10.2 Glycolysis, Pyruvate Dehydrogenase, TCA Cycle, and Oxidative Phosphorylation

10.2 Glycolysis, Pyruvate Dehydrogenase, TCA Cycle, and Oxidative Phosphorylation

Central energy metabolism converts nutrient substrates into ATP through linked catabolic pathways spanning the cytoplasm and mitochondria. Understanding these pathways, their regulatory checkpoints, cellular yields, and toxicological vulnerabilities is paramount for NPLEX Part I candidates.

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Glycolysis: Cytosolic Glucose Oxidation

Glycolysis is a universal cytosolic metabolic pathway that metabolizes 1 molecule of 6-carbon glucose into 2 molecules of 3-carbon pyruvate, generating a net yield of 2 ATP (via substrate-level phosphorylation) and 2 NADH.

The 10 Enzymatic Steps & Key Checkpoints

Glycolysis consists of an energy investment phase (consuming 2 ATP) and an energy generation phase (producing 4 ATP).

Glucose (6C)
  │  [1] Hexokinase / Glucokinase (Consumes 1 ATP)
  ▼
Glucose-6-Phosphate
  │  [2] Phosphoglucose Isomerase
  ▼
Fructose-6-Phosphate
  │  [3] Phosphofructokinase-1 (PFK-1)  <-- RATE-LIMITING STEP! (Consumes 1 ATP)
  ▼
Fructose-1,6-Bisphosphate
  │  [4] Aldolase
  ├───────────────────────────┐
  ▼                           ▼
Glyceraldehyde-3-Phosphate  Dihydroxyacetone Phosphate (DHAP)
  │                           │ [5] Triose Phosphate Isomerase
  └───◄───────────────────────┘
  │ (x2 molecules per glucose)
  │  [6] Glyceraldehyde-3-Phosphate DH (Produces 2 NADH total)
  ▼
1,3-Bisphosphoglycerate
  │  [7] Phosphoglycerate Kinase (Produces 2 ATP total - Substrate Level)
  ▼
3-Phosphoglycerate
  │  [8] Phosphoglycerate Mutase
  ▼
2-Phosphoglycerate
  │  [9] Enolase (Inhibited by Fluoride)
  ▼
Phosphoenolpyruvate (PEP)
  │  [10] Pyruvate Kinase (Produces 2 ATP total - Substrate Level)
  ▼
Pyruvate (3C x2)

Irreversible Glycolytic Steps & Regulation

1. Hexokinase vs. Glucokinase (Step 1):
   • Hexokinase: Expressed in most extrahepatic tissues. Low $K_m$ (high affinity for glucose), low $V_{max}$. Strongly inhibited by its product, Glucose-6-Phosphate (G6P), preventing excessive cellular glucose trapping when energy is abundant.
   • Glucokinase: Expressed exclusively in liver parenchymal cells and pancreatic $\beta$-cells. High $K_m$ (low affinity; operates efficiently only after carbohydrate-rich meals), high $V_{max}$ (capacity to clear large portal glucose loads). Not inhibited by G6P; regulated by glucokinase regulatory protein (GKRP).
2. Phosphofructokinase-1 (PFK-1 - Step 3): The primary rate-limiting enzyme of glycolysis.
   • Positive Allosteric Effectors: High AMP, Fructose-2,6-bisphosphate (F2,6BP). Insulin increases F2,6BP by dephosphorylating the bifunctional enzyme PFK-2/FBPase-2, stimulating PFK-1 and driving glycolysis.
   • Negative Allosteric Effectors: High ATP, Citrate (signals abundant energy/TCA intermediates).
3. Pyruvate Kinase (Step 10): Catalyzes PEP $\rightarrow$ Pyruvate.
   • Activated by: Fructose-1,6-bisphosphate (feed-forward activation).
   • Inhibited by: ATP, Alanine, and Acetyl-CoA. Glucagon induces phosphorylation/inactivation in liver.

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Metabolic Fates of Pyruvate

Pyruvate generated by glycolysis occupies a critical branch point depending on cellular oxygen status, organ tissue type, and metabolic demand:

                        ┌─► Lactate (Anaerobic: LDH; re-oxidizes NADH to NAD+)
                        │
                        ├─► Acetyl-CoA (Aerobic: PDH in mitochondrial matrix)
Pyruvate (3-Carbon) ────┤
                        ├─► Oxaloacetate (Anaplerotic / Gluconeogenesis: Pyruvate Carboxylase)
                        │
                        └─► Alanine (Transamination: ALT; Cahill / Glucose-Alanine Cycle)

1. Aerobic Oxidation to Acetyl-CoA: Transported into mitochondrial matrix for oxidation by the Pyruvate Dehydrogenase (PDH) complex.
2. Anaerobic Reduction to Lactate: Catalyzed by Lactate Dehydrogenase (LDH) in the cytosol ($\text{Pyruvate} + \text{NADH} + \text{H}^+ \rightleftharpoons \text{Lactate} + \text{NAD}^+$). Crucial in mature red blood cells (which lack mitochondria) and exercising skeletal muscle to regenerate cytosolic $\text{NAD}^+$ required to maintain Step 6 of glycolysis.
3. Transamination to Alanine: Catalyzed by Alanine Aminotransferase (ALT) using B6 (PLP). Facilitates nitrogen transport from muscle to liver via the Cahill (Glucose-Alanine) Cycle.
4. Carboxylation to Oxaloacetate: Catalyzed by mitochondrial Pyruvate Carboxylase (requires Biotin B7 and ATP). Serves as an anaplerotic reaction to replenish TCA intermediates or initiate gluconeogenesis.

Pyruvate Dehydrogenase (PDH) Complex

PDH is a massive multienzyme mitochondrial complex ($E_1$ pyruvate dehydrogenase, $E_2$ dihydrolipoyl transacetylase, $E_3$ dihydrolipoyl dehydrogenase) converting Pyruvate to Acetyl-CoA ($3\text{C} \rightarrow 2\text{C} + \text{CO}_2$).

• Five Mandatory Coenzymes:
  1. Thiamine pyrophosphate ($B_1$ / TPP)
  2. Flavin adenine dinucleotide ($B_2$ / FAD)
  3. Nicotinamide adenine dinucleotide ($B_3$ / $\text{NAD}^+$)
  4. Coenzyme A ($B_5$ / CoA)
  5. Lipoic acid (Mnemonic: Tender Florida Nurse Can Lift)
• Clinical / Toxicological Highlight: Arsenic poisoning binds lipoic acid covalently, inhibiting PDH (and $\alpha$-ketoglutarate DH). Causes garlic breath, watery diarrhea, skin hyperpigmentation/hyperkeratosis, QT prolongation, and severe lactic acidosis.

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The Tricarboxylic Acid (TCA) Cycle

Occurring in the mitochondrial matrix, the TCA (Krebs) cycle oxidizes the 2-carbon acetyl unit of Acetyl-CoA to two molecules of $\text{CO}_2$.

Stoichiometry & Yield

For each turn of the TCA cycle (1 Acetyl-CoA derived from $\frac{1}{2}$ glucose molecule):

• 3 NADH (generated by Isocitrate DH, $\alpha$-Ketoglutarate DH, Malate DH)
• 1 $\text{FADH}_2$ (generated by Succinate DH / Complex II)
• 1 GTP (generated by Succinyl-CoA Synthetase via substrate-level phosphorylation)
• 2 $\text{CO}_2$

(Therefore, 1 Glucose molecule yielding 2 Acetyl-CoA generates 6 NADH, 2 $\text{FADH}_2$, 2 GTP, and 4 $\text{CO}_2$ in the TCA cycle).

Key Regulatory Steps

• Isocitrate Dehydrogenase: The rate-limiting enzyme of the TCA cycle (catalyzes Isocitrate $\rightarrow \alpha$-Ketoglutarate). Activated by ADP; inhibited by ATP and NADH.
• $\alpha$-Ketoglutarate Dehydrogenase: Requires the same 5 coenzymes as PDH (TPP, FAD, $\text{NAD}^+$, CoA, Lipoic acid). Inhibited by Succinyl-CoA, NADH, and ATP.
• Succinate Dehydrogenase: Embedded directly in the inner mitochondrial membrane as Complex II of the Electron Transport Chain.

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Electron Transport Chain & Oxidative Phosphorylation

Located within the inner mitochondrial membrane, the Electron Transport Chain (ETC) oxidizes $\text{NADH}$ and $\text{FADH}_2$, transferring electrons along protein complexes to molecular oxygen ($\text{O}_2$), the terminal electron acceptor ($2\text{H}^+ + \frac{1}{2}\text{O}_2 + 2e^- \rightarrow \text{H}_2\text{O}$).

Membrane Complexes & Proton Pumping

As electrons flow through high-affinity carriers, Complexes I, III, and IV pump protons ($\text{H}^+$) from the matrix into the intermembrane space, generating an electrochemical proton gradient (proton-motive force):

P/O Ratios: Oxidation of 1 $\text{NADH}$ yields $\approx 2.5$ ATP; oxidation of 1 $\text{FADH}_2$ yields $\approx 1.5$ ATP.

Pharmacological & Toxicological Inhibitors

   NADH ──► Complex I ──► CoQ ──► Complex III ──► Cyt c ──► Complex IV ──► O2
                │                     │                        │
           Inhibited by          Inhibited by             Inhibited by
            Rotenone             Antimycin A              Cyanide (CN-)
            Amobarbital                                   Carbon Monoxide (CO)
                                                          Sodium Azide (N3-)

1. ETC Complex Inhibitors: Directly block electron flow, arresting oxygen consumption and halting proton pumping.
   • Complex I Inhibitor: Rotenone (pesticide), Amobarbital.
   • Complex III Inhibitor: Antimycin A.
   • Complex IV Inhibitors: Cyanide ($\text{CN}^-$), Carbon Monoxide ($\text{CO}$), Sodium Azide ($\text{N}_3^-$), and Hydrogen Sulfide ($\text{H}_2\text{S}$). Binds $\text{Fe}^{3+}$ in Cytochrome $a_3$, halting respiration.
2. ATP Synthase Inhibitor: Oligomycin directly blocks the $F_0$ proton channel of Complex V, causing an accumulation of protons in the intermembrane space, which eventually stops ETC electron flow due to steep backpressure.
3. Uncoupling Agents: Dissipate the intermembrane proton gradient without inhibiting electron transport. Electron transport continues rapidly, consuming $\text{O}_2$, but ATP synthesis ceases; the potential energy is released entirely as heat.
   • Synthetic Uncouplers: 2,4-Dinitrophenol (2,4-DNP) (historical diet drug causing hyperthermia, tachypnea, diaphoresis, and death), High-dose Salicylates (Aspirin overdose).
   • Endogenous Uncoupler: Thermogenin (UCP-1) in brown adipose tissue of newborns and hibernating mammals, providing non-shivering thermogenesis.