroxyacetone and glyceraldehyde. Then these 2 molecules combine to form fructose-1,6-bp then to Fructose-6-P glucose-6-p glucose. This is how you liver synthesizes glucose from pyruvate. Fructose-1,6-BP fructose-6-p is irreversible. So how the liver do? How the liver bypasses these irreversible reactions? The enzyme that converts PEP to pyruvate (in glycolysis) is pyruvatekinase, but in irreversible reaction, we cannot use the same pathway to convert pyruvate to PEP. Page 10

12 So what is the mechanism to do this? The last enzyme in synthesizing glucose is Glucose-6- phosphatase, it converts Glucose-6-P Glucose, it removes phosphate so this is one gluconeogenic enzyme that is specific for gluconeogenesis to overcome the irreversible reaction of Glucose-6-P to glucose. Hexokinase does not transfer Glucose-6-p to Glucose. So we need another enzyme. Fructose-1,6-Bisphosphotase is the opposite in action for fructokinase 1, phosphofructokinase 1 in glycolysis, phosphorylates fructose to fructose-1,6-bp. it's irreversible. Fructose-1,6-BPase removes the phosphate from carbon#1 of fructose and converts fructose-1,6-bp fructose-6-p, this is the overcome of the second reaction. The third one is called PEPCK (PEP carboxykinase). The fourth enzyme is pyruvate carboxylase. What is the action of it? It carboxylates pyruvate to put the CO2 to pyruvate and convert it to carboxylated pyruvate. So what are the four enzymes that are important and specific for Gluconeogenesis in addition to the reversible enzyme of glycolysis? (Starting from the end) 1-pyruvate carboxylase. 2-PEP carboxykinase. 3-fructose-1,6-BPase. 4-Glucose-6-Pase. Page 11

13 Gluconeogenesis is very important in your body, done by liver to transport glucose not being used in the liver to the brain or to the muscle when it is exercising. This picture represents the gluconeogenesis. We have pyruvate as the end reaction of glycolysis. Now, in order to make PEP from pyruvate, there is no reversible enzyme pyruvate kinase to convert to PEP, there must be a bypass by the liver then bypass this reversible reaction of glycolysis, what is the bypass? First pyruvate carboxylase will add CO2 to pyruvate and convert it to oxaloacetate (oxaloacetic acid). Page 12

14 This reaction is taking place in the mitochondria, you see that ATP is used for carboxylation in order to make glucose we need 2-pyruvate molecule. The second step which is taking place in cytoplasm, is the conversion of oxaloacetate to PEP, PEP carboxykinase requires an ATP like energy compound (GTP), in the same amount of ATP. Here, we need molecules of GTP to convert oxaloacetate to PEP, so we use 2 enzymes in gluconeogenesis to convert pyruvate to PEP. Our body cannot convert pyruvate to PEP by pyruvate kinase, so it needs these 2 enzyme with those extra enzymes to convert pyruvate to PEP. Now we have PEP that is easily reversed to 2-Phosphoglycerate 3- Phosphoglycerate. 2 more ATP molecules are used to convert to 1,3- bisphosphoglycerate, 2 more NADH to convert to Glyceraldehyde-3- P and Dihydroxyacetone and these are combined to produce fructose-1,6-bp, this step also is irreversible. So the liver must have an extra enzyme to convert fructose-1,6-bp to fructose-6-p. The enzyme is fructose-1,6-bpase. Fructose-6-P is one of the intermediates in glycolysis but fructose-6-p cannot be converted reversibly by hexokinase, so the liver requires another extra enzyme called Glucose-6-phosphotase that removes phosphate group from Glucose-6-P to get glucose. *Why Glucose-6-P cannot leave the liver? -because phosphate is negative charged, it cannot enter through the cell membrane. Page 13

15 Gluconeogenesis Reactions of Gluconeogenesis Run as Reverse of Glycolysis Reactions Glucose 11. Glucose-6-Phosphatase 1. Hexokinase G6P 10. Phosphoglucoisomerase 2. Phosphoglucoisomerase F6P 9. F1,6 Bisphosphatase 3. PFK1 F1,6BP 8. Aldolase 4. Aldolase DHAP + G3P 7. Triose Phosphate Isomerase 5. Triose Phosphate Isomerase GA3P 6.. GA3PDH 6. GA3PDH 1,3 BPG 5. Phosphoglycerate Kinase 7. Phosphoglycerate Kinase 3-PG 4. Phosphoglycerate Mutase 8. Phosphoglycerate Mutase 2-PG 3.. Enolase 9. Enolase PEP 2.. PEPCK 10. Pyruvate Kinase 1. Pyruvate Carboxylase Pyr Gluconeogenesis Reactions of Gluconeogenesis Reaction #1 Step 1 of Bypassing Pyruvate Kinase Occurs in Mitochondrion Page 14

16 Gluconeogenesis Reactions of Gluconeogenesis Reaction #1 Step 1 of Bypassing Pyruvate Kinase Occurs in Mitochondrion Pyruvate Carboxylase Pyruvate )Pyr( Oxaloacetate )OAA( Pyruvate carboxylase converts pyruvate to oxaloacetate. That requires Biotin, (whenever you hear carboxylase enzyme, remember that it requires Biotin). Page 15

17 This is the Biotin and how is carries the CO2 that will be added to pyruvate to make oxaloacetate. Page 16