Energy Metabolism and ATP Production
Energy and Metabolism
Energy Definition: Fuels metabolism and enables cells, muscles, and other body tissues to perform work.
Metabolism: A series of chemical reactions where the product of one reaction is the substrate for the next.
Energy Metabolism: Major reactions by which the body obtains and uses energy from food.
Food Sources and Breakdown:
Carbohydrates (CHO) $\rightarrow$ sugars, starches $\rightarrow$ glucoses.
Fats $\rightarrow$ triglycerides $\rightarrow$ glycerol, fatty acids.
Proteins $\rightarrow$ amino acids.
Categories of Energy Metabolism:
Catabolism: Processes that release energy by breaking down larger molecules.
Anabolism: Processes that use released energy to build larger molecules.
Forms of Energy:
Chemical
Nuclear
Electrical
Mechanical
Thermal
Radiant
Chemical Energy:
Chemical energy from carbohydrates, proteins, and fats is primarily used to produce Adenosine Triphosphate (ATP), a high-energy chemical.
Catabolism of glucose is the primary energy source for ATP production.
Adenosine Triphosphate (ATP)
Structure:
ATP has 2 high-energy phosphate bonds (~).
Energy is stored in these bonds until needed.
Phosphate groups are split off by a process called hydrolysis.
Hydrolysis Reactions:
One high-energy bond splits (yielding ADP, adenosine diphosphate):
ATP + H2O \rightarrow ADP + Pi + H^+ (where P_i is inorganic phosphate)
Two high-energy bonds split (yielding AMP, adenosine monophosphate):
ATP + H2O \rightarrow AMP + PPi (where PP_i is inorganic pyrophosphate)
"Energy Currency":
Cells contain a limited quantity of ATP, sufficient for only several seconds of intense exercise.
Cells never completely deplete ATP stores; approximately 30\% of ATP remains in muscle.
ATP is constantly regenerated.
ATP Production Systems
There are 3 primary energy systems that produce ATP:
Phosphagen System (Immediate energy system).
Anaerobic System (Anaerobic glycolysis).
Aerobic System (Citric Acid Cycle / Krebs Cycle / Tricarboxylic Acid Cycle & Electron Transport Chain).
The Phosphagen System
Characteristics:
The simplest and most immediate energy system.
Provides instantaneous energy.
Low complexity (a one-step process).
Very fast maximal rate of ATP production.
Very limited capacity to make ATP.
No lag time for increased ATP production.
Mechanism:
Creatine phosphate (CP) directly provides a phosphate group for ATP resynthesis.
The enzyme Creatine Kinase catalyzes this reaction.
Cells store larger amounts of CP than ATP.
Supplemental creatine can be used to enhance CP stores.
Reaction:
Creatine\,Phosphate + ADP \xrightarrow{Creatine\,Phosphokinase} Creatine + ATP
Anaerobic Pathway (Anaerobic Glycolysis)
Substrates: Only glucose can be used.
Glycogen, the storage form of glucose, can be broken down into glucose through glycogenolysis to enter this pathway.
Process: Glucose is broken down to pyruvate.
Oxygen Availability:
If no oxygen is available, pyruvate is converted to lactic acid.
ATP Yield: Generates 2 molecules of ATP per molecule of glucose.
Reaction for Lactic Acid Formation:
Pyruvate\,(COO^-, C=O, CH3) \xrightarrow{Lactate\,Dehydrogenase} L-Lactate\,(COO^-, HO-C-H, CH3)
In this reaction, NADH + H^+ is oxidized to NAD^+.
Aerobic Pathway
Oxygen Requirement: Occurs when oxygen is available.
Efficiency: Glucose breakdown is highly efficient, producing significantly more ATP.
ATP Yield: Over 90\% of the body's ATP is produced via this pathway.
Speed & Capacity: It is the slowest ATP production system, but it has an unlimited ability to produce ATP.
Metabolic Pathways Involved (5):
Glycolysis
Beta-oxidation (for fatty acid breakdown)
Deamination (for amino acid breakdown)
Citric Acid Cycle (also known as Krebs cycle or Tricarboxylic Acid cycle, TCA cycle)
Electron Transport Chain (ETC)
End Products: ATP, carbon dioxide (CO2), and water (H2O).
Glycolysis (in Aerobic Context)
The net result of glycolysis in the aerobic pathway is 2 ATP and 2 pyruvate molecules.
Pyruvate to Acetyl CoA: Pyruvate reacts with Coenzyme A (CoA) to form acetyl CoA, which can then enter the Citric Acid Cycle.
Citric Acid Cycle (Krebs Cycle / TCA Cycle)
Location: Occurs in the mitochondrial matrix.
Nature: It is a catabolic pathway.
Inputs: Products from the breakdown of carbohydrates, fats, and amino acids (primarily as acetyl CoA) feed into the cycle.
Outputs: Ultimately yields CO2, H2O, and ATP (indirectly through reducing equivalents like NADH and FADH2).
Intermediates: Some intermediate compounds can leave the cycle to participate in other metabolic processes (e.g., gluconeogenesis, synthesis of some amino acids).
Entry Molecule: Acetyl CoA (a 2-carbon molecule) enters the cycle.
Structure: Involves 8 key reactions; it is a true cycle, beginning and ending with the same 4-carbon compound, oxaloacetate (OAA).
First and Final Reactant: Citric acid is involved in the initial condensation and eventual regeneration.
Direct Products per Turn: For each acetyl CoA entering the cycle, it produces 3\,NADH, 1\,GTP (which is functionally equivalent to ATP), and 1\,FADH_2.
Carbon Loss: There is a loss of 2 carbons as CO_2.
Key Reactions and Enzymes:
Acetyl CoA condenses with Oxaloacetate $\rightarrow$ Citrate (catalyzed by Citrate Synthase).
Citrate $\rightarrow$ Isocitrate (catalyzed by Aconitase).
Isocitrate $\rightarrow$ Alpha-ketoglutarate (catalyzed by Isocitrate Dehydrogenase; CO_2 is released, and NADH + H^+ is produced).
Alpha-ketoglutarate $\rightarrow$ Succinyl CoA (catalyzed by Alpha-ketoglutarate Dehydrogenase Complex; CO_2 is released, and NADH + H^+ is produced).
Succinyl CoA $\rightarrow$ Succinate (catalyzed by Succinate Thiokinase/Succinyl CoA Synthetase; GTP is produced from GDP + P_i).
Succinate $\rightarrow$ Fumarate (catalyzed by Succinate Dehydrogenase; FADH_2 is produced).
Fumarate $\rightarrow$ Malate (catalyzed by Fumarase).
Malate $\rightarrow$ Oxaloacetate (catalyzed by Malate Dehydrogenase; NADH + H^+ is produced).
ATP Yield from NADH and FADH2 (within the Electron Transport Chain)
Old Convention (approximate values for ATP produced by ETC):
Each NADH + H^+ yields 3 ATP.
Each FADH_2 yields roughly 2 ATP.
Newer Convention (more precise values):
Each NADH + H^+ yields 2.5 ATP.
Each FADH_2 yields roughly 1.5 ATP.
Regulation of Metabolic Pathways
Metabolic pathways are tightly regulated through several mechanisms:
Lack of substrate(s).
Accumulation of final product.
Enzymes (effects of hormones on enzyme synthesis or activity via covalent modification).
Compartmentalization of reactions within the cell or specific tissues.
Lack of Substrate(s)
Some enzymes are only active if certain substrate concentrations are met.
If there is insufficient substrate concentration, the reaction will not begin.
Accumulation of Final Product
The removal or use of the final product drives the reaction forward.
As long as the product concentration remains low and substrate is sufficient, the reaction continues.
If the product concentration becomes too high, it can inactivate one or more enzymes in the reaction series, often involving allosteric enzymes.
Allosteric Enzymes
"Allo" means "Other."
These enzymes possess allosteric sites (sites other than the active site) where effector molecules can bind.
The binding of positive or negative effectors causes a conformational (shape) change in the enzyme.
This conformational change affects the binding of the substrate at the catalytic site, either enhancing or inhibiting enzyme activity.
Allosteric enzymes are often located at key control points within metabolic pathways.
Example: Citrate, a product of the Krebs cycle, can negatively regulate Phosphofructokinase-1 (PFK-I) in glycolysis, suggesting feedback inhibition.
Effect of Hormones on Enzymes
Slow Effect (minutes or longer): Hormones can regulate enzyme synthesis.
Increased levels of a hormone can increase the synthesis of some enzymes.
Example: Glucokinase in the Liver:
Glucokinase is an enzyme in the liver that phosphorylates glucose, effectively trapping it within the liver cell (Glucose \rightarrow Glu-6-P).
Increased blood glucose levels lead to increased insulin (a hormone).
Increased insulin signals the need for producing more glucokinase to trap glucose in the liver, particularly during states of plenty (feasting).
Conversely, if there is no meal $\rightarrow$ decreased glucose $\rightarrow$ decreased insulin.
Decreased insulin $\rightarrow$ decreased glucokinase $
ightarrow$ less glucose trapped in the liver, allowing it to be distributed to other tissues.This regulation is slow because it involves transcription and translation to synthesize new enzyme molecules.
Quick Effect (milliseconds): Hormones can trigger covalent modification of existing enzymes.
Covalent modification provides rapid control without the need to synthesize more enzyme.
Enzymes can be quickly activated ("turned on") or deactivated ("turned off") in response to hormonal signals, like a light switch.
Mechanisms:
Phosphorylation: Adding a phosphate group (PO_4) can activate or deactivate an enzyme.
Dephosphorylation: Removing a phosphate group (PO_4) can activate or deactivate an enzyme.
Kinase: An enzyme that adds a PO_4 group.
Phosphatase: An enzyme that removes a PO_4 group.
This mechanism avoids the waiting period associated with hormone and enzyme production.
Example:
Decreased glucose $\rightarrow$ increased glucagon (a catabolic hormone) $\rightarrow$ decreased ATP $\rightarrow$ increased AMP.
Glucagon signals cells to release energy.
AMP is converted to cAMP (cyclic AMP), which acts as a secondary messenger inside the cell (the hormone is the first messenger outside the cell).
cAMP activates Protein Kinase A.
Protein Kinase A then phosphorylates and activates target enzymes that control catabolic pathways, such as:
Glycogen breakdown $\rightarrow$ glucose + energy.
Protein breakdown $\rightarrow$ amino acids + energy $\rightarrow$ glucose (gluconeogenesis).
Fatty acid breakdown $\rightarrow$ acetyl CoA $\rightarrow$ energy.
Compartmentalization of Reactions
Definition: The localization of metabolic activity to specific parts of the cell.
Cellular Locations and Examples:
Cytoplasm
Mitochondria: Pyruvate carboxylase, an enzyme in gluconeogenesis, is uniquely located in the mitochondria.
Lysosomes: Proteolytic enzymes are localized here.
Tissue-Specific Localization: Enzymes can also be found only in certain tissues.
Glucokinase is in the liver.
Hexokinase is in the brain and muscle.
Reasons for Compartmentalization:
Separation of Opposing Pathways: Prevents futile cycles, e.g., fatty acid synthesis occurs in the cytosol, while beta-oxidation occurs in the mitochondria.
Regulation of Pathway Activity: By controlling the transport of substrates to specific cellular locations, a pathway can be inhibited (e.g., blocking transport of a substrate stops its pathway).
Glucokinase (Liver) vs. Hexokinase (Brain, Muscle):
Shared Function: Both enzymes perform the same basic function: adding a phosphate to glucose (Glucose $\rightarrow$ Glucose-6-Phosphate) to trap it within the cell.
Tissue Distribution: Glucokinase is primarily in the liver; Hexokinase is found in brain and muscle.
Regulatory Differences:
Glucokinase in Liver:
Activated by high glucose and high insulin, signaling the liver to store glucose as glycogen.
Inhibited by high glucagon.
It is generally "turned off" except when activated by high insulin (reflecting high glucose), preventing the liver from storing glucose unnecessarily.
Hexokinase in Brain and Muscle:
Brain cells absolutely require glucose, so hexokinase is always active to ensure glucose uptake, even when blood glucose is low.
In muscle, hexokinase is only inhibited by an accumulation of Glucose-6-Phosphate (G-6-P), preventing glucose from being trapped if not needed. This feedback inhibition is more likely to occur in muscle than in the brain.
Regulation of the Krebs Cycle (Recap and Detail)
Key regulatory enzymes in the Krebs cycle are often allosterically controlled:
Citrate Synthase (converts Acetyl CoA + OAA $\rightarrow$ Citrate):
Negative effector: NADH.
Isocitrate Dehydrogenase (converts Isocitrate $\rightarrow$ Alpha-ketoglutarate):
Negative effectors: High ATP, high NADH.
Positive effectors: High ADP, AMP, high NAD^+, Ca^{2+}.
Alpha-ketoglutarate Dehydrogenase Complex (converts Alpha-ketoglutarate $\rightarrow$ Succinyl CoA):
Negative effectors: NADH, ATP, Succinyl CoA.
Positive effector: Ca^{2+}.
General ATP Generation Regulation via Krebs Cycle:
Negative effectors: Surplus NADH allosterically inhibits citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.
Positive effectors: Ca^{2+} powerfully activates Pyruvate Dehydrogenase (PDHase), leading to more acetyl CoA entry into the TCA cycle. Ca^{2+} also stimulates isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.
Electron Transport Chain (ETC)
Other Name: Respiratory Chain.
Components: Consists of four protein complexes.
Location: Located on the inner membrane of the mitochondrion.
Overall Function:
Electrons are transferred sequentially through the protein complexes.
Oxygen (O2) serves as the final electron acceptor, leading to the formation of water (H2O).
Electron Carriers (Reducing Equivalents):
Electrons are carried by NADH + H^+ and FADH_2, which are often referred to as reducing equivalents.
NADH:
Can be free-floating in the mitochondrial matrix and cytoplasm.
When NAD^+ is reduced to NADH, NADH diffuses to the ETC.
FADH2:
FAD binds tightly with enzymes (e.g., Succinate Dehydrogenase) in the mitochondrial inner membrane.
FAD is reduced to FADH_2, which is immediately available to the ETC at a different entry point than NADH.
Chemiosmosis and ATP Production
Mechanism: Chemiosmosis is the coupling of the proton-motive force with ATP synthesis.
Energy Release & Proton Pumping:
NADH and FADH_2 release energy when they are oxidized (lose electrons) in the ETC.
This released energy is used to pump H^+ (protons) from the mitochondrial matrix into the intermembrane space.
NADH contributes to pumping approximately 10 protons.
FADH_2 contributes to pumping approximately 6 protons.
Protons are pumped against their electrical and concentration gradients.
ATP Synthase:
The accumulated H^+ in the intermembrane space creates a proton-motive force.
These protons flow down their gradient (via facilitated diffusion) through the ATP Synthase enzyme complex, back into the mitochondrial matrix.
The potential energy from this proton-motive force is then harnessed by ATP Synthase to synthesize ATP from ADP and P_i.
ATP Synthase Structure:
The F_0 subunit is embedded in the inner membrane and consists of a disc of 10 'c' subunits, plus 'a' and 'b' subunits.
The F1 subunit extends into the matrix and contains a 'g' subunit and three 'a' and three 'b' subunits, which are directly involved in the ADP + Pi \rightarrow ATP reaction.
Protons Required per ATP:
Approximately 4\,H^+ are required to produce one ATP molecule (incorporating 3\,H^+ for ATP synthesis by ATP synthase and 1\,H^+ for the coupled transport of ADP and P_i into the mitochondria).
Based on this:
NADH yield: (10\,H^+ pumped) / (4\,H^+ per ATP) = 2.5\,ATP/NADH.
FADH2 yield: (6\,H^+ pumped) / (4\,H^+ per ATP) = 1.5\,ATP/FADH2.
Getting ATP Out of Mitochondria
Synthesized ATP is rapidly transported out of the mitochondrial matrix as it is made.
ADP (a substrate for ATP synthesis) must simultaneously enter the mitochondrial matrix.
The inner mitochondrial membrane is largely impermeable to these molecules.
ATP-ADP Translocase (Antiport): This transporter allows ATP out of and ADP into the mitochondria (in an antiport fashion).
Phosphate Transporter: This transporter allows inorganic phosphate (P_i) to enter the matrix.
Summary: 1\,H^+ is consumed for the overall transfer of ADP and P_i into, and ATP out of, the matrix.
ETC Complexes and Electron Flow (Detailed)
NADH + H^+ and FADH_2 produced in the Krebs cycle are shuttled to the ETC.
Energy is gradually released from the electrons, which is used for:
Heat generation for the body.
ATP synthesis.
Formation of metabolic water.
Complex I (NADH Dehydrogenase):
All NADH + H^+ passes its hydrogens and electrons to Complex I.
NADH is oxidized (loses electrons) to NAD^+ (which recycles).
Contains the cofactor FMN (flavin mononucleotide).
FMN passes electrons through a series of Fe-S (iron-sulfur) complexes.
Role: To transfer two electrons and protons from NADH (in the matrix) to Coenzyme Q (ubiquinone, in the inner membrane).
Energy is released, and 2\,H^+ are pumped across the membrane.
Coenzyme Q (Ubiquinone):
Acts as a mobile electron carrier, accepting electrons from Complex I (from NADH) and Complex II (from FADH2).
Passes electrons one at a time.
Enough energy is utilized as electrons pass through Coenzyme Q to contribute to 1 ATP equivalent (via proton pumping).
Complex II (Succinate Dehydrogenase - SDHase):
FAD is a tightly bound cofactor to SDHase.
SDHase accepts electrons from succinate (from the Krebs cycle), reducing FAD to FADH_2. The electrons are then transferred to Coenzyme Q.
This step does not pump protons across the inner membrane, meaning FADH_2 contributes to fewer ATPs than NADH.
Complex III (Cytochrome c Reductase):
Receives electrons from Coenzyme Q.
Transfers electrons to Cytochrome c, involving intermediate cytochromes b and c_1.
As electrons move, they release energy, contributing to proton pumping (resulting in 1 ATP equivalent for NADH and FADH2).
Cytochromes:
The ETC involves several cytochromes: b, c, aa_3.
Each cytochrome contains an iron (Fe) heme prosthetic group.
Iron exists in two ionization states: Fe^{3+} ("ferric", oxidized) and Fe^{2+} ("ferrous", reduced).
Iron in cytochromes accepts and transfers electrons by undergoing oxidation (Fe^{2+} \rightarrow Fe^{3+})) and reduction (Fe^{3+} \rightarrow Fe^{2+})).
Cytochromes start in the ferric state (+3) and become ferrous (+2) upon accepting an electron.
Complex IV (Cytochrome c Oxidase):
Receives electrons from Cytochrome c (via Cytochrome a and a_3).
Contains 2 copper ions.
Electrons are passed to oxygen (O2), which is the final electron acceptor, forming water (H2O).
Electron movement through Complex IV also releases energy, contributing to proton pumping (resulting in 1 ATP equivalent for NADH).
ATP Yield Summary (from Reducing Equivalents to ETC):
Every NADH + H^+ (entering at Complex I) yields 3 ATP (old convention) or 2.5 ATP (more recent convention).
FADH_2 (entering at Coenzyme Q, bypassing Complex I) yields 2 ATP (old convention) or 1.5 ATP (more recent convention).
Regulation of ATP Generation (Krebs Cycle & ETC Link)
Tight Linkage: The Krebs cycle generates NADH and FADH_2 for the ETC. However, the ETC must be actively accepting these hydrogens to regenerate NAD^+ and FAD, which are essential for the Krebs cycle to continue.
This entire process is dependent on the availability of oxygen as the final electron acceptor in the ETC.
ATP/ADP Ratio: A high ATP/ADP ratio indicates abundant energy.
With less ADP in the mitochondrial matrix, ATP synthase has reduced substrate, which slows down ATP synthesis, and consequently the ETC and Krebs cycle.
Negative Effectors of Krebs Cycle (related to ETC products):
A "surplus" of NADH or acetyl CoA inhibits Pyruvate Dehydrogenase (PDHase), reducing the supply of acetyl CoA to the TCA cycle.
"Surplus" NADH allosterically inhibits citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase within the TCA cycle.
Positive Effectors of Krebs Cycle (related to energy demand):
Ca^{2+} ultimately (and powerfully) activates PDHase, which increases the production of acetyl CoA.
Ca^{2+} also stimulates isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase of the TCA cycle, increasing flux when calcium levels rise (e.g., during muscle contraction, indicating a need for more ATP).