Carbohydrates Metabolism Chapter 4 Carbohydrates Metabolism

§ 1 Overview Carbohydrates in general are polyhydroxy aldehydes or ketones or compounds which yield these on hydrolysis.

Biosignificance of Carbohydrates The major source of carbon atoms and energy for living organisms. Supplying a huge array of metabolic intermediates for biosynthetic reactions. The structural elements in cell coat or connective tissues.

Glucose transporters (GLUT) GLUT1: RBC GLUT4: adipose tissue, muscle

The metabolism of glucose glycolysis aerobic oxidation pentose phosphate pathway glycogen synthesis and catabolism gluconeogenesis

glycogen Glycogenesis Glycogenolysis starch lactate Glycolysis Digestion absorption glucose aerobic oxidation Lactate, amino acids, glycerol Gluconeo-genesis H2O+CO2 Pentose phosphate pathway Ribose, NADPH

§2 Glycolysis

Glycolysis The anaerobic catabolic pathway by which a molecule of glucose is broken down into two molecules of lactate. glucose →2lactic acid (lack of O2) All of the enzymes of glycolysis locate in cytosol.

1. The procedure of glycolysis glycolytic pathway pyruvate lactic acid

1) Glycolytic pathway : G → pyruvate including 10 reactions.

(1) G phosphorylated into glucose 6-phosphate Phosphorylated G cannot get out of cell Hexokinase , HK (4 isoenzymes) , glucokinase, GK in liver ; Irreversible .

Comparison of hexokinase and glucokinase hexokinase glucokinase occurrence in all tissues only in liver Km value 0.1mmol/L 10mmol/L Substrate G, fructose, glucose mannose Regulation G-6-P Insulin Comparison of hexokinase and glucokinase

(2) G-6-P → fructose 6-phosphate

(3) F-6-P → fructose 1,6-bisphosphate The second phosphorylation phosphofructokinase-1, PFK-1

(4) F-1,6-BP → 2 Triose phosphates Reversible

(5) Triose phosphate isomerization G→2 molecule glyceraldehyde-3-phosphate, consume 2 ATP .

(6) Glyceraldehyde 3-phosphate → glycerate 1,3-bisphosphate

(7) 1,3-BPG → glycerate 3-phosphate Substrate level phosphorylation

(8) Glycerate 3-phosphate → glycerate 2-phosphate

(9) Glycerate 2-phosphate → phosphoenol pyruvate

(10) PEP →pyruvate Second substrate level phosphorylation irreversible

2) Pyruvate → lactate

Summary of Glycolysis

Total reaction: Formation of ATP: C6H12O6 + 2ADP + 2Pi 2CH3CHOHCOOH + 2ATP + 2H2O Formation of ATP: The net yield is 2 ~P or 2 molecules of ATP per glucose.

2. Regulation of Glycolysis Three key enzymes catalyze irreversible reactions : Hexokinase, Phosphofructokinase & Pyruvate Kinase.

1) PFK-1 The reaction catalyzed by PFK-1 is usually the rate-limiting step of the Glycolysis pathway. This enzyme is regulated by covalent modification, allosteric regulation.

bifunctional enzyme

2) Pyruvate kinase Allosteric regulation: F-1,6-BP acts as allosteric activator； ATP and Ala in liver act as allosteric inhibitors;

Covalent modification: phosphorylated by Glucagon through cAMP and PKA and inhibited.

3) Hexokinase and glucokinase This enzyme is regulated by covalent modification, allosteric regulation and isoenzyme regulation. Inhibited by its product G-6-P. Insulin induces synthesis of glucokinase.

3. Significance of glycolysis 1) Glycolysis is the emergency energy-yielding pathway. 2) Glycolysis is the main way to produce ATP in some tissues, even though the oxygen supply is sufficient, such as red blood cells, retina, testis, skin, medulla of kidney. In glycolysis, 1mol G produces 2mol lactic acid and 2mol ATP.

§ 3 Aerobic Oxidation of Glucose

The process of complete oxidation of glucose to CO2 and water with liberation of energy as the form of ATP is named aerobic oxidation. The main pathway of G oxidation.

1. Process of aerobic oxidation

1) Oxidative decarboxylation of Pyruvate to Acetyl CoA irreversible; in mitochodria.

Pyruvate dehydrogenase complex: E1 pyruvate dehydrogenase Es E2 dihydrolipoyl transacetylase E3 dihydrolipoyl dehydrogenase thiamine pyrophosphate, TPP (VB1) HSCoA (pantothenic acid) cofactors lipoic Acid NAD+ (Vpp) FAD (VB2)

Pyruvate dehydrogenase complex: HSCoA NAD+

pyruvate dehydrogenase complex The structure of pyruvate dehydrogenase complex

HSCoA

CO2 NADH +H+ NAD+ CoASH

2) Tricarboxylic acid cycle, TCAC The cycle comprises the combination of a molecule of acetyl-CoA with oxaloacetate, resulting in the formation of a six-carbon tricarboxylic acid, citrate. There follows a series of reactions in the course of which two molecules of CO2 are released and oxaloacetate is regenerated. Also called citrate cycle or Krebs cycle.

(1) Process of reactions

Citrate cycle

Summary of Krebs Cycle ① Reducing equivalents

② The net reaction of the TCAC: acetylCoA+3NAD++FAD+GDP+Pi+2H2O → 2CO2+3NADH+3H++FADH2+GTP+ HSCoA ③ Irreversible and aerobic reaction ④ The enzymes are located in the mitochondrial matrix.

⑤ Anaplerotic reaction of oxaloacetate

(2) Bio-significance of TCAC ① Acts as the final common pathway for the oxidation of carbohydrates, lipids, and proteins. ② Serves as the crossroad for the interconversion among carbohydrates, lipids, and non-essential amino acids, and as a source of biosynthetic intermediates.

Krebs Cycle is at the hinge of metabolism.

2. ATP produced in the aerobic oxidation acetyl CoA → TCAC : 3 (NADH+H+) + FADH2 + 1GTP → 12 ATP. pyruvate →acetyl CoA: NADH+H+ → 3 ATP 1 G → 2 pyruvate : 2(NADH+H+) → 6 or 8ATP 1mol G： 36 or 38mol ATP （12＋3 ）×2 ＋ 6（ 8 ）＝36（ 38 ）

3. The regulation of aerobic oxidation The Key Enzymes of aerobic oxidation The Key Enzymes of glycolysis Pyruvate Dehydrogenase Complex Citrate synthase Isocitrate dehydrogenase (rate-limiting ) -Ketoglutarate dehydrogenase

(1) Pyruvate dehydrogenase complex

(3) Isocitrate dehydrogenase (2) Citrate synthase Allosteric activator: ADP Allosteric inhibitor: NADH, succinyl CoA, citrate, ATP (3) Isocitrate dehydrogenase Allosteric activator: ADP, Ca2+ Allosteric inhibitor: ATP (4) -Ketoglutarate dehydrogenase Similar with Pyruvate dehydrogenase complex

Oxidative phosphorylation→TCAC↑ ATP/ADP↑ inhibit TCAC, Oxidative phosphorylation ↓ ATP/ADP↓，promote TCAC， Oxidative phosphorylation ↑

4. Pasteur Effect The key point is NADH ： NADH mitochondria Under aerobic conditions, glycolysis is inhibited and this inhibitory effect of oxygen on glycolysis is known as Pasteur effect. The key point is NADH ： NADH mitochondria Pyr TCAC CO2＋H2O Pyr can’t produce to lactate.

§4 Pentose Phosphate Pathway

1. The procedure of pentose phosphate pathway/shunt In cytosol

1) Oxidative Phase

2) Non-Oxidative Phase Transketolase: requires TPP Transaldolase Ribose 5-p Fructose 6-p Glycolysis Fructose 6-p Xylulose 5-p Xylulose 5-p Glyceraldehyde 3-p Transketolase: requires TPP Transaldolase

2. Regulation of pentose phosphate pathway The net reation: 3G-6-P + 6NADP+ → 2F-6-P + GAP + 6NADPH + H+ + 3CO2 2. Regulation of pentose phosphate pathway Glucose-6-phosphate Dehydrogenase is the rate-limiting enzyme. NADPH/NADP+↑, inhibit; NADPH/NADP+↓, activate.

3. Significance of pentose Phosphate pathway 1) To supply ribose 5-phosphate for bio-synthesis of nucleic acid; 2) To supply NADPH as H-donor in metabolism; NADPH is very important “reducing power” for the synthesis of fatty acids and cholesterol, and amino acids, etc.

NADPH is the coenzyme of glutathione reductase to keep the normal level of reduced glutathione; So, NADPH, glutathione and glutathione reductase together will preserved the integrity of RBC membrane.

Deficiency of glucose 6-phosphate dehydrogenase results in hemolytic anemia. favism NADPH serves as the coenzyme of mixed function oxidases (mono-oxygenases). In liver this enzyme participates in biotransformation.

§5 Glycogen synthesis and catabolism

Glycogen is a polymer of glucose residues linked by  (1→4) glycosidic bonds, mainly  (1→6) glycosidic bonds, at branch points.

1. Glycogen synthesis (Glycogenesis) The process of glycogenesis occurs in cytosol of liver and skeletal muscle mainly.

UDPG: G active pattern, G active donor. In glycogen anabolism, 1 G consumes 2~P. Glycogen synthase: key E.

UDPG

Branching enzyme

2. Glycogen catabolism (glycogenolysis) Phosphorylase: key E; The end products: 85% of G-1-P and 15% of free G; There is no the activity of glucose 6-phosphatase (G-6-Pase) in skeletal muscle.

Debranching enzyme: glucan transferase -1,6-glucosidase

(1→6) linkage Nonreducing ends Glycogen phosphorylase Transferase activity of debranching enzyme (1→6) glucosidase activity of debranching enzyme Glucose

3. Regulation of glycogenesis and glycogenolysis 1) Allosteric regulation In liver: G phosphorylase glycogenolysis In muscle:

2) Covalent modification Glucagon epinephrine Adenylyl cyclase receptor G protein cAMP PKA Phosphorylase Glycogen synthase glycogenolysis Blood sugar glycogenesis

§6 Gluconeogenesis

Concept: The process of transformation of non-carbohydrates to glucose or glycogen is termed as gluconeogenesis. Materials: lactate, glycerol, pyruvate and glucogenic amino acid. Site: mainly liver, kidney.

1. Gluconeogenic pathway The main pathway for gluconeogenesis is essentially a reversal of glycolysis, but there are three energy barriers obstructing a simple reversal of glycolysis.

1) The shunt of carboxylation of Pyr

2) F-1, 6-BP →F-6-P

3) G-6-P →G 2 lactic acid G consume ATP?

gluconeogenesis

2. Regulation of gluconeogenesis Substrate cycle: The interconversion of two substrates catalyzed by different enzymes for singly direction reactions is called “substrate cycle”. The substrate cycle produces net hydrolysis of ATP or GTP.------futile cycle

Key enzymes of gluconeogenesis PEP carboxykinase Pyr carboxylase Fructose-bisphosphatase Glucose-6-phosphatase

3. Significance of gluconeogenesis Replenishment of Glucose by Gluconeogenesis and Maintaining Normal Blood Sugar Level. Replenishment of Liver Glycogen. Regulation of Acid-base Balance.

First stages (cytosol) Second stages (Mt.) Third stages (Mt.)

Lactic acid (Cori) cycle Lactate, formed by the oxidation of glucose in skeletal muscle and by blood, is transported to the liver where it re-forms glucose, which again becomes available via the circulation for oxidation in the tissues. This process is known as the lactic acid cycle or Cori cycle. prevent acidosis；reused lactate

Lactic acid cycle

§6 Blood Sugar and Its Regulation

1. The source and fate of blood sugar

Blood sugar level must be maintained within a limited range to ensure the supply of glucose to brain. The blood glucose concentration is 3.89～6.11mmol/L normally.

2. Regulation of blood sugar level 1）insulin： for decreasing blood sugar levels. 2）glucagon：for increasing blood sugar levels. 3）glucocorticoid: for increasing blood sugar levels. 4）adrenaline：for increasing blood sugar levels.

3. Abnormal Blood Sugar Level Hyperglycemia: > 7.22～7.78 mmol/L The renal threshold for glucose: 8.89～10.00mmol/L Hypoglycemia: < 3.33～3.89mmol/L

Pyruvate as a junction point