9.3 The Pyruvate Dehydrogenase Complex & the Citric Acid (Krebs) Cycle

9.3 The Pyruvate Dehydrogenase Complex & the Citric Acid (Krebs) Cycle

Following glycolysis in the cytosol, pyruvate must be transported across the mitochondrial membranes to undergo oxidative decarboxylation. In the mitochondrial matrix, the carbon skeleton of pyruvate is committed irrevocably to cellular respiration or lipid biosynthesis.

Mitochondrial Import and the Pyruvate Dehydrogenase Complex (PDC)

Pyruvate crosses the porous outer mitochondrial membrane through voltage-dependent anion channels (porins). It then traverses the selectively impermeable inner mitochondrial membrane via the mitochondrial pyruvate carrier (MPC), driven by a proton-symport mechanism down the electrochemical gradient.

Once inside the alkaline matrix, pyruvate encounters the Pyruvate Dehydrogenase Complex (PDC), a massive macromolecular multi-enzyme machine that catalyzes an irreversible oxidative decarboxylation:

The Three Catalytic Subunits and Five Coenzymes

The PDC integrates three distinct enzymatic activities and utilizes five vitamin-derived cofactors (easily remembered by the mnemonic: Tender Love & Care For Neurochemists — TPP, Lipoate, CoA, FAD, NAD):

1. $E_1$: Pyruvate Dehydrogenase (Decarboxylase)
   • Prosthetic Group: Thiamine Pyrophosphate (TPP, Vitamin $\text{B}_1$).
   • Mechanism: The carbanion of TPP performs a nucleophilic attack on the C2 carbonyl of pyruvate. Decarboxylation releases the C1 carboxylate as $\text{CO}_2$, leaving a two-carbon hydroxyethyl intermediate covalently bound to TPP (hydroxyethyl-TPP).
2. $E_2$: Dihydrolipoyl Transacetylase
   • Prosthetic Group: Lipoic Acid (Lipoamide) covalently tethered to a lysine residue, forming a flexible 14-Ångström "swinging arm".
   • Cosubstrate: Coenzyme A (CoA-SH, derived from Pantothenic Acid / Vitamin $\text{B}_5$).
   • Mechanism: The lipoamide swinging arm oxidizes the hydroxyethyl group back to an acetyl moiety while reducing its internal disulfide bond to dithiol form. The acetyl group is transferred onto the lipoamide, forming a high-energy thioester. Next, $E_2$ transfers the acetyl group to the thiol of Coenzyme A, releasing Acetyl-CoA. This leaves the lipoyl swinging arm in a fully reduced, inactive dihydrolipoamide state.
3. $E_3$: Dihydrolipoyl Dehydrogenase
   • Prosthetic Group: Flavin Adenine Dinucleotide (FAD, derived from Riboflavin / Vitamin $\text{B}_2$).
   • Cosubstrate: Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$, derived from Niacin / Vitamin $\text{B}_3$).
   • Mechanism: To regenerate the active oxidized disulfide ring of lipoamide for the next catalytic cycle, $E_3$ transfers two electrons and two protons from dihydrolipoamide onto enzyme-bound FAD, forming $\text{FADH}_2$. Finally, $\text{FADH}_2$ is re-oxidized by transferring its electron pair to mobile $\text{NAD}^+$, generating soluble NADH + $\text{H}^+$.

Toxicology & Clinical Correlation: Arsenic (arsenite) binds with high affinity to the neighboring sulfhydryl groups of dihydrolipoamide, permanently inactivating $E_2$. This arrests both the PDC and the $\alpha$-ketoglutarate dehydrogenase complex, causing severe cellular ATP starvation, severe lactic acidosis, and neurological death.

Regulation of the Pyruvate Dehydrogenase Complex

Because mammalian cells cannot synthesize glucose from acetyl-CoA (the PDC reaction is irreversible and human cells lack the glyoxylate cycle), PDC activity commits carbohydrate carbons to terminal oxidation or lipogenesis. Regulation occurs via two coordinated mechanisms:

1. Covalent Phosphorylation (Primary Control)

• Pyruvate Dehydrogenase Kinase (PDK): Phosphorylates specific serine residues on the $E_1$ subunit, rendering the entire complex catalytically inactive.
  • Activated by: Elevated cellular energy indicators—high [ATP], high [Acetyl-CoA], and high [NADH]. When energy reserves are saturated, PDK shuts down pyruvate entry.
  • Inhibited by: Elevated substrate levels—high [ADP], [Pyruvate], [$\text{NAD}^+$], and [CoA-SH].
• Pyruvate Dehydrogenase Phosphatase (PDP): Dephosphorylates the $E_1$ subunit, restoring the complex to its active form.
  • Activated by: Elevated intracellular $\text{Ca}^{2+}$ (signaling active muscle contraction or neuronal excitation) and insulin in adipose tissue and liver (promoting lipogenic conversion of glucose into fat).

2. Allosteric Feedback Inhibition

Independent of phosphorylation, $E_2$ is allosterically inhibited directly by its product acetyl-CoA, and $E_3$ is allosterically inhibited by its product NADH.

The Citric Acid (Krebs / TCA) Cycle: The Eight Reactions

The Citric Acid Cycle is an eight-step cyclic pathway residing within the mitochondrial matrix. It completely oxidizes the two-carbon acetyl moiety of acetyl-CoA into two molecules of $\text{CO}_2$, capturing released chemical energy in the form of reduced coenzymes (3 NADH and 1 $\text{FADH}_2$) and one high-energy nucleoside triphosphate (1 GTP or ATP).

Step-by-Step Pathway

1. Citrate Synthase (Aldol Condensation): $$\text{Acetyl-CoA (2C)} + \text{Oxaloacetate (4C)} + \text{H}_2\text{O} \longrightarrow \text{Citrate (6C)} + \text{CoA-SH} \quad (\Delta G^{\circ\prime} = -31.5\text{ kJ/mol})$$ Citrate synthase exhibits an induced-fit mechanism: oxaloacetate binds first, creating the binding pocket for acetyl-CoA and preventing premature thioester hydrolysis. Hydrolysis of the energy-rich acetyl-CoA thioester drives the condensation forward irreversibly.
2. Aconitase (Isomerization): $$\text{Citrate} \xrightleftharpoons{\text{[4Fe-4S]}} \text{cis-Aconitate} \xrightleftharpoons{} \text{Isocitrate}$$ Citrate (a tertiary alcohol) cannot be oxidized further without breaking a carbon-carbon bond. Aconitase dehydrates citrate to enzyme-bound cis-aconitate and rehydrates it to isocitrate (a secondary alcohol) utilizing an iron-sulfur ($[4\text{Fe}-4\text{S}]$) cluster.
3. Isocitrate Dehydrogenase (Oxidative Decarboxylation — Rate-Limiting Step): $$\text{Isocitrate (6C)} + \text{NAD}^+ \longrightarrow \alpha\text{-Ketoglutarate (5C)} + \text{CO}_2 + \text{NADH} + \text{H}^+ \quad (\Delta G^{\circ\prime} = -20.9\text{ kJ/mol})$$ This is the rate-limiting step of the entire TCA cycle. Isocitrate is oxidized to an oxalosuccinate intermediate, which spontaneously decarboxylates to yield $\alpha$-ketoglutarate and the first molecule of $\text{CO}_2$ and NADH.
   • Regulation: Allosterically stimulated by ADP and $\text{Ca}^{2+}$; inhibited by ATP and NADH.
4. $\alpha$-Ketoglutarate Dehydrogenase Complex (Second Oxidative Decarboxylation): $$\alpha\text{-Ketoglutarate (5C)} + \text{CoA-SH} + \text{NAD}^+ \longrightarrow \text{Succinyl-CoA (4C)} + \text{CO}_2 + \text{NADH} + \text{H}^+ \quad (\Delta G^{\circ\prime} = -33.5\text{ kJ/mol})$$ This multienzyme complex is homologous in structure, catalytic mechanism, and cofactor dependency (TPP, lipoamide, CoA, FAD, $\text{NAD}^+$) to the pyruvate dehydrogenase complex. It releases the second $\text{CO}_2$ and generates the second NADH, capturing bond energy in the high-energy thioester succinyl-CoA.
   • Regulation: Inhibited by its products (succinyl-CoA and NADH) as well as ATP; stimulated by $\text{Ca}^{2+}$.
5. Succinyl-CoA Synthetase / Succinate Thiokinase (Substrate-Level Phosphorylation): $$\text{Succinyl-CoA (4C)} + \text{GDP (or ADP)} + \text{P}_i \xrightleftharpoons{} \text{Succinate (4C)} + \text{GTP (or ATP)} + \text{CoA-SH} \quad (\Delta G^{\circ\prime} = -2.9\text{ kJ/mol})$$ The exergonic cleavage of the high-energy thioester bond drives the phosphorylation of GDP to GTP (in liver and kidneys) or ADP to ATP (in skeletal and cardiac muscle). Nucleoside diphosphate kinase readily equilibrates these pools ($\text{GTP} + \text{ADP} \rightleftharpoons \text{GDP} + \text{ATP}$).
6. Succinate Dehydrogenase / Complex II (Stereospecific Dehydrogenation): $$\text{Succinate (4C)} + \text{FAD} \xrightleftharpoons{} \text{Fumarate (4C)} + \text{FADH}_2 \quad (\Delta G^{\circ\prime} = 0\text{ kJ/mol})$$ Succinate dehydrogenase is unique among TCA cycle enzymes: it is a flavoprotein directly embedded in the inner mitochondrial membrane, functioning simultaneously as Complex II of the electron transport chain. It oxidizes the central single bond of succinate to a trans double bond, transferring two electrons directly to enzyme-bound FAD. Malonate, a structural analog of succinate, acts as a classic competitive inhibitor.
7. Fumarase (Hydration): $$\text{Fumarate (4C)} + \text{H}_2\text{O} \xrightleftharpoons{} \text{L-Malate (4C)} \quad (\Delta G^{\circ\prime} = -3.8\text{ kJ/mol})$$ Fumarase catalyzes the stereospecific trans-hydration of the double bond, converting fumarate exclusively into L-malate.
8. Malate Dehydrogenase (Final Oxidation & OAA Regeneration): $$\text{L-Malate (4C)} + \text{NAD}^+ \xrightleftharpoons{} \text{Oxaloacetate (4C)} + \text{NADH} + \text{H}^+ \quad (\Delta G^{\circ\prime} = +29.7\text{ kJ/mol})$$ This final reaction regenerates oxaloacetate while yielding the third NADH. Although this reaction is highly endergonic under standard conditions, it proceeds rapidly forward in vivo because oxaloacetate is continually removed by the exergonic citrate synthase reaction (Step 1), maintaining [OAA] at extremely low steady-state concentrations ($<10^{-6}\text{ M}$).

Bioenergetic Stoichiometry & The Amphibolic Nature of the TCA Cycle

Stoichiometric Balance Sheet per Acetyl-CoA

Cumulative Balance Sheet per Glucose Molecule

Because each starting glucose molecule yields 2 pyruvate, the yields through PDC and the TCA cycle are doubled:

• PDC Phase (2 Pyruvate $\to$ 2 Acetyl-CoA): $2\text{ NADH} + 2\text{ CO}_2$
• TCA Cycle Phase (2 Acetyl-CoA oxidized): $6\text{ NADH} + 2\text{ FADH}_2 + 2\text{ GTP} + 4\text{ CO}_2$
• Total Mitochondrial Yield prior to ETC: $8\text{ NADH} + 2\text{ FADH}_2 + 2\text{ GTP} + 6\text{ CO}_2$

Notice that all 6 carbons of the original glucose have been released as $\text{CO}_2$ (2 from PDC, 4 from the TCA cycle).

Amphibolic Nature and Anaplerotic Reactions

The TCA cycle is amphibolic, participating in both catabolic oxidative degradation and anabolic biosynthetic pathways:

• Catabolic Influx: Intermediates are fed by the catabolism of carbohydrates, amino acids (e.g., glutamate $\to \alpha$-ketoglutarate, aspartate $\to$ oxaloacetate), and odd-chain fatty acids (succinyl-CoA).
• Anabolic Efflux (Cataplerosis): Citrate is exported to the cytosol for fatty acid and sterol synthesis; $\alpha$-ketoglutarate and oxaloacetate are transaminated into amino acids; succinyl-CoA provides the pyrrole rings for porphyrin and heme synthesis.
• Anaplerotic (Replenishing) Reactions: If intermediates are siphoned off for biosynthesis, the cycle would stall without replenishing pathways. The preeminent anaplerotic reaction in mammalian tissues is catalyzed by Pyruvate Carboxylase ($\text{Pyruvate} + \text{CO}_2 + \text{ATP} \to \text{Oxaloacetate} + \text{ADP} + \text{P}_i$), activated by acetyl-CoA.

Summary of TCA Cycle Enzymes and Energetics