Unit 2: Biochemistry of Carbohydrates

Unit 2: Biochemistry of Carbohydrates

1. Introduction to Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. They are the most abundant organic molecules in nature and serve as:

• Primary energy source for cells
• Structural components (cellulose in plants, glycoproteins)
• Recognition molecules (cell surface antigens)
• Metabolic intermediates

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2. Definition and Classification

2.1 Definition

Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses) and their derivatives.

2.2 Classification

Carbohydrates are classified based on their complexity:

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3. Isomerism and Optical Activity

3.1 Stereoisomerism

Carbohydrates exhibit multiple types of isomerism:

3.2 Optical Activity

• Carbohydrates are optically active due to presence of chiral centers.
• Specific rotation [α] = α / (l × c) where α = observed rotation, l = path length (dm), c = concentration (g/mL).
• The optical activity is used in quality control of sugar-based pharmaceuticals.

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4. Structural Representation of Sugar Molecules

4.1 Fischer Projections

• Open-chain form
• Carbon chain vertical, most oxidized group (aldehyde or ketone) at or near top
• Horizontal bonds project out of plane

4.2 Haworth Projections

• Cyclic (furanose or pyranose) form
• Ring lies perpendicular to plane, thicker bonds indicate front
• For D-sugars, the terminal CH₂OH is above the ring; for L-sugars, below.

4.3 Chair Conformations

• Six-membered pyranose rings adopt chair conformations
• Substituents can be axial or equatorial
• Most stable conformation has bulky groups equatorial

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5. Chemical Properties of Carbohydrates

5.1 Reactions Due to Carbonyl Group

5.2 Reactions Due to Hydroxyl Groups

5.3 Mutarotation

• Change in optical rotation due to interconversion between α and β anomers in solution.
• Important for understanding stability of sugar-containing formulations.

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6. Pharmaceutical Importance of Carbohydrates

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7. Digestion, Absorption, Metabolism, and Excretion of Carbohydrates

7.1 Digestion

• Mouth: Salivary α-amylase (ptyalin) hydrolyzes α1-4 glycosidic bonds of starch, producing maltose, maltotriose, and dextrins.
• Stomach: Acid inactivates salivary amylase; no significant carbohydrate digestion.
• Small intestine:
  • Pancreatic α-amylase continues starch digestion.
  • Brush border enzymes (maltase, isomaltase, sucrase, lactase) hydrolyze disaccharides and oligosaccharides to monosaccharides.
  • Specific enzymes:
    • Maltase: maltose → glucose + glucose
    • Sucrase: sucrose → glucose + fructose
    • Lactase: lactose → glucose + galactose
    • Isomaltase: α1-6 linkages in branched dextrins

7.2 Absorption

• Monosaccharides (glucose, galactose, fructose) are absorbed by intestinal epithelial cells.
• Transport mechanisms:
  • SGLT1 (sodium-glucose linked transporter): Active transport for glucose and galactose (co-transport with Na⁺).
  • GLUT5: Facilitated diffusion for fructose.
  • GLUT2: Facilitated diffusion across basolateral membrane into blood.

7.3 Metabolism Overview

Carbohydrate metabolism involves pathways that maintain blood glucose homeostasis and provide energy.

Major Pathways:

• Glycolysis: Breakdown of glucose to pyruvate (anaerobic) or further to acetyl-CoA (aerobic).
• Citric Acid Cycle (TCA/Krebs cycle) : Oxidation of acetyl-CoA to CO₂ with production of NADH, FADH₂, and GTP.
• Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH/FADH₂ donate electrons to generate ATP.
• Glycogenesis: Synthesis of glycogen from glucose (storage).
• Glycogenolysis: Breakdown of glycogen to glucose-6-phosphate.
• Gluconeogenesis: Synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, amino acids).
• Pentose Phosphate Pathway (PPP) : Generates NADPH and ribose-5-phosphate.
• Glucuronate Pathway: Produces glucuronic acid for conjugation reactions.

7.4 Excretion

• Normally, glucose is completely reabsorbed in the renal tubules via SGLT2.
• Glucosuria occurs when blood glucose exceeds renal threshold (~180 mg/dL) or due to tubular dysfunction.
• Excess carbohydrates are converted to fat (lipogenesis) or glycogen.

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8. Glycolysis

8.1 Overview

Glycolysis is the metabolic pathway that converts glucose (6C) into two molecules of pyruvate (3C) with net production of ATP and NADH. Occurs in cytoplasm of all cells.

8.2 Phases of Glycolysis

Net reaction: Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

8.3 Key Enzymes and Regulation

8.4 Energetics

• Substrate-level phosphorylation: 2 ATP (net)
• NADH from glycolysis can enter mitochondria and generate additional ATP via ETC (approximately 1.5-2.5 ATP per NADH depending on shuttle).

8.5 Fates of Pyruvate

• Aerobic conditions: Pyruvate converted to acetyl-CoA by pyruvate dehydrogenase complex → enters TCA cycle.
• Anaerobic conditions: Pyruvate reduced to lactate by lactate dehydrogenase (regenerates NAD⁺).
• In yeast: Pyruvate decarboxylated to acetaldehyde, then reduced to ethanol.

8.6 Dysregulation of Glycolysis

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9. Feeder Pathways of Glycolysis

Other carbohydrates and metabolites can enter glycolysis at various points:

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10. Pentose Phosphate Pathway (PPP)

10.1 Overview

The pentose phosphate pathway (also called hexose monophosphate shunt) is an alternative route for glucose oxidation. It occurs in cytoplasm and has two main functions:

1. Generation of NADPH for reductive biosynthesis (fatty acids, cholesterol, steroids) and antioxidant defense (glutathione reduction).
2. Production of ribose-5-phosphate for nucleotide synthesis.

10.2 Phases

10.3 Tissue Distribution

• High activity in tissues requiring NADPH: liver, adipose tissue, adrenal cortex, lactating mammary gland, erythrocytes.
• Also active in tissues requiring ribose-5-P: rapidly dividing cells (bone marrow, tumors).

10.4 Regulation

• G6PD is the rate-limiting enzyme.
• Inhibited by NADPH (high NADPH/NADP⁺ ratio).
• Induced by insulin and dietary carbohydrates.

10.5 Dysregulation of PPP

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11. Glucuronate Pathway

11.1 Overview

Also called the uronic acid pathway, this pathway converts glucose to glucuronic acid, ascorbic acid (in animals that can synthesize it), and pentoses. It is active in liver.

11.2 Key Steps

1. Glucose-6-phosphate → Glucose-1-phosphate (phosphoglucomutase)
2. Glucose-1-phosphate + UTP → UDP-glucose (UDP-glucose pyrophosphorylase)
3. UDP-glucose → UDP-glucuronate (UDP-glucose dehydrogenase)
4. UDP-glucuronate can be used for conjugation reactions (phase II metabolism) or converted to D-glucuronate → L-gulonate → ascorbic acid (in some animals; humans lack L-gulonolactone oxidase, cannot synthesize vitamin C)

11.3 Pharmaceutical Importance

• Glucuronidation: UDP-glucuronate is the donor for glucuronidation of drugs, bilirubin, hormones, and xenobiotics (UGT enzymes).
• Detoxification: Increases water solubility for excretion.
• Bilirubin metabolism: Conjugation with glucuronic acid is essential for bilirubin excretion.
• Drug interactions: Induction or inhibition of UGT enzymes affects drug clearance.

11.4 Dysregulation

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12. Glycogen Metabolism

12.1 Glycogenesis (Synthesis of Glycogen)

Location: Liver and muscle (cytoplasm)

Steps:

1. Glucose → Glucose-6-phosphate (hexokinase/glucokinase)
2. Glucose-6-phosphate → Glucose-1-phosphate (phosphoglucomutase)
3. Glucose-1-phosphate + UTP → UDP-glucose (UDP-glucose pyrophosphorylase)
4. Glycogen synthase transfers glucose from UDP-glucose to non-reducing end of glycogen (α1-4 linkage)
5. Branching enzyme (amylo-1,4→1,6-transglucosidase) creates α1-6 branches every 8-12 residues

Regulation:

• Glycogen synthase activated by insulin (dephosphorylation) and glucose-6-phosphate
• Inhibited by glucagon (liver) and epinephrine (muscle) via cAMP-dependent phosphorylation

12.2 Glycogenolysis (Breakdown of Glycogen)

Steps:

1. Glycogen phosphorylase cleaves α1-4 linkages, releasing glucose-1-phosphate
2. Debranching enzyme has two activities:
   • 4:4 transferase (transfers three glucose residues from branch to nearby chain)
   • α1-6 glucosidase (hydrolyzes α1-6 linkage to release free glucose)
3. Glucose-1-phosphate → Glucose-6-phosphate (phosphoglucomutase)
4. In liver, glucose-6-phosphatase converts G6P to free glucose (released into blood); muscle lacks this enzyme (G6P used locally)

Regulation:

• Glycogen phosphorylase activated by glucagon (liver), epinephrine (muscle), AMP, and calcium
• Inhibited by ATP and glucose-6-phosphate

12.3 Glycogen Storage Diseases (Glycogenoses)

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13. Gluconeogenesis

13.1 Overview

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, glucogenic amino acids). It occurs mainly in liver (90%) and kidney (10%).

13.2 Precursors

• Lactate (from muscle/RBCs via Cori cycle)
• Glycerol (from lipolysis)
• Glucogenic amino acids (e.g., alanine, glutamine) – all except leucine and lysine
• Propionate (from odd-chain fatty acids, some amino acids)

13.3 Key Enzymes Bypassing Glycolysis Irreversible Steps

Gluconeogenesis uses most glycolytic enzymes in reverse, but three irreversible steps of glycolysis must be bypassed:

Pyruvate carboxylase requires biotin; converts pyruvate to oxaloacetate in mitochondria. Oxaloacetate is reduced to malate (to exit mitochondria) or transaminated to aspartate, then reconverted to OAA in cytoplasm.

PEPCK converts OAA to PEP (using GTP).

13.4 Regulation

• Hormonal control: Glucagon and cortisol stimulate gluconeogenesis; insulin inhibits.
• Substrate availability: Increased precursors (alanine, lactate) stimulate.
• Allosteric regulation: Acetyl-CoA activates pyruvate carboxylase; AMP inhibits fructose-1,6-bisphosphatase.

13.5 Energetics

Synthesis of one glucose from two pyruvate requires 6 ATP equivalents (4 ATP + 2 GTP) and 2 NADH.

13.6 Dysregulation

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14. Citric Acid Cycle (TCA Cycle, Krebs Cycle)

14.1 Overview

The citric acid cycle is the final common pathway for oxidation of carbohydrates, fatty acids, and amino acids. It occurs in the mitochondrial matrix and produces:

• Energy: GTP (ATP equivalent)
• Reducing equivalents: NADH, FADH₂ (enter ETC for ATP production)
• Intermediates for biosynthesis (anaplerotic reactions)

14.2 Reactions of TCA Cycle

14.3 Regulation

• Citrate synthase: Inhibited by ATP, NADH, succinyl-CoA; activated by ADP.
• Isocitrate dehydrogenase: Activated by ADP, Ca²⁺; inhibited by ATP, NADH.
• α-Ketoglutarate dehydrogenase: Inhibited by succinyl-CoA, NADH; activated by Ca²⁺.

14.4 Energetics per Acetyl-CoA

• 3 NADH → 7.5 ATP (assuming 2.5 ATP/NADH)
• 1 FADH₂ → 1.5 ATP
• 1 GTP → 1 ATP Total: ~10 ATP per acetyl-CoA

14.5 Anaplerotic Reactions

Replenish TCA intermediates:

• Pyruvate → Oxaloacetate (pyruvate carboxylase)
• Pyruvate → Malate (malic enzyme)
• Transamination of aspartate → oxaloacetate
• Glutamate → α-ketoglutarate

14.6 Dysregulation of TCA Cycle

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15. Energetics of Various Metabolic Processes of Carbohydrates

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16. Brief Overview of Carbohydrate Metabolic Disorders

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Summary Tables

Table 1: Major Pathways of Carbohydrate Metabolism

Table 2: Hormonal Regulation of Carbohydrate Metabolism

Table 3: Important Enzyme Deficiencies in Carbohydrate Metabolism

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References

1. Lippincott Williams & Wilkins. (2020). Lippincott's illustrated reviews: Biochemistry. (Chapters on Carbohydrate Metabolism)

2. Berg, J. M., Tymoczko, J. L., & Gatto, G. J. (2019). Stryer's biochemistry (8th ed.). W. H. Freeman and Company. (Chapters on Carbohydrates and Metabolism)

3. Nelson, D. L., & Cox, M. M. (2017). Lehninger principles of biochemistry (7th ed.). W. H. Freeman and Company. (Chapters on Carbohydrates, Glycolysis, TCA, and related pathways)

4. Rodwell, V. W., Bender, D. A., Botham, K. M., Kennelly, P. J., & Weil, P. A. (2017). Harper's illustrated biochemistry (31st ed.). McGraw-Hill Education. (Chapters on Carbohydrate Metabolism and Disorders)

5. Devlin, T. M. (2016). Textbook of biochemistry with clinical correlations (8th ed.). Wiley-Liss. (Chapters on Carbohydrate Metabolism and Clinical Correlations)

6. Scriver, C. R., et al. (2001). The metabolic and molecular bases of inherited disease (8th ed.). McGraw-Hill. (For glycogen storage diseases and other inborn errors)

7. Kumar, V., Abbas, A. K., & Aster, J. C. (2020). Robbins & Cotran pathologic basis of disease (10th ed.). Elsevier. (Clinical correlations)

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Recommended Textbooks for Further Reading:

• Lippincott Williams & Wilkins. (2020). Lippincott's illustrated reviews: Biochemistry. (Excellent for visual learning and clinical correlations)
• Rodwell, V. W., et al. (2017). Harper's illustrated biochemistry (31st ed.). (Strong clinical emphasis)
• Nelson, D. L., & Cox, M. M. (2017). Lehninger principles of biochemistry (7th ed.). (Comprehensive and detailed)