BIOSCI 101 Lecture 4

Setting the Stage – Daily ATP Demand & Energy Analogies

• A resting adult (≈70–85 kg) hydrolyses ≈$70\,\text{kg}$ of ATP per day.

  • ≈$7\,\text{kg}$ (10 %) is consumed by the heart alone.

  • Chemical energy equivalent ≈ $280$ AA batteries or enough heat to boil several litres of water.

• Laboratory-grade ATP costs ≈$160 \$/\text{g}$; keeping an 85 kg lecturer running would cost ≈$14\,000,000\$/\text{day}$ – an illustrative, not literal, figure.

• Elite marathoner Eliud Kipchoge (52 kg body mass) is estimated to turn over ≈$40\,\text{kg}$ ATP during a sub-2-h marathon – almost his entire body mass.

Oxidation–Reduction (Redox) Refresher

• Mnemonic: “LEO says GER”
  • Loss of Electrons = Oxidation (LEO)
  • Gain of Electrons = Reduction (GER)

• Electrons are usually abstracted from C–H or C–OH bonds (not from C–C directly → reason for multi-step pathways).

• Hydrogen atoms carry one electron; removing an H removes that e⁻.

• Typical redox pair example: $\text{Malate}\;\rightarrow\;\text{Oxaloacetate}+2\,\text{H}^+ + 2\,\text{e}^-$ (citric acid cycle preview).

Enzymes & Pathways – Why All the Steps?

• Direct combustion of glucose would release energy explosively (cf. the Hindenburg); cells extract energy incrementally via enzyme-coupled pathways.

• Metabolic control = homeostasis; pathways permit:

  • Fine-tuned regulation.

  • Branch points for biosynthesis.

  • Safe capture of energy in phosphate or redox bonds.

• Enzymes are often named -dehydrogenase (remove/add H & e⁻) or -kinase (transfer phosphate).

ATP Economics (Terminology)

• Substrate-level phosphorylation (SLP): ATP formed by direct transfer of a high-energy phosphate from a substrate to ADP.

• Oxidative phosphorylation (OXPHOS): ATP formed via the electron-transport chain and proton motive force (mitochondria; later lecture).

Glycolysis – Universal Sugar Splitter

• Occurs in cytosol of all known life forms; anaerobic; 10 enzyme-catalysed steps.

• Overall: $\text{Glucose}\,(\text{C}<em>6)\;\rightarrow\;2\;\text{Pyruvate}\,(\text{C}</em>3) + 2\;\text{ATP}_{\text{net}} + 2\;\text{NADH}$.

• Serves to:

  • Trap glucose inside cell.

  • Prime molecule so later steps can harvest energy.

  • Supply intermediates for other pathways (e.g.DHAP for lipid synthesis).

1. Glucose Entry & Trapping

• GLUT transporters (GLUT1–4): alternating-conformation carriers; bidirectional but passive.

• Immediate phosphorylation keeps glucose from diffusing back out.
  $\text{ATP}+\text{Glucose}\xrightarrow{\text{Hexo/Glucokinase}}\text{ADP}+\text{Glucose-6-P}$

Hexokinase vs Glucokinase (GK)

Michaelis–Menten Basics

• Velocity vs [S] is hyperbolic.
  $K<em>m$ = [S] at $\tfrac12V</em>{max}$ – reflects affinity.
  High $K<em>m$ ⇒ low affinity; low $K</em>m$ ⇒ high affinity.

• Named for Leonor Michaelis & Maud Menten (who also pioneered gel electrophoresis, 1912).

2. Energy-Investment Phase (Steps 1–3)

1. Hexokinase / Glucokinase – ATP 1 invested; Glucose-6-P produced (irreversible).

2. Phosphoglucose Isomerase – Glc-6-P ⇌ Fru-6-P (ring rearrangement; prepares C-1 for P addition).

3. Phosphofructokinase-1 (PFK-1) – ATP 2 invested; Fru-6-P → Fru-1,6-bisP (irreversible, COMMITMENT step).

3. Cleavage Phase (Steps 4–5)

4. Aldolase – Splits Fru-1,6-bisP → DHAP + Glyceraldehyde-3-P (GAP).

5. Triose-Phosphate Isomerase (TPI) – Rapidly interconverts DHAP ⇌ GAP so pathway proceeds with two GAP molecules.

4. Energy-Payoff Phase (Steps 6–10)

6. GAP Dehydrogenase (GAPDH) – Only redox step.
   • Adds inorganic Pi (no ATP cost) to GAP → 1,3-bisP-glycerate.
   • Reduces $\text{NAD}^+\;\rightarrow\;\text{NADH}+\text{H}^+$ (2 per glucose).

7. Phosphoglycerate Kinase – SLP yields 2 ATP (first return) → 3-P-glycerate.

8. Phosphoglycerate Mutase – Shifts phosphate to C-2 → 2-P-glycerate.

9. Enolase – Dehydration (−H₂O) → Phosphoenolpyruvate (PEP).

10. Pyruvate Kinase – SLP yields 2 ATP (second return); PEP → Pyruvate (irreversible).

ATP & Reducing-Power Ledger

Regulation of Glycolysis

• Irreversible Steps = Control Points: HK/GK, PFK-1, Pyruvate Kinase.

• PFK-1 Allosteric Modulators
  • ATP (high) → inhibitory (feedback)
  • AMP/ADP → stimulatory (reflects low energy)
  • Citrate → inhibitory (signals citric-acid-cycle sufficiency)
  • ↓pH (↑H⁺ from lactate) → inhibitory (protects cell).

NAD⁺/NADH – The Soluble Electron Shuttle

• Structure: Adenosine-diphosphate linked to nicotinamide ribonucleotide (vitamin B₃ derived); retains ribose 2′-OH ⇒ RNA heritage.

• Redox equation
  $\text{NAD}^+ + 2\,\text{e}^- + 2\,\text{H}^+ \rightleftharpoons \text{NADH} + \text{H}^+$

• Transports 2 e⁻ (as hydride + free H⁺) through aqueous cytosol to mitochondria (aerobic) or to fermentation enzymes (anaerobic).

Fate of Pyruvate

1. Aerobic (O₂ Present)

• Pyruvate → Acetyl-CoA (pyruvate dehydrogenase) → enters citric acid cycle → OXPHOS.
  • Slow but yields ≈30–32 ATP/glucose (31–73 % efficiency depending on conditions).

2. Anaerobic (O₂ Absent or High-Demand Muscle)

• Need to re-oxidise NADH to keep GAPDH running.

a) Lactate Fermentation (animals, some microbes)

$\text{Pyruvate} + \text{NADH} \xrightarrow{\text{Lactate DH}} \text{Lactate} + \text{NAD}^+$

• Lactate accumulates → ↓pH & activates nociceptors ("burn" sensation).

• Cori Cycle: muscle lactate → blood → liver; liver reconverts 2 lactate → glucose (costs 6 ATP) → blood – provides glucose during recovery.

b) Ethanol Fermentation (yeast; some fish under anoxia)

$\text{Pyruvate} \rightarrow \text{Acetaldehyde} + \text{CO}_2 \xrightarrow{\text{Alcohol DH}} \text{Ethanol}$

• Restores NAD⁺; CO₂ forms bubbles in bread, beer, champagne.

• Goldfish in frozen lakes produce ethanol to survive prolonged anoxia – “too drunk to drive” analogy.

Substrate-Level Phosphorylation vs Oxidative Phosphorylation

• SLP sites in glycolysis: Phosphoglycerate Kinase (step 7) & Pyruvate Kinase (step 10).
  • Direct phosphate transfer from a high-energy substrate.

• OXPHOS (mitochondria): Will utilise the 2 NADH (cytosolic shuttles) to generate ~5 ATP/glucose when O₂ is available.

Thought Experiment – Draining DHAP

• If another pathway siphons DHAP:

  1. TPI equilibrium pulls GAP → DHAP (back reaction).

  2. Aldolase equilibrium pulls Fru-1,6-bisP → GAP + DHAP.

  3. PFK flux increases; GAPDH may reverse, consuming NADH & ATP.
     ⇒ Demonstrates why branch-point control is vital.

Evolutionary & Historical Nuggets

• Glycolysis predates atmospheric O₂ (≈2.4 Ga) – explains its universality.

• NAD⁺’s ribose suggests metabolism first evolved in an RNA world.

• Enzyme kinetics framework (Michaelis–Menten) devised 1913; Maud Menten also pioneered protein electrophoresis.

Practical, Ethical & Humorous Asides

• Students need not memorise every structure; focus on mechanisms & control points – multiple-choice exam will supply names.

• Spontaneous human combustion myths explained by nitrocellulose-stiffened Victorian clothing acting as wick + body fat fuel.

• Lecturer’s “pet pics” & bad weather acknowledged; previous exam questions often contain humour – worthwhile practice.

Quick-Fire Summary Sheet

• ATP investment: 2  • ATP payoff: 4  • Net: 2 ATP/glucose.

• Reducing power: 2 NADH (≈5 ATP potential via OXPHOS).

• Irreversible/Regulatory enzymes: HK (I–III) or GK, PFK-1, Pyruvate Kinase.

• Main control ligand: ATP (inhibits PFK); AMP (activates PFK); citrate & low pH (inhibit).

• Anaerobic rescue: Lactate (animals), Ethanol + CO₂ (yeast/fish).

• Efficiency: Anaerobic ≈2–4.5 % of glucose’s theoretical energy; Aerobic up to ≈31–73 % (depending on ΔG₍ATP₎ used).