Comprehensive notes: Bioenergetics and Substrate Utilization
ATP-generating pathways: power ranking and key regulators
Power (ATP per second) ordering among major pathways:
Creatine phosphate (CP or phosphocreatine) provides the highest power.
Glycolysis provides the next highest power.
Oxidative phosphorylation provides the lowest power among the three, but its power depends on substrate availability (carbs vs fats).
Approximate relationships: CP ≈ 2× glycolysis ≈ 4× oxidative phosphorylation in the context of their relative speeds; glycolysis is roughly twice as powerful as oxidative phosphorylation.
Three guiding factors that regulate any metabolic pathway:
Substrates (mass-action effects): more substrate speeds up the pathway.
Enzymes (enzyme availability/activity): more active enzymes speed up flux; fewer enzymes slow it down.
Product levels (N-products) and allosteric regulation: accumulation of products inhibits enzymes; substrates can activate enzymes.
Mechanisms include allosteric activation/inhibition and classic product inhibition.
Takeaway: when evaluating pathway capacity, always consider substrate, enzyme activity, and product inhibition/regulation.
Capacity limits for the three major energy systems
Phosphocreatine (fossil creatine) system:
Capacity is limited by the finite stores of phosphocreatine (substrate availability).
Once phosphocreatine is depleted, ATP generation from this pathway collapses until stores are replenished.
Typical time scale: on the order of ~10 seconds for depletion under intense demand.
Glycolysis (anaerobic):
Substrate: glucose (glycogen can shorten the start, since muscle glycogen is readily available early in exercise).
Initial capacity estimates (resting to ~60s of heavy effort) suggest ~30–100 s range for high-intensity glycolytic flux before major fatigue mechanisms set in.
Primary limiter shifts away from substrate (glucose) and toward enzyme activity and metabolite regulation.
Rate-limiting enzyme (classically) is phosphofructokinase (PFK).
Regulation of PFK:
Activated by low energy state: high ADP, high AMP.
Inhibited by high energy state: high ATP, high citrate.
During sustained high-intensity exercise, glycolysis is increasingly inhibited by buildup of metabolites (e.g., hydrogen ions H⁺, inorganic phosphate Pᵢ) and changes in redox state, which slow flux.
Lactate production occurs when pyruvate accumulates faster than it can enter mitochondria; lactate helps regenerate NAD⁺ by reoxidizing NADH to NAD⁺, sustaining glycolysis.
Lactate management is also tied to the Cori cycle (lactate shuttling to liver for gluconeogenesis) and mitochondrial entry of pyruvate via PDH (pyruvate dehydrogenase).
Oxidative phosphorylation:
Capacity limited by substrate availability (glycogen stores; fat stores can support longer durations).
Duration typically cited as up to ~90 minutes when glycogen stores begin to deplete, leading to a slowdown in carbohydrate oxidation.
Fat oxidation capacity is theoretically available for days due to adipose triglyceride stores, but is limited by transport into mitochondria (beta-oxidation rate) and other regulatory factors.
In general, oxidative phosphorylation does not accumulate fatigue via metabolite buildup to the same extent as glycolysis, but reactive oxygen species (ROS) can contribute to fatigue signaling and adaptation.
Summary: CP is fastest but short-lived; glycolysis is fast but limited by metabolites and enzyme regulation; oxidative phosphorylation is slower but sustained, limited by substrate availability and mitochondrial capacity.
Carbohydrate metabolism: glycolysis, glycogen, and pyruvate fate
Carbohydrate sources:
Glycogen in muscle provides immediate substrate early in exercise; glycogenolysis breaks glycogen into glucose units for glycolysis.
Glycogenolysis is the breakdown of glycogen (glycogen lysis). The enzyme is often glycogen phosphorylase; debranching enzymes handle branches.
Net products of glycolysis per one glucose molecule (from Tuesday’s material):
Glucose (6 carbons) → two molecules of pyruvate (3 carbons each):
Net ATP produced by substrate-level phosphorylation:
Electron carriers produced:
Pyruvate is the end product of glycolysis.
Pyruvate fate (two main routes):
Anaerobic/fermentation pathway: pyruvate → lactate to regenerate NAD⁺ so glycolysis can continue when oxygen is limiting or mitochondrial entry is slow.
Aerobic pathway: pyruvate enters mitochondria and is converted to acetyl‑CoA by pyruvate dehydrogenase (PDH), yielding NADH for the downstream Krebs cycle.
Lactate and its roles:
Lactate production serves two critical purposes when pyruvate accumulates:
Mass-action relief: continue glycolysis and provide substrate-level ATP (two ATP per glucose via glycolysis).
NAD⁺ regeneration: lactate formation reoxidizes NADH to NAD⁺, enabling sustained glycolysis by keeping NAD⁺ available for glyceraldehyde-3-phosphate dehydrogenase.
Lactate transporters (monocarboxylate transporters, MCTs) shuttle lactate out of the cytosol to neighboring fibers, the heart, or the liver.
Lactate utilization options:
In oxidative muscles or heart: lactate can be taken up and oxidized to lactate dehydrogenase (reforming pyruvate) and then enter the mitochondria for oxidation.
In liver: lactate can be converted back to glucose via gluconeogenesis (Cori cycle), with glycerol from fat and certain amino acids as other gluconeogenic substrates.
Cori cycle: lactate produced in muscle is shuttled to liver, converted to glucose, and returned to muscle for ATP production; cycle helps maintain blood glucose during intense exercise.
Pyruvate dehydrogenase (PDH) and the glycolysis–Krebs transition:
Pyruvate produced by glycolysis is converted to acetyl‑CoA by the PDH complex:
Pyruvate + CoA + NAD⁺ → Acetyl‑CoA + CO₂ + NADH + H⁺
PDH is a key regulatory step that gates entry of glycolytic energy into the Krebs cycle.
Regulation of PDH is strongly tied to the redox state (NADH/NAD⁺ ratio) and energy status:
High NADH/NAD⁺ (high redox ratio) inhibits PDH, slowing entry of acetyl‑CoA into Krebs.
Low redox ratio (more NAD⁺ available) speeds PDH and pyruvate flux into oxidative metabolism.
Energetic yields for glucose (summary):
Theoretical maximum from complete oxidation of one glucose molecule:
Glycolysis substrate-level ATP:
NADH produced: NADH total (2 from glycolysis, 2 from PDH, 6 from Krebs)
FADH₂ produced: FADH₂ total (from Krebs)
GTP (substrate-level ATP) produced: GTP
Oxidative phosphorylation energetics (P/O):
Each NADH yields ~ (practically slightly less due to shuttle losses)
Each FADH₂ yields ~
Theoretical ATP yield:
Actual yield in many cells close to due to shuttle losses and proton leak.
Important note: the difference between theoretical and actual ATP is largely due to energetics of transporting NADH into the mitochondria (shuttle systems) and proton leakage.
Glycogenesis and storage of carbohydrate:
When energy is surplus, glucose can be stored as glycogen (glycogenesis):
Primary storage sites: liver (~100 g) and muscle (~500 g).
Liver glycogen maintains blood glucose; muscle glycogen serves muscle-specific energy and is not readily released to blood.
Glycogenolysis is the breakdown of glycogen to glucose units for glycolysis during energy demand.
Glucose/glycogen metabolism connections:
Glucose → glycolysis → pyruvate → lactate or acetyl‑CoA (via PDH) → Krebs → ETC.
The rate and direction of these pathways depend on energy status, substrate availability, and regulation by metabolites and enzymes.
Fat metabolism: lipid energy pathways and yield
Fat storage and mobilization:
Triglycerides: glycerol backbone + three fatty acids stored in adipose tissue and intramuscular stores.
Lipolysis (lipid breakdown) releases glycerol and free fatty acids (FFAs) into cytosol.
Glycerol can be fed into glycolysis (via glycerol-3-phosphate pathway) to generate ATP; FFAs undergo beta-oxidation in mitochondria.
Excess fat storage as triglycerides can also occur in adipose tissue and to a lesser extent in muscle (intramuscular triglycerides, IMTGs).
Transport into mitochondria (rate-limiting step):
Fatty acids must be activated and then transported into the mitochondrial matrix via the carnitine shuttle.
Key transporter: carnitine palmitoyltransferase I (CPT-1) at the outer mitochondrial membrane; CPT-2 functions inside the matrix.
CPT-1 is the rate-limiting step of mitochondrial fatty acid entry and oxidation.
The idea behind carnitine supplementation is to enhance transport into mitochondria, though practical efficacy varies.
Activation and beta-oxidation:
Activation cost: fatty acid activation requires the consumption of 2 ATP equivalents up front.
Beta-oxidation cycles shorten the fatty acid by 2 carbons per cycle, yielding acetyl‑CoA, NADH, and FADH₂ per cycle.
For an 18-carbon fatty acid (C18):
Number of beta-oxidation cycles:
Each cycle yields: 1 NADH and 1 FADH₂. So total beta-oxidation yields plus 9 acetyl‑CoA (since 18 carbons yield 9 acetyl‑CoA).
Acetyl‑CoA enters the Krebs cycle and yields energy via NADH, FADH₂, and GTP:
Per acetyl‑CoA in Krebs:
For 9 acetyl‑CoA:
Energetic yield for an 18-carbon fatty acid (example):
Activation cost:
Beta-oxidation: and
Krebs cycle with 9 acetyl‑CoA: , ,
Total oxidative phosphorylation yield (NADH/FADH₂ + Krebs contributions):
Including activation cost and combining all sources yields approximately:
Note: The commonly cited total for complete fatty acid oxidation of an 18-carbon fatty acid is higher than 92 ATP; the detailed accounting above matches the typical classroom calculation when counting all NADH, FADH₂, and acetyl‑CoA contributions with their exact shuttle efficiencies. In the provided material, the calculated total after all steps and shuttles comes to about – ATP for an 18-carbon fatty acid, when counted with the full NADH/FADH₂ pool from both beta-oxidation and Krebs and including acetyl‑CoA-derived NADH and FADH₂; the key point is fats yield far more ATP per molecule than carbohydrates but slower to mobilize and oxidize.
Energy density and practical implication:
Energy density: fats ≈ , carbs ≈ , making fats a very energy-dense fuel source.
Fats provide large energy stores but are energetically less accessible quickly; carbs provide rapid ATP production via glycolysis and oxidative pathways.
The body can store vast fat energy reserves, supporting prolonged activity and endurance.
Ketogenesis:
When fat oxidation is high and carbohydrate availability is low, ketone bodies can be produced (ketogenesis) and used by tissues such as the brain and heart as alternative energy sources.
This helps spare protein and maintain energy when carbohydrate intake is limited.
Intramuscular triglycerides (IMTG):
IMTG stores can contribute to fat oxidation during endurance exercise; their contribution increases with duration and training adaptations.
Practical note on fat and carbohydrate interaction:
High carbohydrate availability inhibits fatty acid transport into mitochondria and reduces fat oxidation, even at rest, and particularly at higher intensities.
Conversely, long-duration, low-carbohydrate or high-fat diets can shift substrate utilization toward greater fat oxidation (until intensity limits pace or performance).
Protein metabolism as an energy source
Protein contribution to energy:
Protein energy yield is similar to carbohydrate, approximately , but depends on which amino acids are used.
In normal conditions, proteins contribute a small portion to total energy (typically < 10%), with smaller fractions in well-fed situations.
During starvation or energy scarcity, protein contribution can rise substantially.
Protein breakdown and conversion:
Proteins are first broken down by proteolysis into amino acids (protein lysis).
The amino group is removed as ammonia (toxic) and is converted to urea for excretion via the kidneys (urea cycle).
The remaining carbon skeletons can enter glycolysis or the Krebs cycle to yield energy.
Practical note:
The body generally prefers non-protein fuels for energy; protein catabolism is a backup or adaptive response to energy shortage.
Oxidative capacity, training, and substrate utilization
Oxidative capacity (an in vitro measure):
Defined as the maximum rate at which muscle tissue can consume oxygen in a controlled chamber (isolated tissue measure).
Measured as microliters of O₂ per gram per hour per tissue volume/specific tissue context.
Strongly correlated with SDH activity (succinate dehydrogenase), a Krebs cycle enzyme and component of complex II in the ETC.
Higher SDH activity indicates greater mitochondrial density and oxidative enzyme content, predicting greater oxidative capacity.
SDH as a marker:
SDH stands for succinate dehydrogenase, a Krebs cycle enzyme that also participates in the electron transport chain (complex II).
It serves as a practical proxy for mitochondrial content and oxidative capacity.
VO₂ max and endurance performance:
VO₂ max is the maximum rate of oxygen consumption and is a primary measure of aerobic fitness.
Generally, higher oxidative capacity correlates with higher VO₂ max.
Other cardiovascular factors (heart pumping capacity, capillary density, myoglobin content) also influence endurance performance.
Training effects on oxidative capacity:
Endurance/combined training improves oxidative capacity in both type I (slow-twitch) and type II (fast-twitch) muscle fibers.
Young individuals may show less improvement in type II fibers for some measures, whereas older individuals may show more noticeable gains in oxidative capacity with training.
Improvements come from increases in mitochondria, mitochondrial enzyme content (e.g., SDH), capillary density, and myoglobin.
Interplay of substrate utilization and oxidative capacity:
At rest, fat oxidation predominates due to abundant fat stores and low immediate energy demands.
With increasing intensity, carbohydrate oxidation rises because carbs provide ATP more rapidly than fat (roughly twice as fast).
The shift from fat to carbohydrate as a primary fuel with increasing intensity is partly due to transport/logistics constraints in fat oxidation, notably transport into mitochondria via CPT and reduced adipose blood flow at high intensities.
The crossover concept and dietary influence:
Crossover point: the exercise intensity where carbohydrate oxidation overtakes fat oxidation, approximately around 70% VO₂ max in many individuals.
Training shifts this crossover to the right: more fat can be used at higher intensities when well-trained.
Diet also shifts substrate utilization:
High-carbohydrate, low-fat diet increases carbohydrate usage even at lower intensities (e.g., ~70% of energy from carbs at 20% VO₂ max in some subjects).
Low-carbohydrate, high-fat diets shift reliance toward fat, moving the crossover to higher intensities (e.g., around 85% VO₂ max in some cases).
Adaptations contributing to greater fat oxidation with training:
Increased CPT transporters and improved transport of fatty acids into mitochondria (rate-limiting step becomes less limiting).
Enhanced mitochondrial density and oxidative enzyme activity (including SDH) and improved capillary density.
Changes in substrate availability and hormonal regulation that optimize fat oxidation during prolonged exercise.
Summary of practical implications for endurance performance:
Endurance improvements depend on boosting oxidative capacity and fat oxidation ability to spare glycogen during prolonged efforts.
Training, diet, and individual variability all shape the relative contribution of carbs and fats during exercise.
Substrate utilization during exercise: integration and practical takeaways
Core idea: All three pathways (CP, glycolysis, oxidative phosphorylation) are active simultaneously; the relative contribution of each depends on intensity and duration.
Substrate availability, metabolite levels, and enzyme activity ultimately determine the flux through each pathway.
At rest, fat oxidation predominates due to ample fat stores and energy requirements being modest.
As intensity rises, carbohydrate oxidation becomes more dominant due to faster ATP generation from carbohydrates.
With continued exercise, CP stores are depleted early; glycolysis ramps up quickly but is constrained by metabolite buildup; oxidative phosphorylation becomes more prominent as mitochondria can process pyruvate and acetyl‑CoA more efficiently.
High-intensity exercise (≈30 s):
Immediate availability of phosphocreatine drives rapid ATP production via mass-action effects.
ATP and AMP rise, upregulating glycolysis and phosphofructokinase (PFK).
Pyruvate accumulates; if not shuttled into mitochondria efficiently, lactate production increases to sustain glycolysis and regenerate NAD⁺.
As lactate accumulates and pyruvate delivery into mitochondria improves, oxidative phosphorylation contributes more and glycolysis slows due to acid buildup.
Rest to exercise transitions and substrate selection:
During rest and early recovery, fat oxidation is favored; glycogen stores are replenished (glycogen synthesis).
With longer exercise duration, carbohydrate availability decreases (glycogen depletion) and fat oxidation becomes more important, though the capacity is limited by transport and blood flow.
Practical diet and training implications:
Training enhances fat oxidation capacity by increasing CPT transporters and mitochondrial oxidative capacity.
Diet composition affects substrate utilization; high-carb diets favor carbohydrate metabolism, while high-fat diets favor fat metabolism (to a point and with individual variation).
Quick practical recap: key equations and numbers to remember
ATP yields (glucose oxidation, theory vs. reality):
Theoretical ATP from one glucose:
Glycolysis:
Pyruvate to Acetyl‑CoA (PDH): (≈ )
Krebs cycle: per glucose, 2 turns →
Oxidative phosphorylation:
Total theoretical:
Actual typical yield: about due to shuttle losses and proton leak.
Net contributions summarized:
Fat oxidation tally (example for an 18-carbon fatty acid):
Activation:
Beta-oxidation cycles:
Acetyl‑CoA produced:
Krebs outputs per acetyl‑CoA:
Total oxidative yield before activation:
ATP equivalents from NADH and FADH₂:
From Krebs GTP:
Sum before activation cost:
Net after activation:
Energy density (storage fuels):
Fat:
Carbohydrate:
Key terminology to know:
Glycogenolysis (glycogen lysis): breakdown of glycogen to provide glucose for glycolysis.
Glycogenesis: synthesis of glycogen from glucose.
Lipolysis: breakdown of triglycerides into glycerol and free fatty acids.
Beta-oxidation: mitochondrial fatty acid oxidation that yields acetyl‑CoA, NADH, and FADH₂.
CPT-1 (and CPT-2): carnitine shuttle transporting long-chain fatty acids into mitochondria (rate-limiting step).
PDH: pyruvate dehydrogenase, the transition from glycolysis to Krebs cycle by producing acetyl‑CoA and NADH.
SDH: succinate dehydrogenase, Krebs cycle enzyme and complex II of the ETC; used as a proxy for oxidative capacity.
Cori cycle: lactate produced in muscle is converted to glucose in the liver and returned to muscle.
Ketogenesis: production of ketone bodies when fat oxidation is high with low carbohydrate availability.
Practical implications for exam prep:
Be able to explain the regulatory role of ADP/AMP, ATP, citrate, NADH/NAD⁺ in controlling glycolysis and PDH flux.
Be able to calculate ATP yield from glycolysis and aerobic metabolism with/without shuttle losses; know the approximate theoretical vs. actual values.
Understand the concept of the crossover point and how training and diet shift substrate utilization during exercise.
Recognize the roles of lactate beyond a waste product and the consequences of lactate shuttling between tissues (Cori cycle, lactate oxidation).
Recall regulatory enzymes and transport steps that limit fat oxidation (CPT-1) and how endurance training increases oxidative capacity through mitochondrial and capillary adaptations.