Learn & Review: Biochemistry: Metabolism - Anabolic, Catabolic - Insulin, Glucagon - Amino Acids

Biochemistry: The Chemistry of Life

This video series introduces the fundamental concepts of biochemistry, focusing on the metabolism of carbohydrates, proteins, and fats, and the structure and function of amino acids.

I. Introduction to Biochemistry and Metabolism

• Biochemistry: The study of the chemistry of life.
• Dietary Components: Our diet consists of fats, proteins, and carbohydrates.
• Metabolism: The body's process for dealing with food.
  • Anabolism: Building up complex molecules from simpler ones (endergonic, requires energy).
  • Catabolism: Breaking down complex molecules into simpler ones (exergonic, releases energy).
• Enzymes: Biological catalysts that increase the rate of chemical reactions without being consumed.
• Playlists: Two playlists are available:
  • "Biochemistry for the MCAT" (shorter, for exams like MCAT, DAT, NEET).
  • "Biochemistry New" (longer, for aspiring medical professionals like doctors and nurses).

II. Digestion and Breakdown of Macronutrients

• Digestion: The process of breaking down large food molecules into smaller ones that can be absorbed.
• Carbohydrates:
  • Broken down into disaccharides (e.g., maltose, sucrose, lactose) and then monosaccharides (e.g., glucose, fructose, galactose).
  • Examples: Starch (plant origin), Glycogen (animal origin).
• Proteins:
  • Broken down into polypeptides, oligopeptides, dipeptides, and finally amino acids (monopeptides).
• Fats (Triglycerides):
  • Broken down into glycerol and free fatty acids (or monoacylglycerol).
• Pancreas: The "hero" of digestion, producing enzymes for all macronutrients.
• Digestive Enzymes:
  • Carbohydrates: Amylase
  • Proteins: Peptidase, Trypsin
  • Fats: Lipase
  • Rule of Thumb: Enzymes often end in "-ase".

III. Metabolism: Anabolism vs. Catabolism

• Metabolism: Derived from the Greek word "metabolite," meaning "to change."
• Anabolism (Building Up):
  • Endergonic: Energy enters the process.
  • Example: Building glucose molecules into a large glycogen molecule.
  • Associated with the feeding state.
  • Hero Hormone: Insulin.
• Catabolism (Tearing Down):
  • Exergonic: Energy exits the process.
  • Example: Breaking down glycogen into glucose.
  • Associated with the fasting state.
  • Hero Hormones: Glucagon and its friends (epinephrine, cortisol, thyroid hormone).
• Regulatory vs. Counter-Regulatory Hormones:
  • Insulin is a regulatory hormone.
  • Glucagon, epinephrine, cortisol, and thyroxine are counter-regulatory hormones.

IV. Insulin Land vs. Glucagon Land (Feeding vs. Fasting State)

• Insulin World (Feeding State - Abundance):
  • Anabolic: Builds up.
  • Protein Synthesis (Proteogenesis): Amino acids → Proteins.
  • Glycogen Synthesis (Glycogenesis): Glucose → Glycogen.
  • Lipogenesis (Lipid Synthesis): Free fatty acids → Triglycerides.
  • Glycolysis: Glucose is broken down for immediate energy (ATP).
• Glucagon World (Fasting State - Scarcity):
  • Catabolic: Breaks down.
  • Protein Breakdown (Proteolysis): Proteins → Amino acids.
  • Gluconeogenesis: Making glucose from non-carbohydrate sources (e.g., amino acids).
  • Glycogenolysis: Glycogen → Glucose.
  • Lipolysis (Lipid Breakdown): Triglycerides → Free fatty acids and glycerol.
  • Ketogenesis: Production of ketone bodies (acetone, acetoacetic acid, beta-hydroxybutyrate) from fat breakdown.
    • Ketone bodies are acidic, leading to ketoacidosis.
    • Insulin is the major anti-ketogenic hormone. Absence of insulin leads to ketone body formation (e.g., Diabetic Ketoacidosis).

V. The Central Hub: Acetyl-CoA

• Acetyl-CoA: The central molecule in metabolism, acting as a hub where pathways for carbohydrates, proteins, and lipids converge.
• TCA Cycle (Krebs Cycle/Citric Acid Cycle): Acetyl-CoA enters this cycle in the mitochondria, producing ATP and capturing hydrogen carriers (NADH, FADH2).
• Electron Transport Chain (ETC): Occurs in the mitochondria, utilizing the captured hydrogen to generate significant amounts of ATP.
• Metabolic Flow:
  • Carbohydrates → Glucose → Pyruvate → Acetyl-CoA (via Pyruvate Dehydrogenase).
  • Proteins → Amino Acids → Acetyl-CoA.
  • Fats → Triglycerides → Glycerol (gluconeogenic) + Free Fatty Acids (via Beta-oxidation) → Acetyl-CoA.
  • Lipid breakdown also yields ketone bodies, which can be converted to Acetyl-CoA.

VI. Energy Content of Macronutrients

• Carbohydrates: 4 kcal/gram
• Proteins: 4 kcal/gram
• Fat: 9 kcal/gram (highest energy density, preferred for long-term storage).
• Alcohol: 7 kcal/gram

VII. Amino Acids: Building Blocks of Proteins

• Structure: All amino acids share a central alpha-carbon bonded to an amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom (H), and a variable side chain (R-group).
• Amphoteric: Amino acids can act as both acids and bases due to the presence of both amino and carboxylic acid groups.
• Chirality: Most amino acids are chiral (except glycine) because their alpha-carbon is bonded to four different groups.
  • L-amino acids: Predominant form in human bodies.
  • D-sugars: Predominant form of sugars in human bodies.
• Exceptions:
  • Glycine: Achiral, has H as the R-group.
  • Cysteine: L-amino acid but R-configuration.
• GABA (Gamma-aminobutyric acid): An exception where the amino group is on the gamma-carbon, not the alpha-carbon.
• Types of Amino Acids:
  • Proteogenic Amino Acids: The 20 amino acids incorporated into proteins, coded by genetic codons.
  • Non-proteogenic Amino Acids: Not incorporated into proteins, not coded by genetic codons (e.g., ornithine, homocysteine).
• The Famous 20 Proteogenic Amino Acids: (List provided in the transcript, including abbreviations).
• Classification by Metabolic Fate:
  • Glucogenic: Can be converted to glucose.
  • Ketogenic: Can be converted to ketone bodies.
  • Both Glucogenic and Ketogenic: Can be converted to either.
• Classification by Essentiality:
  • Essential Amino Acids (9): Cannot be synthesized by the body; must be obtained from the diet (Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Methionine, Threonine, Histidine, Lysine).
  • Nonessential Amino Acids (11): Can be synthesized by the body (Alanine, Aspartic acid, Glycine, Glutamic acid, Asparagine, Glutamine, Serine, Tyrosine, Arginine, Cysteine, Proline).
  • Semi-essential/Conditionally Essential: Become essential under certain conditions (e.g., disease). Examples include Tyrosine (essential in Phenylketonuria), Arginine, Cysteine, Glycine, Glutamine, Proline.

VIII. Classification of Amino Acids by Side Chain (R-group)

1. Nonpolar, Nonaromatic Side Chains:

   • Include: Alanine, Glycine, Methionine, Proline, and Branched-Chain Amino Acids (Valine, Leucine, Isoleucine).
   • Glycine: Smallest, simplest, achiral, involved in collagen formation.
   • Alanine: Can be converted to/from pyruvate (ALT enzyme, Vitamin B6 cofactor), part of the Cahill cycle.
   • Methionine: Sulfur-containing, start codon AUG codes for it, ultimate methyl group donor (SAMe).
   • Branched-Chain Amino Acids (Valine, Leucine, Isoleucine): Smallest branched chain is Valine. Maple Syrup Urine Disease affects metabolism of these. Isoleucine has two chiral centers.
   • Proline: Cyclic structure, introduces kinks in peptide chains, an imino acid, essential for collagen.

2. Aromatic Side Chains: Contain a benzene ring.

   • Include: Phenylalanine, Tyrosine, Tryptophan.
   • Tryptophan: Double ring structure, precursor to niacin (Vitamin B3 → NAD+), serotonin, and melatonin.
   • Phenylalanine: Precursor to Tyrosine (by adding a hydroxyl group via Phenylalanine hydroxylase). Relatively nonpolar.
   • Tyrosine: Derived from Phenylalanine, precursor to DOPA, dopamine, norepinephrine, epinephrine (catecholamines), and thyroid hormone. Relatively polar due to the hydroxyl group. Phenylalanine hydroxylase deficiency leads to Phenylketonuria (PKU). Tyrosinase deficiency leads to albinism.

3. Polar, Nonaromatic Side Chains:

   • Include: Serine, Threonine, Cysteine, Asparagine, Glutamine.
   • Serine & Threonine: Hydroxyl (-OH) containing. Threonine has a methyl group and two chiral centers.
   • Cysteine: Sulfur (-SH) containing, forms disulfide bonds (important for protein structure).
   • Asparagine & Glutamine: Amide (-CONH2) containing, important for hydrogen bonding and nitrogen transport.

4. Negatively Charged (Acidic) Side Chains:

   • Include: Aspartic Acid (Aspartate), Glutamic Acid (Glutamate).
   • Have a carboxylate group (-COO-) at physiological pH.
   • Aspartate: Negatively charged, involved in nitrogen elimination in the urea cycle.
   • Glutamate: Negatively charged, an excitatory neurotransmitter.

5. Positively Charged (Basic) Side Chains:

   • Include: Arginine, Lysine, Histidine.
   • Mnemonic: "Her Leggings Are So Basic" (Histidine, Lysine, Arginine).
   • Arginine: Positively charged, can produce nitric oxide (vasodilator), has three nitrogens.
   • Lysine: Positively charged, terminal primary -NH3+ group, precursor to carnitine (fatty acid transport).
   • Histidine: Positively charged, has an imidazole ring, physiological buffer (pKa ~6), found in histones.

IX. Amino Acid Derivatives

• Phenylalanine → Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine (Catecholamines).
• Glycine → Protoporphyrin → Heme (in Hemoglobin).
• Tryptophan → Serotonin → Melatonin; Tryptophan → Niacin (NAD+).
• Histidine → Histamine (by decarboxylation).
• Glutamate → GABA (by decarboxylation).
• Collagen Synthesis: Requires Glycine, Proline, and Lysine.

X. Protein Metabolism

• Anabolism (Insulin Land): Amino acids are linked via peptide bonds (condensation/dehydration reaction) to form dipeptides, oligopeptides, and polypeptides (proteins).
• Catabolism (Glucagon Land): Proteins are broken down into amino acids via hydrolysis (adding water), catalyzed by hydrolytic enzymes (e.g., peptidases, trypsin, chymotrypsin).
• Peptide Bond: An amide linkage formed between the carboxyl group of one amino acid and the amino group of another, releasing water.
• Hydrolysis: The reverse reaction, breaking peptide bonds by adding water.
• Protein Breakdown during Starvation: The body breaks down muscle proteins as a last resort for energy, yielding amino acids.
  • Amino acids undergo transamination and deamination.
  • The amino group enters the urea cycle (converted to urea and excreted).
  • The carbon skeleton can be used for gluconeogenesis or ketogenesis.