Metabolism and Energy Production

Mitochondrial DNA Inheritance

Unlike genomic DNA, which is inherited from both parents, mitochondrial DNA is only inherited maternally (from the mother’s oocyte).

Mitochondria - Metabolic Center of the Cell

Reference: Pfanner, N et al. 2019. Nature Reviews Molecular Cell Biology (September 24th, 2025).

Core Concept of Energy Metabolism

Chemical Bonds
: Chemical bonds (shared electrons) store energy that can be transformed into more readily usable forms, primarily in Hydrogen and Phosphate bonds (ATP, Glucose, NADH).

Overview of Metabolism

Definition
: Metabolism is the sum of all biochemical processes in living organisms.

Categories of Metabolism
:

Catabolism
: Breakdown of larger molecules to smaller molecules. It includes oxidative reactions and releases energy.

Anabolism
: Synthesis of smaller molecules into larger ones. It includes reductive reactions and absorbs energy.

Amphibolic Pathways
: Pathways that include both catabolic and anabolic processes.

Examples
:

Catabolic: Glycogen is broken down into glucose monomers.

Anabolic: Glucose monomers are polymerized into glycogen.

Catabolism Fuels Anabolism

Catabolic reactions are essential as energy cannot be created or destroyed; they fuel anabolic processes.

Core Concepts
: Enzyme activity and redox cycles (oxidation/reduction) are central to metabolism.

Metabolic Pathways

Components
:

Metabolites
:

Reactants
: Starting molecules in a biochemical reaction.

Products
: Ending molecules in a biochemical reaction.

Enzymes
: Catalyze reactions in a specific manner, regulated once reactants are depleted.

Some enzymes catalyze reactions in both directions depending on cellular conditions.

Reaction Types
:

Exergonic Reaction
:

Energy is released and it is spontaneous.

Notation: \Delta G < 0 (amount of energy released).

Endergonic Reaction
:

Energy is required making it nonspontaneous.

Notation: \Delta G > 0 (amount of energy required).

Redox Reactions

Oxidation
: Loss of electrons through removal of Hydrogen or addition of Oxygen.

Reduction
: Gain of electrons through addition of Hydrogen or removal of Oxygen.

Non-hydrogen compounds may be considered oxidized or reduced based on electron transport via Hydrogen atoms.

Energy in Biological Molecules

Macromolecules store energy in covalent bonds, utilizing reducing equivalents to transfer energy as electrons.

Nucleotide triphosphates provide readily usable energy via high energy phosphate bonds (ATP, GTP, UTP, CTP).

Fatty Acid Oxidation
: The process that involves oxidation and reduction throughout metabolism to capture energy efficiently.

Energy Production in Metabolic Pathways

Glycolysis
:

Initial steps require ATP, followed by energy capture in subsequent phases generating ATP and reducing equivalents.

Protein Synthesis
: Involves charging tRNAs with amino acids using GTP, facilitating energy transfer to peptide bonds.

Role of Mitochondria in ATP Generation

Mitochondria execute oxidative metabolism, utilizing macronutrients (carbohydrates, fats, proteins) to generate ATP and consuming oxygen (oxidative phosphorylation).

Mitochondrial Morphology

Two membranes and regions (inner membrane, outer membrane, mitochondrial matrix, intermembrane space, cristae) facilitate metabolism.

Impacts of environmental factors cause adaptations within mitochondrial morphology, affecting metabolic function.

Environmental Effects on Mitochondrial Networks

Mitochondrial fission occurs due to obesity-related lipid changes, while fusion enhances mitochondrial function due to exercise-related lipid adaptations.

Reference: Hood, D et al. 2019. Annual Review of Physiology.

Carbohydrate Metabolism Overview (Glycolysis and Glycogen)

Core Concept of Carbohydrate Metabolism

Centrality of Glucose
: Acts as a core link among macronutrients in energy metabolism.

Key Metabolites and Pathways
:

Glycolysis
: Converts glucose to pyruvate, yielding ATP and NADH.

Glycogenolysis
: Breakdown of glycogen to glucose.

Glycogenesis
: Synthesis of glycogen from glucose.

Gluconeogenesis
: Generation of glucose from non-carbohydrate sources.

Overview: Carbohydrate Metabolism

Catabolic Pathways
:

Glycogenolysis

Glycolysis

Pyruvate oxidation

TriCarboxylic Acid (TCA) / Citric acid cycle (CAC)

Electron Transport Chain

Anabolic Pathways
:

Gluconeogenesis

Glycogenesis

Glycogen Breakdown and Fate of Glucose

Glycogen phosphorylase cleaves Glucose-1-P monomers from the end of glycogen chains using inorganic phosphate (phosphorolysis).

Glucose-1-P is isomerized to Glucose-6-P. No energy is captured in these steps.

The fate of glucose varies depending on tissue type:

Liver
: Glucose can leave to circulation.

Skeletal Muscle
: Glucose cannot leave to circulation because it lacks glucose-6-phosphatase.

Energy Phases in Glycolysis

Energy Investment Phase
:

2 ATP utilized to "prime" glucose for catabolism (e.g., Hexokinase, Phosphofructokinase).

Traps G6P in the cell.

Commits F1,6BP to glycolysis.

Energy Payoff Phase
:

Generates 4 ATP (net gain of 2 ATP for the pathway).

Generates 2 NADH.

Generates 2 Pyruvate + 2 H ext{2}O.

Enzymes like dehydrogenases move hydrogen atoms (electrons) and mutases move functional groups.

ATP molecules synthesized via substrate-level phosphorylation.

Pyruvate moves to pyruvate oxidation in mitochondria.

Gluconeogenesis and Glycogenesis Details

Gluconeogenesis
: Converts precursors like glycerol, lactate, and pyruvate, and oxaloacetate to glucose, especially during periods of low carbohydrate intake.

Glycogenesis
: Involves converting G6P to glycogen via several enzymes like phosphoglucomutase, UDP-glucose phosphorylase, and glycogen synthase.

Tricarboxylic Acid Cycle (TCA)

Core Concept of TCA Cycle

Oxygen as Terminal Electron Acceptor
: Oxygen is the terminal electron acceptor in the electron transport chain, enabling much higher ATP generation than glycolysis alone.

Pyruvate Oxidation

Glucose is catabolized to pyruvate by glycolysis (2 pyruvate, net 2 ATP, 2 NADH).

Pyruvate then enters the mitochondria for further oxidation.

Pyruvate Fates Depending on Oxygen Status

Low oxygen (anaerobic)
:

Pyruvate is converted to Lactate via Lactate Dehydrogenase.

NAD^{ ext{+}} is regenerated from NADH.

Lower net energy yield as substrates cannot enter TCA or ETC.

Normal oxygen (aerobic)
:

Pyruvate is converted to Acetyl-CoA via Pyruvate Dehydrogenase.

More NADH is generated (1 per pyruvate molecule).

Higher net energy yield as substrates can enter TCA and ETC.

The TriCarboxylic Acid Cycle (TCA)

Major source of reducing equivalents
that enter the Electron Transport Chain.

Per Acetyl-CoA molecule (which is processed twice per glucose):

3 molecules of NADH (NAD^{ ext{+}} \rightarrow NADH)

1 molecule of FADH ext{2} (FAD \rightarrow FADH ext{2})

1 molecule of GTP (GDP \rightarrow GTP) (can be converted to ATP)

2 CO ext{2} (from isocitrate and \alpha-ketoglutarate)

Oxaloacetate is regenerated.

Types of Reactions in TCA Cycle

Synthesis
: Synthase (e.g., Fumarase); water combines with two molecules.

Isomerization
: Isomerase (e.g., Mutase); molecule rearranged, no atoms lost.

Dehydrogenation
: Dehydrogenase; removes hydrogen (electron) to reducing equivalent.

Phosphorylation
: Synthetase (Kinase); adds phosphate to a molecule (substrate level).

Synthetase
: Succinyl-CoA Synthetase; synthesizes new molecules using energy from phosphate-phosphate bond.

TCA Cycle Reactions (8 total)

Condensation to form Citrate [Citrate Synthase]

Isomerization of Citrate \rightarrow Isocitrate [Aconitase]

Oxidation of Isocitrate \rightarrow \alpha-ketoglutarate [Isocitrate Dehydrogenase]

Oxidation of \alpha-ketoglutarate \rightarrow Succinyl-CoA [\alpha-Ketoglutarate Dehydrogenase]

Substrate level phosphorylation; uses energy of CoA bond [Succinyl-CoA Synthetase]

Oxidation of Succinate to Fumarate [Succinate Dehydrogenase] (
Enzyme is part of the ETC complex II!
)

Condensation to form Malate [Fumarase]

Oxidation of Malate \rightarrow Oxaloacetate [Malate Dehydrogenase]

Interconnections with Other Metabolism

Metabolite Convergence
: Carbohydrate, Protein, and Fat metabolism converge at TCA, determining ATP production based on entry point.

Gluconeogenic Precursors
: TCA cycle provides precursors, particularly through oxaloacetate.

Lipogenesis Precursors
: High TCA cycle activity promotes lipogenesis (synthesis of lipids).

Reducing Equivalents Transport

The route by which NADH (e.g., from glycolysis) enters the mitochondria determines how much ATP will be derived in ETC.

Shuttles
:

Glycerol-Phosphate Shuttle

Malate-Aspartate Shuttle

Electron Transport Chain (ETC)

Core Concept of ETC

Oxygen as Terminal Electron Acceptor
: Oxygen is the terminal electron acceptor in the electron transport chain, allowing for significantly more ATP generation than glycolysis alone.

Glucose Metabolism to ETC

Glucose: Derived from glycogenolysis, gluconeogenesis, or diet.

Initial oxidation to pyruvate via glycolysis.

Reduction to lactate (low oxygen, cytosol) OR oxidation to acetyl-CoA (normal oxygen, mitochondria).

Many oxidation steps in TCA.

The carbon skeleton stays with TCA, while NADH/FADH\text{2} go to the Electron Transport Chain.

ETC Complexes and Electrochemical Potential

Electrons are passed from reducing equivalents (NADH at Complex I, FADH\text{2} at Complex II, Coenzyme Q at Complex III, Cytochrome c at Complex IV).

This flow of electrons pumps protons into the intermembrane space (e.g., 4 H^{\text{+}} at Complex I and III, 2 H^{\text{+}} at Complex IV; Complex II pumps 0 H^{\text{+}}).

ETC Complex Properties

Complexes are integral, hydrophobic proteins.

Coenzyme Q is a hydrophobic protein.

Cytochrome c is hydrophilic.

Complexes associate in lipid rafts.

Protons cannot cross the membrane without a transporter.

Oxygen and Water Formation

Oxygen is the terminal electron acceptor, forming water.

This is why oxygen status regulates which metabolic pathways are utilized.

ATP Synthase: Chemiosmotic Coupling

Chemiosmotic coupling
: Process by which chemical potential energy (proton gradient) becomes chemical energy (ATP).

Oxidative phosphorylation
: For every 4 protons transported, 1 ATP is generated.

Uncoupling in ETC

Uncoupling
: Disconnect between ETC and ATP Synthase.

Uncoupling Proteins (UCP) can lead to energy loss as heat instead of ATP synthesis.

Protein Anabolism

Core Concept of Protein Anabolism

Amino Acid Interchangeability
: Amino acids are interchangeable with many metabolites in core energy metabolism (pyruvate and TCA pathways).

Protein Metabolism and Nitrogen

Macromolecules containing nitrogen
: Protein, Nucleic acids.

Nitrogen catabolism (mostly amino acids) is energy-intensive.

Nitrogen anabolism revolves around transfer of amino and methyl groups.

Essential vs. Non-Essential Amino Acids

Essential
: Not synthesized in sufficient amounts (Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine).

Non-Essential
: Synthesized in sufficient amounts (Alanine, Arginine, Asparagine, Aspartate, Glutamine, Glycine, Proline, Serine, Cysteine, Glutamate, Tyrosine).

Essential amino acids must be consumed in the diet.

Essentiality can vary by organism, life stage, and health condition.

Amino Acid Carbon Skeletons and Precursors

Amino acids are categorized into "families" based on shared structures.

Precursor molecules derived from other metabolic pathways:

Glycolysis
: 3-Phosphoglycerate (Serine), Pyruvate (Alanine).

Tricarboxylic Acid Cycle
: Alpha-Ketoglutarate (Glutamate), Oxaloacetate (Aspartate).

Pentose Phosphate Pathway
: Most mammals cannot synthesize tyrosine and histidine from PPP intermediates.

Glutamate in Amino Acid Synthesis

Glutamate and glutamine are synthesized from ammonium assimilation and transamination reactions.

Ex: \alpha-Ketoglutarate + NH\text{4} ^{\text{+}} \rightarrow Glutamate (Glutamate Dehydrogenase).

Ex: Glutamate + NH\text{4} ^{\text{+}} + ATP \rightarrow Glutamine + ADP + Pi (Glutamine Synthetase).

Transamination Reactions

Transamination
: Occurs in both cytosol and mitochondria.

Enzyme names: Transaminase (glutamate or glutamine).

Reaction type: Transfers an amino group from one amino acid to the \alpha-keto acid, forming a new amino acid and \alpha-keto acid.

This is how non-essential amino acids receive their amino group; does not require ATP.

Aspartate and Alanine Synthesis

Aspartate Synthesis (AST)
: Amino group from glutamate transaminated to oxaloacetate, forming aspartate and \alpha-ketoglutarate.

Alanine Synthesis (ALT)
: Amino group from glutamate transaminated to pyruvate, forming alanine and \alpha-ketoglutarate.

AST and ALT are clinical markers for hepatic damage.

Serine Synthesis

Serine is synthesized from 3-phosphoglycerate.

Requires dehydrogenation, transamination, and dephosphorylation.

Serine is converted to Glycine (via serine hydroxymethyltransferase) and Cysteine (via sulfur transfer from methionine).

Methylation and Cysteine Synthesis

Methionine
: Essential AA vital for transmethylation (CH\text{4} transfer).

Cysteine Synthesis
: Involves activation of methionine to S-adenosylmethionine (SAM), methylation of another metabolite, hydrolysis to homocysteine, condensation with serine to cystathionine, and finally hydrolysis to Cysteine.

Protein Catabolism

Core Concept of Protein Catabolism

Amino Acid Interchangeability
: Amino acids are interchangeable with many metabolites in core energy metabolism (pyruvate and TCA pathways).

Removal of Amino Groups for Catabolism

When carbohydrate levels are low or excess amino acids are consumed, amino acids are oxidized for energy.

Two types of reactions remove nitrogen:

Transamination
: Movement of amino group from one carbon skeleton to another.

Deamination
: Removal to free ammonium (NH\text{4}).

Amino Acid Fates

Glucogenic
: Enters a pathway that can lead to formation of glucose (yields pyruvate or oxaloacetate; can enter TCA cycle).

Ketogenic
: Enters a pathway that can lead to formation of ketones (yields acetyl-CoA or acetoacetyl-CoA; cannot become glucose without a \geq4 carbon intermediate).

Both
: Enters pathways that can become either glucose or ketones.

Glucogenic AA examples
: Aspartate, Asparagine, Alanine, Glycine, Serine, Threonine, Cysteine, Glutamate, Glutamine, Arginine, Proline, Histidine, Valine, Methionine.

Ketogenic AA examples
: Leucine, Lysine.

Both AA examples
: Phenylalanine, Tryptophan, Tyrosine, Isoleucine.

TCA Cycle Requirement for Acetyl-CoA

Acetyl-CoA requires oxaloacetate (a \geq4 carbon intermediate) to enter the TCA cycle.

Two carbons are fully oxidized and lost as CO\text{2} during reactions 3 and 4 of TCA (not the acetyl-CoA carbons in the first cycle).

Net carbon addition/loss is 0, meaning acetyl-CoA cannot become oxaloacetate directly.

Amino Acid Entry Points into Metabolism

Amino acids enter energy metabolism at multiple points:

Glycolysis / Gluconeogenesis arm
: Glucogenic.

Lipogenesis arm
: Ketogenic.

Pyruvate / TCA arm
: Glucogenic.

Nitrogenous Waste Management

Ammonia (NH\text{3}) is a weak acid; high [NH\text{4}}^{\text{+}}, resulting from deamination, can lead to metabolic alkalosis.

High [NH\text{4}}^{\text{+}}} also pushes glutamate/glutamine synthesis.

Mammals excrete nitrogen in the non-toxic form of urea via the Urea Cycle.

Urea Cycle: Nitrogen Elimination Pathway

First nitrogen
: From carbamoyl-P.

Synthesized in mitochondria from free ammonium (from deamination) and CO\text{2}. Requires 2 ATP.

Ornithine reacts with carbamoyl-P to form citrulline, which is exported to the cytosol.

Second nitrogen
: From aspartate.

Aspartate reacts with citrulline (requires 2 ATP equivalents) to form arginosuccinate.

Arginosuccinate splits into arginine and fumarate.

Arginine is hydrolyzed to form Urea (contains 2 nitrogens) and regenerates ornithine, which is transported back to mitochondria.

Linking Urea and TCA Cycles

Net cost of urea cycle
: 4 ATP.

Fumarate (product of urea cycle) can enter TCA, eventually becoming oxaloacetate.

Produces one NADH, yielding \sim2.5 ATP in ETC, which is \simhalf of the energy needed for the Urea Cycle.

Oxaloacetate can be transaminated to aspartate (nitrogen from Glutamate), bringing nitrogen back to the urea cycle and producing another fumarate.

Lipid Anabolism

Core Concept of Lipid Anabolism

Energy-Intensive Process
: Lipid synthesis is an energy-intensive process, involving bicarbonate, acetyl-CoA carboxylase, and building units for fatty acids and glycerol.

Overview: Lipid Metabolism

Energy Storage (Anabolism)
:

Lipids stored as triglycerides (glycerolipids) in adipose tissue (subcutaneous, visceral).

Involves Fatty Acid Synthase enzyme.

Energy Liberation (Catabolism)
:

Breakdown of glycerolipids into glycerol (for glycolysis/gluconeogenesis) and fatty acids.

Involves lipase enzymes.

Steps in Lipid Metabolism

Lipid Anabolism (Organismal energy high)
:

Acetyl-CoA reacts with oxaloacetate to form citrate in mitochondria.

Citrate is exported from mitochondria and reconverted to Acetyl-CoA in the cytosol.

Fatty acid synthase complex synthesizes acetoacetyl-acyl carrier protein (ACP).

Fatty acid synthase complex synthesizes butyryl-ACP from acetoacetyl-ACP.

Two carbons at a time are added to butyryl-ACP until palmitate (16C) is formed.

Palmitate reacts with glycerol to form triglycerides.

Lipid Catabolism (Organismal energy low)
:

Cells receive signal to initiate lipolysis.

Triacylglycerol lipase cleaves fatty acids from glycerol in the cytosol.

Fatty acids are activated in the cytosol.

"Fatty acids" are transported into the mitochondria.

"Fatty acids" are oxidized in the mitochondria (beta-oxidation) to capture energy as reduced reducing equivalents.

Fatty Acid Synthesis from Acetyl-CoA

Starting material
: Acetyl-CoA (from pyruvate dehydrogenase, ketogenic amino acids).

Acetyl-CoA is condensed with oxaloacetate to form citrate (TCA cycle), which can be transported out of the mitochondria.

In the cytosol, citrate is converted back into Acetyl-CoA and oxaloacetate.

Fatty Acid Synthase (FAS) Complex

FAS is a multi-enzyme complex.

Central Reactions in Fatty Acid Anabolism

Transferase
: Moves entire functional groups (e.g., Acetyl transferase moves acetyl group from acetyl-CoA to ACP).

Reductase
: Catalyzes reduction, often consuming reduced reducing equivalent (e.g., \beta-Ketoacyl-ACP reductase utilizes NADPH to reduce ketones).

Malonyl-CoA for FAS

Most acetyl-CoA is converted to malonyl-CoA for fatty acid synthesis.

Primary building block
: Malonyl-CoA (requires 1 ATP per malonyl-CoA).

Regulation
: Activity of acetyl-CoA carboxylase determines if fatty acids are synthesized.

How Fatty Acid Synthase Works with ACP

FAS complex needs one acetyl-CoA and one malonyl-CoA bound to acyl-carrier protein (ACP) to begin.

Acetyl group transferred from acetyl-CoA to ACP, then to \beta-ketoacyl-SACP-synthase (KSase).

Malonyl group transferred from malonyl-CoA to ACP.

Acetyl group from acetyl-KSase is transferred to malonyl-ACP to form acetoacetyl-ACP.

Acetoacetyl-ACP is then progressively reduced to butyryl-ACP (requiring NADPH and dehydration).

Fatty Acid Elongation

Progressive addition of two carbons from each malonyl-CoA grows the fatty acid chain until palmitate (16-carbon saturated fatty acid) is formed.

Uses 2 NADPH to add each 2C.

After hydrolysis, the fatty acid can be further modified (more carbons, double bonds) in the endoplasmic reticulum.

Recap: Fatty Acid Synthesis (Palmitate)

For each molecule of palmitate formed:

8 molecules of acetyl-CoA (comes from, e.g., 4 glucose molecules).

7 acetyl-CoA converted to malonyl-CoA (= 7 ATP invested).

7 additions of malonyl-CoA (= 14 NADPH invested).

If NADPH enters ETC at complex I, each NADPH pumps 10 protons (Total ATP equivalents INVESTED: 42 ATP).

Triacylglycerol Synthesis

Glycerol backbone may come from lipolysis or glycolysis.

Initial phosphorylation requires energy (ATP or NADH, depending on glycerol source).

Progressive addition of fatty acyl-CoAs to hydroxyl groups (not phosphorylated) forms a new triacylglycerol.

Lipid Catabolism

Core Concept of Lipid Catabolism

High ATP Yield
: Fatty acid oxidation yields a very high amount of ATP, in the form of reduced reducing equivalents, with each acetyl-CoA being further oxidized in TCA and ETC.

Overview of Lipid Catabolism

Lipid Anabolism (Organismal energy high)
: Utilizing acetyl-CoA and glycerol to ultimately form triglycerides (TAG) for energy storage.

Lipid Catabolism (Organismal energy low)
:

Cells receive signal to initiate lipolysis.

Triacylglycerol lipase cleaves fatty acids from glycerol in the cytosol.

Fatty acids are activated in the cytosol.

"Fatty acids" are transported into the mitochondria.

"Fatty acids" are oxidized in the mitochondria (beta-oxidation) to capture energy as reduced reducing equivalents.

Lipolysis: Signaling Cascade

Occurs in response to low energy (e.g., hormones activate adenylate cyclase).

Activation forms cyclic AMP (cAMP).

cAMP activates Protein Kinase.

Protein Kinase phosphorylates TAG Lipase to its active form.

TAG/DAG/MAG Lipases cleave fatty acids from glycerol.

FAs continue to activation, glycerol to glycolysis.

Fatty Acid Activation

Fatty acids cleaved from triacylglycerol are activated in the cytosol.

Activation via covalent bonding with CoA (high energy thioester bond).

Energy provided by ATP \rightarrow AMP.

Acyl-CoA is then ready for mitochondrial transport.

Fatty Acid Transport into Mitochondria

Fatty acyl-CoAs must cross both the outer and inner mitochondrial membranes for \beta-oxidation.

Outer Membrane
: Involves Acyl-CoA Synthetase, Voltage-dependent anion channel (VDAC), Carnitine acyltransferase 1 (CPT-1).

Inner Membrane
: Involves Carnitine-Acylcarnitine Translocase (CACT), Carnitine acyltransferase 2 (CPT-2).

CoA can diffuse back across the outer membrane via VDAC.

Beta-oxidation: Oxidation of Fatty Acids

Occurs at the beta carbon within the mitochondria.

Reactions in one cycle
:

Acyl-CoA dehydrogenase oxidizes the \alpha-\beta bond, reducing FAD to FADH\text{2}. Results in unsaturated, trans FA.

Enol-CoA hydratase hydroxylates the beta carbon using water.

L-Hydroxyacyl-CoA dehydrogenase oxidizes the beta carbon, reducing NAD^{\text{+}} to NADH.

Thiolase cleaves acetyl-CoA from the fatty acid, and a new acyl-CoA is formed (2 carbons shorter).

The cycle repeats until the fatty acid is completely converted to acetyl-CoA.

Odd-Chain Fatty Acids

Final cycle of beta-oxidation for odd-numbered fatty acids results in propionyl-CoA (instead of acetyl-CoA).

Propionyl-CoA enters the TCA cycle at Succinyl-CoA (involving propionyl-CoA carboxylase, methylmalonyl-CoA racemase, methylmalonyl-CoA mutase).

Recap: Fatty Acid Catabolism (Palmitate)

For each molecule of palmitate (16C) oxidized:

Activation with CoA
: Invested 1 ATP to AMP (= 2 ATP equivalents).

\beta-Oxidation
: Yields 8 Acetyl-CoA, 7 FADH\text{2}, 7 NADH.

Net ATP Yield
: \sim106 ATP (e.g., 28 ATP from FADH\text{2} and NADH, 80 ATP from 8 Acetyl-CoA via TCA/ETC, minus 2 ATP for activation).

Ketogenesis

Occurs when circulating glucose is low and \beta-oxidation is high (producing lots of acetyl-CoA).

Steps
:

Thiolase condenses two acetyl-CoA molecules to acetoacetyl-CoA.

A third acetyl-CoA is condensed with acetoacetyl-CoA to produce HMG-CoA.

HMG-CoA lyase removes an acetyl-CoA to yield acetoacetate (a ketone body).

Acetoacetate can be metabolized to \beta-hydroxybutyrate (NADH-dependent, reversible) or spontaneously decarboxylated to form acetone.

BASICS OF AEROBIC AND ANAEROBIC METABOLISM

Core Concept of Aerobic/Anaerobic Metabolism

Oxygen as Terminal Electron Acceptor
: Oxygen is the terminal electron acceptor in the electron transport chain, providing significantly more ATP than glycolysis alone.

What is Metabolism?

The biochemical basis of all life processes (creation, maintenance, and function of cells and tissues).

Catabolism
: Degradation of larger molecules, oxidative reactions, releases energy.

Anabolism
: Synthesis of smaller molecules, reductive reactions, absorbs energy.

Amphibolic
: Pathways including both catabolic and anabolic processes.

Nutrition and Metabolism

Macronutrients
: Hold and provide metabolizable energy.

Carbohydrate (glucose): \sim4 kcal/g.

Protein (amino acids): \sim4 kcal/g.

Fat (fatty acids): \sim9 kcal/g.

Micronutrients
: Help liberate metabolizable energy (do NOT provide energy).

B vitamins: Act as cofactors to enzymes (e.g., Niacin (B3, NADH) and Riboflavin (B2, FADH\text{2}) accept electrons; Pantothenic acid (B5, Coenzyme A) carries Acetate).

Importance of Oxygen

The body utilizes different metabolic pathways based on oxygen status.

Aerobic
: Sufficient oxygen; utilized by "slow" energy generation pathways.

Anaerobic
: Insufficient oxygen; utilized by "fast" energy generation pathways.

Highest energy-yielding pathways require O\text{2}.

Oxygen Requirement of Metabolic Pathways

Oxygen is required for the MOST energy from macronutrients.

Oxygen is the terminal electron acceptor of the Electron Transport Chain.

Reducing equivalents (NADH, FADH\text{2}) carry electrons.

Electron passing pumps protons into the intermembrane space, generates H\text{2}O.

Proton return via ATP synthase generates ATP.

Oxygenation Status and Respiratory Quotient (RQ)

Oxygenation status determined by organismal activity (Rest: high O\text{2}; Exercise: low O\text{2}).

Respiratory quotient (RQ)
= CO\text{2} produced (exhaled) : O\text{2} consumed (inhaled).

Carbohydrate: 1.00

Protein: 0.80

Mixed Diet: \sim0.85

Fat: 0.70

Alcohol: 0.66

Lower RQ indicates more O\text{2} consumed per CO\text{2} produced (e.g., fat metabolism).

Energy Generation by RQ

Fat metabolism requires the most oxygen per amount of carbon (lower RQ) and provides relatively more energy (net ATP) because it absolutely requires oxygen.

Core ATP Synthetic Pathways
:

Immediate System
: Phosphocreatine. No oxygen. Yields 1 net ATP. Predominates for \sim5 sec.

Anaerobic System
: Glycolysis + Lactate Fermentation. No oxygen. Yields 2 net ATP (without TCA). Predominates for 5-45 sec.

Aerobic System
: TCA Cycle + ETC. Consumes oxygen.

Glucose: Yields 36 net ATP. RQ = 1.0. Predominates in exercise > 45 sec.

Fatty Acids (Beta-oxidation to TCA): Yields \sim130 net ATP (for palmitate). RQ = 0.70.

Pathways Requiring Oxygen

Pathways that require oxygen (generate reduced reducing equivalents and utilize ETC):

"Aerobic System"

Tricarboxylic Acid Cycle (TCA, Citric Acid Cycle): Central to Carbohydrate, Protein, and Fat catabolism.

Beta Oxidation: Central to Fat catabolism, produces Acetyl-CoA (and propionyl-CoA), which feeds into TCA and ETC.

Pathways NOT Requiring Oxygen

Pathways that do not require oxygen:

"Immediate System" (creatine phosphate lysis).

"Anaerobic System"

Glycolysis: Conversion of glucose to pyruvate. Can proceed anaerobically or aerobically.

Fate of Pyruvate Depends on O\text{2} Status

Low oxygen (lactate fermentation)
:

Pyruvate is anaerobically converted to Lactate via Lactate Dehydrogenase.

NAD^{\text{+}} is regenerated (oxidized).

Lower net energy yield.

Normal oxygen (aerobic respiration)
:

Pyruvate is aerobically converted to Acetyl-CoA via Pyruvate Dehydrogenase.

More NADH is generated (1 per pyruvate molecule).

Higher net energy yield.

Glycolysis During Exercise (Muscle & Liver)

Liver
: Provides glucose to Muscle by breaking down glycogen (Glycogenolysis) or synthesizing glucose (Gluconeogenesis).

Muscle
: Utilizes glucose to produce ATP and Lactate.

Lactate Cycling
: Lactate is taken up by Liver, which then synthesizes new glucose from it via Gluconeogenesis (Cori Cycle).

Equine Gastrointestinal Physiology and Energy Metabolism

Horses are hindgut fermenters; digestion relies on microbial fermentation.

Cecum
: Primary site for fermentation of dietary fiber into short-chain fatty acids (acetate, propionate, butyrate) and organic acids (lactate).

Gut Microbial Metabolism and Exercise Endurance

Lactate from exercising muscle diffuses into the gut lumen.

Metabolized by specific gut microbes (e.g.,
Veillonella
spp.) into SCFAs.

Butyrate \rightarrow beta oxidation.

Propionate \rightarrow gluconeogenesis.

Higher propionate during exercise facilitates higher running endurance (Scheiman et al 2019 – Nature Medicine).

Equine Microbiota and Fitness

Higher gut microbiota diversity associated with cardiometabolic fitness in elite trained horses, elevating expression of mitochondrial genes for:

\beta-oxidation.

Electron transport chain.

Ca\text{2} ^{\text{+}} translocation (muscle contraction).

Mitophagy (mitochondrial turnover).

Biogenesis (new mitochondria).

Reference: Mach et al 2022 – Communications Biology.

Hormones – Intercellular Coordination of Metabolism

Core Concept of Hormonal Regulation

Coordinate Metabolism
: Hormones coordinate metabolism across the entire body, dictating which energy source should be utilized and which behaviors should be conducted to balance energy.

Cell-to-Cell Coordination via Hormones

Metabolism can differ between body parts (e.g., exercising muscle vs. liver).

Hormones help coordinate metabolism across the body to resolve these differences.

Hormones and Signaling

What are hormones?
Signaling molecules produced in one part of the body, acting in another.

Categories
: Polypeptides, AA derivatives, Steroids, Signaling lipids.

Produced by the endocrine system (found in every organ, every organ has hormonal responses).

Chemical signaling types
:

Autocrine
: Same cell.

Direct
: Connected cells.

Paracrine
: Nearby cells.

Endocrine
: Far away cells.

Key Hormones Regulating Metabolism

Leptin
:

Meter for stored energy
.

Primarily produced by adipose tissue.

Secreted in response to insulin and glucocorticoids.

Stimulates phosphorylation of AMP-activated protein kinase.

Acts on Muscle/Adipose
: Induces glucose uptake and \beta-oxidation; reduces fatty acid synthesis.

Acts in Brain
: Reduces appetite (inhibits specific hypothalamus neurons).

Epinephrine
:

Mobilizes energy in response to stress
.

Primarily produced by the adrenal gland (smaller amounts from sympathetic neurons).

Secreted in response to stressful stimuli.

Stimulates production of cAMP (adenylate cyclase).

Acts in Liver
: Enhances glycogenolysis (increased blood glucose).

Acts in Muscle
: Stimulates glycolysis and \beta-oxidation.

Glucagon
:

Responds to low blood glucose
.

Produced by the pancreas
.

Secreted in response to low blood glucose
.

Stimulates production of cAMP (adenylate cyclase).