Metabolism and Carbohydrate Metabolism

Metabolism

• Metabolism is the sum total of all biochemical reactions in a living organism.

• There are two types of metabolism:

  • Anabolism: Metabolic reactions where small molecules join to form larger ones, requiring energy.

  • Catabolism: Metabolic reactions where large molecules break down into smaller ones, producing energy.

Metabolic Pathway

• A metabolic pathway is a series of consecutive biochemical reactions converting a starting material into an end product.

  • Linear pathways generate a final product through a series of reactions.

  • Cyclic pathways regenerate the first reactant in a series of reactions.

• Major metabolic pathways are similar across all life forms.

Nucleotide-Containing Compounds

• Includes Adenosine Phosphates (ATP, ADP, AMP), Flavin Adenine Dinucleotide (FAD), Nicotinamide Adenine Dinucleotide (NAD+), and Coenzyme A (CoA).

Adenosine Phosphates

• ATP is the most important energy-rich compound in a cell.

• At physiological pH, ATP has a charge of -4 due to the removed protons on the triphosphate group.

• Energy-rich compounds release energy after hydrolysis due to particular structural features.

• Hydrolysis of ATP in water transfers a phosphoryl group from ATP to a water molecule, producing adenosine diphosphate (ADP) and inorganic phosphate (Pi).

• The transfer of a phosphoryl group from ATP to water releases energy; measured by free energy change, \Delta G.

  • Negative \Delta G indicates energy is released (exergonic).

  • Positive \Delta G indicates energy is absorbed (endergonic).

  • \Delta G represents \Delta G measured under standard conditions.

ATP-ADP Cycle

• ATP acts as an immediate donor of free energy.

• The turnover rate of ATP is high, with an ATP molecule typically hydrolyzed within 1 minute of formation.

• A human body hydrolyzes about 40 kg of ATP every 24 hours at rest, and up to 0.5 kg per minute during strenuous exertion.

• ATP must be continuously regenerated from ADP for cellular work to occur.

• ATP regeneration sources:

  1. Oxidative phosphorylation: Primary source in aerobic organisms.

  2. Glycolysis: Net formation of two phosphate molecules from lactate produced from one glucose molecule.

  3. Citric acid cycle: One phosphate molecule generated directly at the succinate thiokinase step.

• Phosphagens (e.g., creatine phosphate in vertebrates, arginine phosphate in invertebrates) act as storage forms of high-energy phosphate.

Flavin Adenine Dinucleotide

• A coenzyme needed in metabolic redox reactions.

• Contains the B vitamin riboflavin.

• The ADP subunit is the activating factor.

• Exists in two forms:

  1. Oxidized form: FAD

  2. Reduced form: FADH2

• FAD + 2H^+ + 2e^- \rightarrow FADH_2

Nicotinamide Adenine Dinucleotide

• Contains the B vitamin nicotinamide.

• Exists in two forms:

  1. Oxidized form: NAD^+

  2. Reduced form: NADH

• NAD^+ + 2H^+ + 2e^- \rightarrow NADH + H^+

Coenzyme A

• CoA-SH

• A derivative of the B vitamin pantothenic acid.

• The sulfhydryl group is the active portion.

• Involved in the transfer of acetyl groups in metabolic pathways.

• An acetyl group is the portion of an acetic acid molecule (CH_3-COOH) that remains after the -OH group is removed.

Biochemical Energy Production

• Energy for the human body is obtained from ingested food.

• Involves a multistep process with several catabolic pathways.

• Four general stages:

  1. Digestion

  2. Acetyl group formation

  3. Citric acid cycle

  4. Electron transport chain (ETC) and oxidative phosphorylation

Stage 1: Digestion

• Begins in the mouth, continues in the stomach, and completes in the small intestine.

• End products:

  • Glucose and other monosaccharides from carbohydrates

  • Amino acids from proteins

  • Fatty acids and glycerol from fats and oils

Stage 2: Acetyl Group Formation

• Small molecules from digestion are further oxidized.

• Primary products: Two-carbon acetyl units (attached to coenzyme A to form acetyl CoA) and reduced coenzyme NADH.

Stage 3: Citric Acid Cycle

• Occurs inside mitochondria.

• Acetyl groups are oxidized to produce CO_2 and energy.

• Forms of energy release:

  • Lost as heat

  • Carried by reduced coenzymes NADH and FADH2

• CO_2 exhaled during breathing primarily comes from this stage.

• Also known as:

  • Krebs cycle (after Hans Adolf Krebs)

  • Tricarboxylic acid cycle

• Amphibolic

• The final common pathway for oxidation of carbohydrates, proteins, and lipids.

Citric Acid Cycle - Steps

• 8 steps:

  1. Formation of Citrate (Citrate synthase)

  2. Formation of Isocitrate (Aconitase)

  3. Oxidation and decarboxylation of Isocitrate (Isocitrate dehydrogenase)

  4. Oxidation and decarboxylation of \alpha-Ketoglutarate (alpha-Ketoglutarate dehydrogenase)

  5. Thioester Bond Cleavage in Succinyl CoA and Phosphorylation of GDP – Succinyl CoA synthetase/Succinyl thiokinase

  6. Oxidation of Succinate (Succinate dehydrogenase)

  7. Hydration of Fumarate (Fumarase)

  8. Oxidation of l-Malate to Regenerate Oxaloacetate (Malate dehydrogenase)

Citric Acid Cycle - Key Points

• Major pathway for ATP formation.

• Provides substrates for gluconeogenesis, amino acid synthesis, and fatty acid synthesis.

• Occurs in all cells with mitochondria, specifically in the mitochondrial matrix.

• Substrate: Acetyl CoA.

• Products: 2 CO_2, 1 GTP, 3 NADH, and 1 FADH2.

• Rate-limiting step: Isocitrate to \alpha-ketoglutarate (Enzyme: Isocitrate dehydrogenase).

• Oxidation (producing NADH or FADH2) occurs in four steps:

  • Isocitrate to \alpha-Ketoglutarate - NADH

  • \alpha-Ketoglutarate to Succinyl-Coa - NADH

  • Succinate to Fumarate – FADH2

  • Malate to Oxaloacetate - NADH

• Decarboxylation (removal of CO_2) occurs in two steps:

  • Isocitrate to \alpha-Ketoglutarate

  • \alpha-Ketoglutarate to Succinyl-Coa

• GTP production:

  • Succinyl CoA to Succinate

Regulation of the Citric Acid Cycle

• Regulated by:

  • Citrate synthetase (inhibited by ATP and NADH, activated by ADP)

  • Isocitrate dehydrogenase (inhibited by NADH, activated by ADP)

  • \alpha-ketoglutarate dehydrogenase complex (inhibited by Succinyl CoA, NADH and ATP, activated by ADP)

• High ATP levels reduce the cycle's operation; low ATP levels stimulate it.

Stage 4: Electron Transport Chain and Oxidative Phosphorylation

• Occurs inside mitochondria.

• NADH and FADH2 supply hydrogen ions and electrons for ATP production.

• Molecular O2 (inhaled via breathing) is converted to H2O.

Electron Transport Chain Details

• Series of protein complexes and molecules transferring electrons from donors to acceptors via redox reactions.

• Couples electron transfer with proton transfer across a membrane.

• Complex I: NADH–Coenzyme Q reductase

• Complex II: Succinate–Coenzyme Q reductase

• Complex III: Coenzyme Q–Cytochrome C reductase

• Complex IV: Cytochrome C Oxidase

Oxidative Phosphorylation

• As electrons move along the electron transport chain, about 52.6 kcal/mol of free energy is released.

• ATP is synthesized from ADP and Pi. This synthesis is linked to the oxidation of NADH and FADH2.

• Takes place at three different locations along the electron transport chain.

Energy Changes in the Electron Transport Chain

• During oxidative phosphorylation:

  • Conversion of NADH to NAD^+ generates 2.5 ATP molecules from ADP.

  • Conversion of FADH2 to FAD generates 1.5 ATP molecules.

• Every acetyl CoA molecule entering the citric acid cycle produces:

  • 3 NADH molecules

  • 1 FADH2 molecule

  • 1 GTP molecule (equivalent to ATP)

• 10 ATP molecules are formed per acetyl CoA catabolized

The Chemiosmotic Hypothesis

• Oxidations of the electron transport chain and the synthesis of ATP involves a flow of protons (H^+.

• The flow of electrons causes H^+ ions to be pumped from the matrix across the inner membrane, creating a difference in H^+ concentration and electrical potential across the membrane.

• Protons flow back through the membrane through a channel formed by the enzyme F1-ATPase

• The flow of protons drives the phosphorylation reaction, and provides energy for ATP synthesis.

Uncouplers

• Compounds that increase the permeability of the inner mitochondrial membrane to protons

• Electron transport proceeds rapidly without establishing a proton gradient

• Effects:

  • \uparrow oxygen consumption

  • \downarrow NADH/NAD+ and FADH2/FADH ratio

  • \downarrow ATP synthesis

• Examples:

  • Synthetic: 2,4 dinitrophenol, aspirin

  • Uncoupling protein: Thermogenin (brown fat), Cyanide, CO_2

Carbohydrate Metabolism

• Begins in the mouth with \alpha-amylase catalyzing the hydrolysis of \alpha-glycosidic linkages

• Primary site is within the small intestine, where pancreatic \alpha-amylase breaks down polysaccharide chains

• The final step occurs on the outer membranes of intestinal mucosal cells with enzymes like maltase, sucrase, and lactase

Post-Digestion

• Glucose, fructose, and galactose are absorbed into the bloodstream and transported to the liver.

• In the liver, fructose and galactose are converted to glucose or compounds metabolized by the same pathway as glucose

Blood Sugar Levels

• Glucose is the most plentiful monosaccharide in blood.

• Normal fasting blood sugar level (8-12 hours) is 70-110 mg/100mL (or mg/dL).

• Maximum level reaches 140-160 mg/100 mL about 1 hour after a carbohydrate-containing meal, returning to normal after 2-2.5 hours

• Hypoglycemia: Blood sugar levels below the normal fasting level, leading to dizziness, fainting, convulsions, and shock.

• Hyperglycemia: Blood sugar levels above the normal fasting level.

• Renal threshold: Blood glucose levels above about 180 mg/100 mL, where glucose is excreted in the urine (glucosuria).

Liver's Role

• The liver regulates blood glucose levels.

• When blood glucose levels rise, the liver removes glucose from the bloodstream, converting it to glycogen or triglycerides for storage.

• When blood glucose levels are low, the liver converts stored glycogen to glucose and synthesizes new glucose from non-carbohydrate sources (gluconeogenesis).

Glycolysis

• A series of ten reactions converting a glucose molecule into two pyruvate molecules

Steps in Glycolysis

1. Phosphorylation Using ATP - Hexokinase

2. Isomerization – Phosphoglucoisomerase

3. Phosphorylation Using ATP – Phosphofructokinase

4. Cleavage – Aldolase

5. Isomerization – triose phosphate isomerase

6. Oxidation and Phosphorylation Using Pi - glyceraldehyde 3-phosphate dehydrogenase

7. Phosphorylation of ADP – phosphoglycerokinase

8. Isomerization – phosphoglyceromutase

9. Dehydration – enolase

10. Phosphorylation of ADP - pyruvate kinase

Key Aspects

• Major pathway for glucose metabolism, converting glucose into 3-carbon compounds to provide energy.

• Most common type: Embden-Meyerhof-Parnas pathway.

• Occurs in the cytosol of all mammalian cells.

• Substrate: Glucose.

• End-products: 2 molecules of either pyruvate or lactate.

• Net gain of 2 ATP.

• Two steps produce ATP via substrate-level phosphorylation:

  • 1,3-Bisphosphoglycerate \rightarrow 3-Phosphoglycerate (Enzyme: Phosphoglycerate kinase)

  • Phosphoenolpyruvate \rightarrow Pyruvate (Enzyme: Pyruvate kinase)

• Rate-limiting step: Fructose-6-phosphate \rightarrow Fructose-1,6-bisphosphate (Enzyme: Phosphofructokinase-1)

Regulation of Glycolysis

• Regulated by three enzymes:

  • Hexokinase: Inhibited by high concentrations of glucose-6-phosphate (feedback inhibition).

  • Phosphofructokinase: Inhibited by high concentrations of ATP and citrate and activated by high concentrations of ADP and AMP.

  • Pyruvate kinase: Inhibited by high concentrations of ATP.

Fates of Pyruvate

• Three things that can happen to pyruvate after glycolysis:

  • Oxidation to acetyl CoA under aerobic conditions.

  • Reduction to lactate under anaerobic conditions.

  • Reduction to ethanol under anaerobic conditions for some prokaryotic organisms.

Reduction to Lactate

• Under anaerobic conditions, pyruvate is reduced to lactate as a means of regenerating NAD^+

• Buildup of lactate in the muscles causes muscle pain and cramps and triggers an increase in the rate and depth of breathing

Reduction to Ethanol

• Organisms including yeast regenerate NAD^+ by alcoholic fermentation

• Decarboxylation of pyruvate produces acetaldehyde

• Acetaldehyde is then reduced by NADH to form ethanol

ATP Yield

• Complete oxidation of glucose yields 32 ATP molecules.

• In muscle and brain cells, 30 ATP overall.

Energy Efficiency

• Glucose oxidation liberates 686 kcal/mol, whereas the synthesis of 32 mol of ATP stores 234 kcal/mol.

• The efficiency of the energy storage is 34%

Glycogen Synthesis

• Excess glucose is converted into glycogen in a process called glycogenesis.

• Glycogen is stored primarily in the liver and muscle tissue

• The liver can store about 110 g of glycogen, and the muscles can store about 245 g

Steps in Glycogen Synthesis

1. Isomerization – phosphoglucomutase

2. Activation Formation of UDP-glucose. - UDP-glucose pyrophosphorylase

3. Linkage to Chain – Glycogen synthase

4. Formation of branches – amylo \alpha(1\rightarrow4) \rightarrow \alpha(1\rightarrow6) transglucosidase

Key Points

• Synthesis of glycogen

• Occurs in the liver and muscle cytosol

• Substrate is \alpha-D-glucose

• Product is Glycogen

• Rate-limiting step: Elongation of glycogen chains, i.e., creation of \alpha-(1,4) glycosidic bonds (Enzyme: Glycogen synthase)

Glycogenolysis

• Glycogenolysis is the breakdown of glycogen back into glucose.

• Glycogenolysis can occur in the liver (and kidney and intestinal cells) but not in muscle tissue

• The first step is the cleaving of the a(1-4) linkages, catalyzed by glycogen phosphorylase.

• A debranching enzyme hydrolyzes the a(1-6) linkages

• In the second step, phosphoglucomutase isomerizes glucose 1-phosphate to glucose 6-phosphate.

• In the final step, glucose 6-phosphate is hydrolyzed to free glucose by the enzyme glucose 6-phophatase

Steps in Glycogenolysis

1. Phosphorolysis – Glycogen phosphorylase

2. Isomerization – Phosphoglucomutase

3. Removal of Branches - \alpha(1\rightarrow4)\rightarrow \alpha(1\rightarrow4) glucantransferase; amylo-\alpha(1\rightarrow6) glucosidase

Glycogen in Muscles and the Liver

• Muscle cells lack glucose 6-phophatase and cannot form free glucose from glycogen but carries out the first two steps of glycogenolysis to produce glucose 6-phosphate

• In the liver, glycogen is broken down all the way to form free glucose

Gluconeogenesis

• The supply of glucose in the form of liver and muscle glycogen can be depleted by about 12-18 hours of fasting

• Gluconeogenesis is the process of synthesizing glucose from noncarbohydrate materials.

• The carbon skeletons of lactate, glycerol, and certain amino acids are used to synthesize pyruvate, which is then converted to glucose

Gluconeogenesis Steps

• STEPS 1 & 2: PYRUVATE \rightarrow OAA \rightarrow PEP

  • Pyruvate \rightarrow Oxaloacetate

    • Enzyme: Pyruvate carboxylase

    • Requires biotin and ATP

  • Oxaloacetate \rightarrow Phosphoenolpyruvate

    • Enzyme: Phosphoenolpyruvate Carboxykinase

    • Requires GTP

• STEP 9: FRUCTOSE-1,6-BP \rightarrow FRUCTOSE-6-P

  • Enzyme: Fructose-1,6-bisphosphatase

  • Rate-limiting step of gluconeogenesis

• STEP 11: GLUCOSE-6-PHOSPHATE \rightarrow GLUCOSE

  • Enzyme: Glucose 6-phosphatase

  • Final step shared with glycogenolysis

  • Present in liver and kidney, to release glucose into the bloodstream

CORI Cycle

• About 90% of gluconeogenesis occurs in the liver.

• Very little takes place in the brain, skeletal muscle, or heart

• Gluconeogenesis involving lactate is especially important under anaerobic conditions

• During exercise, lactate levels increase in muscle tissue, and some diffuses into the blood.

Regulation of Carbohydrate Metabolism

• Requires that metabolic pathways be responsive to cellular conditions and that energy is not wasted in producing unneeded materials

• Besides the regulation of enzymes at key control points, the body also uses three important regulatory hormones:

  • Epinephrine

  • Glucagon

  • Insulin

Insulin

• Enhances the absorption of glucose from the blood into the cells of active tissues

• Increases rate of synthesis of glycogen, fatty acids, and proteins

• Stimulates glycolysis

Glucagon

• Activates the breakdown of glycogen in the liver, thereby increasing blood glucose levels

• Insulin and glucagon work in opposition to each other

Epinephrine

• Stimulates glycogen breakdown in muscles

• Increases heart rate, constricts blood vessels, and dilates air passages

Carbohydrate Metabolism Hormones