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Energy Systems

Objectives• Bioenergetics ~ energy transfer

a) Within the bodyb) During exercise

• ATP regeneration

• Sources of energy during exercise:a) Anaerobicb) Aerobic

Bioenergetics

Energy Transfer• Direct transfer of chemical energy required for

all forms of biological worka) Energy = the capacity for work

Dynamic state related to changeAs work load increases, energy transfer increases

• Bioenergetics & Thermodynamicsa) Process of converting food stuffs into “harnessed”

energy

1st Law of Thermodynamics• Energy is neither created nor destroyed, but is

transformed from one form to anothera) As the body undergoes transformations, energy is

changed from one form to anotherConservation of energy

Energy in food Heat

ATP

Mechanical Chemical

Potential & Kinetic EnergyTotal (heat) energy of a system includes both:

1. Potential energy• Bound in a specific form

2. Kinetic energy• Harnessing of potential energy (energy of motion)• Biosynthesis results from harnessing energy

Individual atoms are joined to synthesize biologic compounds

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Energy Release & Conservation• Exergonic reactions (-∆G)

a) Physical or chemical release of energy‘Gibbs free energy’

b) Measurement of ‘free energy’

G = H - TS

G = free energy

H = Enthalpy (potential energy)

T = Absolute temperature (ºC + 273)

S = Unavailable energy due to randomness

Energy Release & Conservation• Endergonic reactions

a) Chemical processes that store or absorb energy

• Processes may be coupleda) Exergonic + Endergonic

∆G = ∆ H - T ∆ S

~ -∆G 68kcal/molH2 + O H2O

H2 + O H2Oex:

~ +∆G 68kcal/mol

Examples of ∆G for Important Molecules

-10.3 ∆GCreatine Phosphate

-7.3 ∆GATP

-5.0 ∆GGlucose-1-phosphate

-3.3 ∆GGlucose-6-phosphate

2nd Law of Thermodynamics• All reactions proceed in the direction of:

a) Increased entropyb) The release of free energy

• The more –∆G, the greater the release of free energy

• Direction & amount of free energy may be modified by alteringa) Substrate concentrationb) Product concentration

Rate of Bioenergetics• Enzymes function as:

a) Biological Catalysts – speed up chemical reactions without being involved in the reaction or altering the free energy release

b) Couplers – provide means to couple reactions

c) Regulators of metabolism

• Factors influencing enzyme function:a) pHb) Temperaturec) Availability of substrate and enzyme

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Figure 5.7Major affects on enzyme activity

Enzymes (cont.)

• Mode of actiona) Lock and key mechanism (active site)b) Enzyme-substrate complex

After entering active site catalyzes reaction & end product produced

• Enzymes increase or decrease the likelihood of a reaction occurringa) Allosteric modification – can be activated or

inhibitedPFK (glycolysis) & Phosphorylase (glycogenolysis)

Figure 5.8

Alterations in enzyme concentration Coenzymes• Complex, non-protein organic substance

a) Ironb) Zincc) B vitamins

• Require less specificity than enzymesa) Affects various numbers of reactionsb) May serve as a temporary carrier

Nicotinamide adenine dinucleotide (NAD+)Flavin adenine dinucleotide (FAD)

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Hydrolysis & Condensation Rx1. Hydrolysis reactions

• Catabolizes complex organic molecules (CHO, lipids, proteins)

Specific enzymes for each molecule

• Splits chemical bonds by adding H+ and OH-

Digestive enzymes

2. Condensation reactions• Synthesis of molecules or anabolic process• Reverse of hydrolysis

Peptide bondsFigure 5.9

Figure 5.9

Electrons, protons & oxidation-reduction reactions

• Electronsa) Negatively charged subatomic particles circulating

around the atom nucleusb) Essential for atoms to form covalent (sharing) bondsc) During many chemical reactions

Electrons are either removed or added to molecules

• Molecules that lose 1 or more electrons are oxidized, whereas molecules that gain electrons are reduced

• Oxidation involves the loss of electrons, & reduction involves the gaining of electrons

• Reactions occur together and often termed oxidation-reduction or redox reactions

A:e + B A + B:e

Pyruvate + NADH + H+ lactate + NAD+example:

Q – which molecule was reduced & which oxidized in the direction of lactate production?

(Reduced) (Oxidized)

Figure 5.11

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Bioenergetics Review• Law’s of Thermodynamics

• Exergonic & endergonic rxa) Potential & kinetic energy

• Enzymes & coenzymes

• Hydrolysis, condensation & redox reactions

Energy Transfer

Food Energy Figure 6.2

Adenosine Triphosphate (ATP)

ATP + H2O ADP + PiATPase

∆G = -7.3kcal/mol

ATP – Energy Currency of the Cell• Design & function of skeletal muscle metabolism

is to meet rapidly meet the ATP demandex: Muscle contraction can ↑ cellular ATP demand by

100 foldCould deplete resting [ATP] in as little as 2 – 3 seconds of intense exercise

• Skeletal muscle has sensitive biochemical controls of metabolic pathways involving the sudden activation and inhibition of specific enzymes

ATP Regeneration• Skeletal muscle can produce the necessary ATP

for muscle contraction from 1 or a combination of three metabolic reactions/pathways:

1. Phosphagen System

2. Glycolysis

3. Mitochondrial Respiration

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Anaerobic vs. Aerobic

Immediate Sources of ATP

(Anaerobic)

1. Phosphocreatine• Most immediate means for ATP regeneration

Requires only 1 enzyme, creatine kinase (CK)

• Actually 2 “coupled” reactions

• Responsive to immediate changes in ADP & ATP concentrations

Energy Reservoirs

CrP + ADP + H+ ATP + CrCK

CrP Cr + Piexergonic

ADP + H+ + Pi ATPendergonic

Energy Reservoirs (cont.)

2. Andenylate Kinase• Reforms ATP using two ADP molecules

Results in ATP and AMPAdenylate kinase drives this reaction

• AMP acts as an important regulatorActivator of the allosteric enzymes phosphorylase(glycogenolysis) and phosphofructokinase (PFK)

ADP + ADP ATP + AMPAK

Important By-products of the Phosphogen Systems

• PCr hydrolysis and adenylate kinase reaction generate:a) Pi

b) AMPc) ADP

• By-products stimulate: a) Glycogenolysisb) Glycolysisc) Respiratory pathways of mitochondria

By-products

Exercise Specifics• ATP – PCr system

a) Provides ATP for short-term, high-intensity movements & is rapidly depleted

What type of exercises What type of exercises include these systems?include these systems?

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Glycogenolysis & Glycolysis

Glycogen & Glycogen Stores1. Liver

• Reservoir for blood glucose & brain

2. Muscle• Does not release to blood (only for muscle

metabolism)

Biochemically efficient – no net ATP costGlucose requires one ‘extra’ ATP

Glycogenolysis• Catabolism of glycogen requires

a) Removal of glucose units from glycogenb) Addition of Pi

End product – glucose-6-phosphate

• Pi & Ca2+ major regulators of glycogenolysisa) Epinephrine also ↑

• Major enzyme required is phosphorylase

Glycolysis• Glucose (glucose-6-phosphate) → pyruvate

• Rapid rate of net 2 ATP

• Produces 2 NADH + 2 H+

• Provides substrate for other pathwaysa) Pyruvateb) Lactate

• Transported into the cell by GLUT proteins

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Via glycerol-phosphate shuttle &malate-aspartateshuttle

Via glycerol-phosphate shuttle &malate-aspartateshuttle

• Pyruvate reduction to lactate:

a) Lactate dehydrogenase (LDH)

b) Depends on availability of NADHNADH/NAD+ (redox potential)

c) Reduction of pyruvate to lactate helps to buffer the solution

‘absorbing’ excess protons (H+)

Pyruvate + NADH + H+ lactate + NAD+LDH

Figure 6.12

Sport Application • Glycolysis resulting in lactate formation (lactic acid lactic acid

systemsystem)

• Lactate accumulationa) Blood Lactate Threshold (LT)

Lactate production exceeds clearanceAverage for untrained = ~ 55% max aerobic capacity (Davis JA et al., JAP 1979)

Training increases lactate threshold

Figure 7.2

Sport Application (cont.)

• Lactate-producing capacity increases with anaerobic traininga) Increased intramuscular glycogen stores

Allows for increased glycolysis

b) Increased glycolytic enzymesTraining increases ~ 20%

c) Increased ability to recruit Type IIb fibers

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Lactate Is Not a Waste Product!1. Lactate shuttle

• Converted to pyruvate and oxidized as an energy source in another cell

2. Gluconeogenesis • Converted back to glucose in the liver in Cori Cycle

Figure 6.13

Sources of ATP (Aerobic & Steady-State)

Pyruvate Dehydrogenase Complex• Pyruvate entry into mitochondria is converted to

acetyl CoA by a series of linked enzymes known collectively as pyruvate dehydrogenase

• Glycolysis to TCA cycle

• Irreversible step

Pyruvate + NAD+ + CoA Acetyl CoA + NADH + H+ + CO2

Pyruvate dehydrogenase complex

acetyl CoA → TCA cycle• Combined products of the TCA cycle from acetyl

CoA are:a) 2 CO2

b) 1 ATPc) 3 NADH + H+

d) 1 FADH2

• All CO2 produced in energy metabolism are accounted for from pyruvate dehydrogenase reaction and 2 reactions of the TCA cycle

Support ATP regeneration during oxidative phosphorylation

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Figure 6.15

TCA key points• TCA cycle is ‘ultimate furnace’

a) Amino acidsb) Glucosec) Fatty acidsd) Ketones

• Main electron transferring pathway (redoxreactions – energy conserved in NADH & FADH)

acetyl CoA

TCA key points (cont.)

• Each ‘turn of cycle’: one acetyl-CoA enters and 2 CO2 leave

• Synthesis of citrate which is an inhibitor for glycolysis at PFKa) Mainly during rest and post exercise recovery

• Oxaloacetate is regenerated with each turn

Electron Transport Chain (ETC)• Involves the biochemical use of O2 to regenerate

ATP in mitochondria

• ETCa) Series of electron receivers located along the inner

mitochondrial membrane that sequentially receive and transfer electrons to the final electron receiver –oxygen

• Consumption of oxygen, and formation of water and ATP during the ETC is termed oxidative phosphorylation

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Summary Summary (cont.) – Regulation of Energy Metabolism

• Overall energy state dictates the direction of metabolic pathways

• Rate-limiting modulators:1. ATP2. ADP3. cAMP4. NAD5. Calcium6. pH

• It is the relative concentrations that are important: NADH/NAD+ & ATP/ADP

Application & Measurement of Energy

Metabolism

Sport Application & ConceptsOxygen Consumption During Exercise:

• Oxygen consumption (VO2)a) Pulmonary oxygen uptakeb) Oxygen is measured at lung, not tissues (debatable)c) Steady-state

When oxygen demand is met by oxygen deliveryBlood lactate doesn’t accumulateExercise may continue at this rate until limitations other than oxygen alter performance

Sport Application & Concepts (cont.)

Oxygen Deficit:

• As exercise begins:a) Oxygen demand increases immediatelyb) Oxygen consumption lags behind

• Oxygen deficita) Quantitative expression of difference between

oxygen consumed and the amount that would have been consumed had steady state been reached right from the start.

Figure 7.3

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Sport Application & Concepts (cont.)

Oxygen Deficit (cont.):

• Trained Individualsa)Reach steady-state more rapidlyb)Have a smaller oxygen deficit

More rapid increase in cardiac outputLarger percentage of blood directed to active muscleTraining induced cellular adaptations

1. Increased capillary density2. Increased number mitochondria3. Increased oxidative enzymes

Sport Application & Concepts (cont.)

Maximal Oxygen Consumption (VO2max):

• Maximal volume of oxygen one can consumeMaximal oxygen uptakeMaximal aerobic powerAerobic capacity

• Provides a quantitative measure of capacity for aerobic ATP resynthesis

Figure 7.5

Muscle Fiber Types• Fast- and slow-twitch muscle fibers

a) Slow-twitch = Type IHighest aerobic capacityLowest glycolytic capabilities

b) Fast-twitch = Type IIType IIa: Medium glycolytic and aerobic capabilitiesType IIb: Highest glycolytic capacity & lowest aerobic capacity

Type II Type I

O2 Consumption During Recovery• Metabolic dynamics of recovery oxygen

consumption

• Excess postexercise oxygen consumption (EPOC):a) Oxygen cost of adjustments in:

VentilationCirculating hormonesBlood circulationTemperatureO2 reloading with muscle

Speed of recovery depends on recovery mode

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Energy from Fat

Energy Release From Fat• Adipocytes

a) Site of fat storage and mobilizationb) Fat is stored primarily as triglycerides

• Mobilizationa) First step in utilizing fatty acids is Lipolysisb) Triglycerides are split into:

Fatty acidsGlycerol

c) Hormone Sensitive Lipase (HSL) drives lipolysis

Transport and Uptake of Fatty Acids• Fatty acids from Lipolysis are FFA

a) Bound to Albumin for transport in plasma

• FFA are taken up by muscle cellsa) FFA are activated to fatty Acyl CoAb) Acyl CoA binds to Carnitine for transport into

mitochondriac) Carnitine Acyltransferase drives this reaction

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Fatty Acids From Lipoproteins• Lipoproteins also transport triglycerides

• Lipoprotein Lipase (LPL) catalyzes hydrolysis of these triglycerides

• LPL is located on surface of surrounding capillaries

Circulating triglycerides

Circulating triglycerides

Oxidation of fat w/in muscle• Beta Oxidation

a) Cleaves two-carbon compounds from fatty AcylCoA molecule

b) Two-carbon acetyl groups enter Citric Acid Cyclec) Oxidation produces NADH

• Fate of Glycerola) Conversion to Pyruvate via glycolytic actionb) Gluconeogenesis

Converted to Glucose in Liver

Hormonal Effects• Lipolysis is stimulated by:

a) Epinephrineb) Norepinephrinec) Glucagond) Growth Hormone

• Intracellular mediatora) cAMP activates hormone sensitive lipase

Energy from Protein

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Branched-chain Amino Acids• AA play contributory role as energy substrates

during endurance and heavy resistance exercises

• Requires removal of nitrogen from AAa) Liver – deaminationb) Muscle – transamination

• Depend on enzyme concentration (training)

Figure 1.25