Metabolism 3.1 Energy Production from Carbohydrates (3)
Learning outcomes (Lecture Outline & Goals)
Roles of the TCA cycle
Describe its function in metabolism.
Regulation of the TCA cycle
Explain how it is controlled.
Key features of oxidative phosphorylation
Describe its characteristics.
Electron transport and ATP synthesis
Explain these processes and their coupling.
Uncoupling of these processes
Describe when, why, and how uncoupling occurs in some tissues.
Comparison of oxidative phosphorylation and substrate-level phosphorylation
Distinguish between these two mechanisms.
Quick recap: key molecules in cellular energy status
Adenosine Triphosphate (ATP)
Energy source for cellular use and storage; a nucleoside triphosphate with high-energy bonds.
ATP:ADP or ATP:AMP ratios
Regulate many metabolic activities.
AMP-activated protein kinase (AMPK)
Energy sensor that maintains cellular energy homeostasis.
cAMP
A second messenger signalling molecule.
Adenosine
Acts as a hormone/neurotransmitter.
Last week’s learning context
Pentose phosphate pathway, glycolysis, and intermediates linking to carbohydrate metabolism
Key intermediates include G-6-P, G-1-P, G-3-P, F-6-P, 5C sugar phosphates, pyruvate, lactate; glycogen involvement; connection to glycolysis.
Pyruvate Dehydrogenase (PDH) and its regulation
Pyruvate fate after glycolysis
Pyruvate, the end product of glycolysis, is transported from the cytosol to mitochondria.
It is converted to acetyl-CoA via the Pyruvate Dehydrogenase (PDH) complex.
PDH is located in the mitochondrial matrix, converting 3-carbon pyruvate to 2-carbon acetyl-CoA with CO2 release and NADH generation.
PDH structure and cofactors
PDH complex is a large multi-enzyme complex comprising 3 enzymes and several cofactors.
Cofactors required: thiamine pyrophosphate (TPP), FAD, NAD+, CoA, and lipoic acid.
Vitamin B-family factors are essential, making PDH sensitive to B-vitamin deficiencies.
PDH-catalysed reaction (overall)
The reaction (per acetyl-CoA formed) is: ext{pyruvate} + ext{CoA} + ext{NAD}^+ \ ightarrow ext{acetyl–CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+
Stoichiometry: Pyruvate (3C) → Acetyl-CoA (2C) + CO2 + NADH.
PDH regulation (control by phosphorylation state and energy signals)
Activators: pyruvate, CoA, NAD+, ADP; insulin (via activating PDH phosphatase, promotes dephosphorylation and activation).
Inhibitors: acetyl-CoA, NADH, ATP, fatty acids.
Phosphorylation state controls activity: PDH kinase phosphorylates and inhibits PDH; PDH phosphatase dephosphorylates and activates PDH.
PDH deficiency (clinical)
A rare X-linked defect, it is the most common cause of congenital lactic acidosis.
Consequence: no acetyl-CoA formation and limited aerobic energy production; pyruvate accumulates and is reduced to lactate anaerobically.
Clinical presentation: neurological and muscular abnormalities; may be fatal in the neonatal period.
Management: dietary restriction of carbohydrates and proteins, ketogenic diet, and vitamin B supplementation.
Notably more common in men due to X-linked inheritance.
The TCA Cycle (Stage 3) and its Regulation
Overview and role
Central pathway in intracellular catabolism, occurring in mitochondria.
It is oxidative and exergonic, oxidizing 2 acetyl-CoA (2 C each) to 4 CO2.
Produces reducing equivalents: 6 NADH + H+ and 2 FADH2; 2 GTP (ATP) synthesized per glucose (two turns of the cycle per glucose).
Oxygen is required; intermediates serve as precursors for biosynthetic pathways.
Stoichiometry per acetyl-CoA (one turn)
Overall per cycle (one acetyl-CoA): ext{CH}3 ext{CO–S–CoA} + 3 ext{NAD}^+ + ext{FAD} + ext{GDP} + ext{P}i + 2 ext{H}2 ext{O} \ ightarrow 2 ext{CO}2 + ext{CoA} + 3 ext{NADH}^+ + ext{H}^+ + ext{FADH}_2 + ext{GTP}
Since 2 acetyl-CoA are formed per glucose, the total cycle outcome is doubled: 6 NADH, 2 FADH2, 2 GTP per glucose (before releasing reducing equivalents to the ETC).
Energy yield and substrate-level phosphorylation
2 cycles per glucose → 2 GTP (substrate-level phosphorylation) per glucose.
Regulation of the TCA cycle (key enzymes and regulatory signals)
Citrate synthase (Reaction 1): Activated by acetyl-CoA; inhibited by citrate.
Isocitrate dehydrogenase (Reaction 4): Activated by ADP; inhibited by ATP and NADH.
α-ketoglutarate dehydrogenase (Reaction 5): Activated by ADP; inhibited by ATP, NADH, succinyl-CoA.
Biosynthetic role of TCA intermediates
Citrate/isocitrate/aconitate provide precursors for fatty acid synthesis and other lipids.
α-Ketoglutarate: amino acid synthesis and transamination reactions.
Succinyl-CoA: heme synthesis.
Oxaloacetate: gluconeogenesis (in liver) and amino acid synthesis; malate supports malate–aspartate shuttle and gluconeogenesis.
Overall, TCA integrates energy production with biosynthesis.
Summary view of the TCA cycle (key points)
Central hub linking carbohydrate, fat (via acetyl-CoA), and protein metabolism.
Occurs in mitochondria; oxidative, producing NADH and FADH2 for ATP generation via oxidative phosphorylation.
Produces 2 GTP (ATP equivalents) per glucose via substrate-level phosphorylation; 6 NADH and 2 FADH2 for oxidative phosphorylation.
Intermediates are siphoned off for biosynthesis as needed.
Oxidative Phosphorylation (Stage 4): Electron Transport and ATP Synthesis
Core concept
Electrons from NADH and FADH2 are transferred through a chain of protein complexes (ETC) to O2, releasing free energy in steps.
This energy is used to pump protons across the inner mitochondrial membrane, creating a proton gradient (proton-motive force, pmf).
The pmf drives ATP synthesis via ATP synthase (F1F0-ATPase), coupling electron transport to ATP production.
Electron transport chain (ETC) architecture
Complex I (NADH dehydrogenase): transfers electrons from NADH to ubiquinone; pumps protons.
Complex II (succinate dehydrogenase): transfers electrons from FADH2 to ubiquinone; does not pump protons.
Complex III (cytochrome bc1 complex): transfers electrons to cytochrome c; proton pumping occurs.
Complex IV (cytochrome c oxidase): transfers electrons to O2, forming water; pumps protons.
The reduced carrier (NADH or FADH2) donates electrons to the chain; oxidized carriers are regenerated.
Proton gradient and ATP synthase
Proton translocation across the inner membrane creates a proton-motive force (pmf).
ATP synthase uses the energy of proton flow back into the matrix to convert ADP + Pi into ATP.
Stoichiometry: approximately 2.5 ATP per NADH and ~1.5 ATP per FADH2 (depending on conditions and coupling efficiency).
The energy stored in pmf is partly converted to ATP and partly dissipated as heat in some tissues.
Energetics and ATP yield from NADH and FADH2
NADH oxidation to O2 yields ~\Delta G^\theta = -220\frac{\text{kJ}}{\text{mol}} and produces ~2.5 ATP.
FADH2 oxidation to O2 yields ~\Delta G^\theta = -152\frac{\text{kJ}}{\text{mol}} and produces ~1.5 ATP.
In glycolysis, PDH, and the TCA cycle, the total reducing equivalents (per glucose) are 10 NADH and 2 FADH2.
Overall energy flow: only a portion of the energy from NADH/FADH2 is captured as ATP; the rest is released as heat or used to maintain pmf.
Energy accounting and efficiency (summary from the notes)
NADH-linked energy: ~35% of the energy released from NADH is captured as ATP (≈ 2.5 ATP per NADH; ~77.5 kJ per NADH).
FADH2-linked energy: ~31% of the energy released from FADH2 is captured as ATP (≈ 1.5 ATP per FADH2; ~46.5 kJ per FADH2).
The remainder is lost as heat due to imperfect coupling between electron transport and ATP synthesis.
Net ATP yield from glucose (classic accounting in this course)
Glycolysis: 2 ATP (substrate-level) and 2 NADH → ~5 ATP total.
PDH: 2 NADH → ~5 ATP.
TCA cycle: 2 GTP (ATP) + 6 NADH + 2 FADH2 → ~20 ATP from reduced cofactors (per glucose: 10 NADH → 25 ATP; 2 FADH2 → 3 ATP; plus 2 ATP from GTP).
TOTAL ≈ 32 ATP per glucose molecule.
Expressed as a sum: \text{Total ATP per glucose} \ = 2 \,(\text{Glycolysis ATP}) + 2 \,(\text{GTP from TCA}) + 10 \,(\text{NADH}) \times 2.5 + 2 \,(\text{FADH2}) \times 1.5 = 32 \,\text{ATP}
Efficiency and tissue variation
Oxidative phosphorylation is highly efficient but can vary with tissue type.
Brown adipose tissue (BAT) engages extra heat generation via uncoupling.
Substrate-Level vs Oxidative Phosphorylation: Key distinctions
Oxidative phosphorylation
Produces ATP from ADP and Pi using energy from a proton gradient created by ETC.
Requires membrane-associated complexes (inner mitochondrial membrane).
Energy coupling occurs via pmf; most energy conserved as ATP, some lost as heat depending on coupling efficiency.
Oxygen is required (final electron acceptor).
Substrate-level phosphorylation
Produces ATP directly from a substrate without a proton gradient or membrane-bound ATP synthase.
Occurs in cytosol and mitochondrial matrix (glycolysis and TCA step(s)).
Can occur in absence of oxygen (to a limited extent).
Comparative efficiency
Oxidative phosphorylation: higher overall ATP yield but variable efficiency depending on coupling; significant heat production when uncoupled.
Substrate-level phosphorylation: direct ATP production but lower overall energy yield per substrate.
Uncoupling and Heat Production
Uncoupling concept
Uncouplers increase inner mitochondrial membrane permeability to protons, dissipating the pmf and eliminating the drive for ATP synthesis.
Heat is generated instead of ATP production; can be lethal if uncontrolled.
Synthetic uncouplers
Dinitrophenol (DNP) and dinitrocresol (DNC) are classic examples.
Result: proton gradient collapses; ATP synthesis declines; heat production rises.
Natural uncoupling: UCP1 (thermogenin) in brown adipose tissue (BAT)
Activated by cold exposure and noradrenaline (norepinephrine).
Process:
1) Lipolysis of triglycerides → fatty acids.
2) Fatty acids provide reducing power for oxidative phosphorylation and activate UCP1.
3) UCP1 shuttles protons from intermembrane space to matrix, bypassing ATP synthase.Consequence: electron transport proceeds, but ATP production is uncoupled from the proton gradient; energy released as heat (non-shivering thermogenesis).
Physiological relevance
BAT thermogenesis contributes to heat generation in cold environments.
Uncoupling serves as a mechanism for heat production in humans and some mammals.
Oxidative Phosphorylation: Clinical and Pathophysiological Aspects
Tight coupling is essential for efficient ATP production
When coupling is disrupted, energy supply becomes compromised; cells may rely more on glycolysis and produce lactate (anaerobic metabolism) under some conditions.
Oxidative phosphorylation diseases (mtDNA and nuclear DNA mutations involved)
Leber hereditary optic neuropathy (LHON): mutations in complex I subunit genes; presents with progressive vision loss.
Leigh syndrome: mutations in genes encoding components of the ATP synthase or other ETC components; presents with neurodegeneration and psychomotor decline in infancy.
MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes): mtDNA mutations affecting energy production; heterogeneous clinical presentation.
Energy Balance and Head-to-Head: Glucose Catabolism in Numbers
Overall oxidation of glucose to CO2 and H2O (theoretical \Delta G^\theta)
\text{C}6 \text{H}{12} \text{O}6 + 6 \text{O}2 \ ightarrow 6 \text{CO}2 + 6 \text{H}2 \text{O}
\Delta G^\theta_\text{overall (glucose oxidation)} \ = -2870\text{ kJ/mol}
Energy accounted so far (substrate-level phosphorylation)
Glycolysis: 2 ATP net (4 ATP produced; net = 2 ATP) and 2 NADH → ~5 ATP.
PDH: 2 NADH → ~5 ATP.
TCA cycle: 2 GTP (ATP) and 6 NADH + 2 FADH2 → ~20 ATP from reduced cofactors.
Substrate-level phosphorylation total: ~4 ATP (glycolysis + TCA) => cited as ~-124 kJ/mol energy.
Rest of energy is stored in reduced cofactors for oxidative phosphorylation
Remaining energy after substrate-level phosphorylation: approximately -2746 kJ/mol stored in NADH + H+ and FADH2.
Net ATP yield summary (glucose -> 32 ATP in this model)
NADH: 10 total from glycolysis, PDH, and TCA → 10 × 2.5 = 25 ATP.
FADH2: 2 total → 2 × 1.5 = 3 ATP.
LTP: 4 ATP.
Total: 25 + 3 + 4 = 32 ATP per glucose molecule.
Expression of ATP yields (summary form)
\text{ATP}{\text{NADH}} = 2.5 \times n{\text{NADH}}
\text{ATP}{\text{FADH}2} = 1.5 \times n{\text{FADH}2}
\text{Total ATP} = 32 \text{ per glucose} \ (\text{in this framework})
Biosynthetic Roles of TCA Intermediates
TCA intermediates provide carbon skeletons for multiple biosynthetic pathways
Citrate is a precursor for fatty acids and sterols (via acetyl-CoA) and for other biosynthetic routes.
α-Ketoglutarate is a precursor to several amino acids via transamination.
Succinyl-CoA is a precursor for heme synthesis.
Oxaloacetate can feed gluconeogenesis (in liver) and amino acid synthesis.
Malate and oxaloacetate also participate in shuttle systems (malate–aspartate shuttle) and replenishment (anaplerotic reactions).
Connections to Foundational Principles and Real-World Relevance
Centrality of mitochondria in energy metabolism
Oxidative metabolism sits at the heart of energy production, integrating carbohydrate, fat, and protein catabolism.
Regulation integrates energy status signals
NAD+/NADH, ADP/ATP ratios, allosteric modulators, and hormonal signals (e.g., insulin effects on PDH via phosphatases).
Pathophysiology in energy disorders
PDH deficiency and mtDNA mutations illustrate how energy failure manifests in neurological and muscular symptoms; mitochondrial diseases can be inherited and have multisystem effects.
Pharmacological and nutritional implications
Ketogenic diets for PDH deficiency; understanding uncoupling informs metabolic heat production and potential therapeutic strategies (e.g., thermogenesis).
Key Equations and Expressions (LaTeX)
PDH-catalysed reaction
\text{pyruvate} + \text{CoA} + \text{NAD}^+ \ ightarrow \text{acetyl–CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+
TCA cycle (one acetyl-CoA turn)
\text{CH}3 \text{CO–S–CoA} + 3 \text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}i + 2 \text{H}2 \text{O} \ ightarrow 2 \text{CO}2 + \text{CoA} + 3 \text{NADH}^+ + \text{H}^+ + \text{FADH}_2 + \text{GTP}
ATP yield from NADH and FADH2 (discrete yields)
\text{ATP per NADH} = 2.5 \text{ ATP per FADH}_2 = 1.5
Total ATP from glucose (summary)
\text{Total ATP per glucose} = 2 \ (\text{glycolysis}) + 2 \ (\text{GTP from TCA}) + 10 \times 2.5 \ + 2 \times 1.5 = 32 \text{ ATP}
Proton motive force (pmf) across the inner mitochondrial membrane
\text{pmf} = \ \Delta \psi - \frac{RT}{F} \ln \bigg( \frac{[\text{H}^+]{\text{in}}}{[\text{H}^+]{\text{out}}} \bigg)
Overall oxidative phosphorylation (conceptual)
Energy from pmf drives ATP synthesis via ATP synthase (F1F0-ATPase): \text{ADP} + \text{P}_i \xrightarrow{\text{ATPsynthase}} \text{ATP}
Energy of glucose oxidation (thermodynamics)
\Delta G^\theta_\text{overall} \,=\,-2870\ \text{kJ/mol}
Short note on terminology
Oxidative phosphorylation vs substrate-level phosphorylation
Oxidative phosphorylation depends on a proton gradient and membrane-bound enzyme complexes.
Substrate-level phosphorylation transfers a phosphate to ADP directly from a high-energy substrate, without a gradient.
Quick literature pointers (conceptual, not exhaustive)
PDH deficiency and ketogenic diet as therapeutic approach
LHON, Leigh syndrome, MELAS as illustrative oxidative phosphorylation diseases
UCP1 in brown adipose tissue and non-shivering thermogenesis as a physiological uncoupling mechanism
Connections to prior and subsequent topics
Links to glycolysis
NAD+/NADH balance, lactate production under redox stress.
Anticipated discussion
Metabolic regulation in fasting/feeding states and energy demand variations.
Foundations for exploring
Metabolic diseases and pharmacological targets in energy metabolism.