Lecture 9.1 and 9.2: Muscle Energy Metabolism Flashcards
Energy Transfer
Moving System Toward vs. Away From Equilibrium
- move system toward equilibrium: energy is produced
- move system away from equilibrium: energy is required (work)
- low thermodynamic stability in products and reactants
- high thermodynamic stability (lower energy) at equilibrium
Steady State Conditions
A → B
- there is a way to replenish ‘A’ at the same rate it is converted to ‘B’
- there is a way to remove ‘B’ at the same rate it is produced from ‘A’
In living cells, do reactions ever ‘finish’?
no – kept from reaching equilibrium
- reactions are always at some point away from equilibrium, but moving towards equilibrium
- if equilibrium is reached, cell dies
ATP Hydrolysis
ATP + H2O → ADP + Pi + H+
- amount of energy is proportional to the difference in thermodynamic stability of all reactants and all products
- hydrolysis products are more strongly bonded than reactants
ATP Hydrolysis
What is resonance stabilization?
there is greater resonance stabilization in products than reactants
ATP Hydrolysis
What are charge repulsion effects?
there is more ‘bond weakening’ by charge effects in reactants than in products
What are the 2 mechanisms through which ATP is generated?
- substrate level phosphorylation (SLP)
- oxidative phosphorylation (Ox Phos)
What are the 3 main substrates used for substrate level phosphorylation (SLP)?
- creatine phosphate
- glucose/glycogen
- lipid
How is ATP generated via SLP with creatine phosphate?
ATP generated via removal of phosphate, which can then be used in muscle contraction
How is ATP generated via SLP with glucose/glycogen?
ATP generated via oxidation
How is ATP generated via SLP with lipids?
ATP generated via oxidation
Describe the ATP yield for glucose oxidation.
glucose is converted into 2 pyruvate via glycolysis in cell cytosol
- produces 2 ATP
- substrate level phosphorylation
pyruvate is converted into 2 acetyl-CoA in mitochondrial matrix
- acetyl-CoA enters TCA cycle and produces 2 ATP
- substrate level phosphorylation (pyruvate is phosphorylated)
other products of glycolysis are used in electron transport in mitochondrial inner membrane
- produces various amounts of ATP
- oxidative phosphorylation
What is the actual vs. theoretical yield for glucose oxidation? Why?
actual ATP yield (32) is lower than the theoretical yield (36) due to involvement of H+ in pyruvate and ADP transport
Amount of Available Substrate in White Muscle (Fast Contraction)
creatine phosphate > glycogen > lipid > ATP > glucose
- ATP: muscle [ATP] = < 5 mmol/kg
- creatine phosphate: muscle [CrP] = around 40 to 50 mmol/kg
- glucose/glycogen: muscle glucose = 2 mmol/kg, muscle glycogen = 25 to 30 mmol/kg
- lipid: muscle triacylglycerol = 20 mmol/kg
Where are the sources of the main substrates that support ATP production?
muscles have stored sources for ATP production, or sources get transported through blood to muscle tissue
Source of Main Substrates that Support ATP Production
Where are carbohydrate sources?
- glucose from muscle stores of glycogen – main store
- glucose from blood
Source of Main Substrates that Support ATP Production
Where are lipid sources?
- fatty acids are stored in muscle and adipose tissues as triacylglycerol
- fatty acids from lipoproteins from the blood
What does mitochondrial beta-oxidation do?
breaks fatty acids into 2 carbon acetyl-CoA, which enter TCA and produce ATP
What substrate does glycolysis use?
only carbohydrates
What substrate does oxidative phosphorylation use?
can oxidize carbohydrates or lipids (or amino acids)
Carbohydrates vs. Lipids
Which substrate can be more rapidly mobilized?
glycogen
Carbohydrates vs. Lipids
Which substrate is a more dense source of energy?
lipids
- cell can produce approximately 10x more ATP from triglyceride droplets than from the same volume of glycogen granules
Carbohydrates vs. Lipids
Which substrate is associated with mitochondria?
- triglyceride droplets are always associated with mitochondria
- only some glycogen granules are associated with mitochondria, while others are dispersed within myofibrils
Carbohydrate Pathway: How are carbohydrates converted into ATP?
- glycogen is broken down into glucose 1-P by glycogen phosphorylase
- glucose 1-P is converted into pyruvate via glycolysis and yields ATP
- pyruvate is transported into mitochondria
- pyruvate dehydrogenase converts pyruvate into acetyl-CoA
- pyruvate may also sometimes be converted into lactate
- acetyl-CoA enters TCA cycle, which yields electrons that are used in oxidative phosphorylation to make ATP that is used in muscle contraction
Phosphocreatine Pathway: How is phosphocreatine converted into ATP?
- converted into creatine
- generates ATP molecule that can be used for muscle contraction
Lipid Pathway: How are lipids converted into ATP?
- fatty acids can be transported to mitochondria with the help of carnitine
- beta-oxidation produces acetyl-CoA
- acetyl-CoA enters TCA cycle, which yields electrons that are used in oxidative phosphorylation to make ATP that is used in muscle contraction
Which substrate pathway is used during physical activity?
all three pathways are often used in combination to meet the energy demands of varying exercise intensities and durations
- choice of which pathway predominates depends on the specific requirements of the exercise and the availability of substrates and oxygen
What are the sources of acetyl-CoA production during muscle metabolism?
carbohydrates, fatty acids, and amino acids
- amino acids can be converted into pyruvate and then acetyl-CoA, OR can be converted directly into acetyl-CoA and be used as a source during muscle metabolism – depends on type and duration of exercise, etc. which determines which pathway is used
What are the advantages of phosphocreatine hydrolysis to replenish ATP during muscle activity?
- speed – provides rapid energy for short bursts of high-intensity exercise
- anaerobic – no oxygen requirement
What are the disadvantages of using glycolysis to replenish ATP during muscle activity?
- inefficient ATP production
- lactic acid production/muscle soreness
What are the advantages of using oxidative phosphorylation to replenish ATP during muscle activity?
- efficiency in ATP production/extended period of time/no lactic acid production
What are the 3 main categories of enzymes?
- near equilibrium
- allosteric
- flux generating
What are near equilibrium enzymes?
- obeys Michaelis-Menten kinetics
- operates a near equilibrium
- high muscle enzyme concentration and kcat
- small changes in substrate concentration ( [P]/[S] ) affects flux through enzyme
- ie. aldolase
What are allosteric enzymes?
- classic sigmoidal shape
- operates far from equilibrium
- lower enzyme concentration in muscle tissue
- binding of +/- modulators to enzyme at sites other than active sites affect enzyme function – affects relationship between [S] and VO, shifts sigmoidal curve
- ie. phosphofructokinase, hexokinase
When are hexokinase and phosphofructokinase allosterically regulated?
in the preparatory phase of glycolysis
What does phosphofructokinase do?
catalyzes phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate (in glycolysis)
What are the positive and negative regulators of phosphofructokinase?
- positive regulators: binding AMP and ADP, fructose 2,6-bisphosphate when cell is in low energy level
- negative regulators: ATP, citrate
What does hexokinase do?
catalyzes phosphorylation of glucose by ATP to glucose-6-P (in glycolysis)
What are flux generating enzymes?
- operates in an ‘on/off’ type fashion
- operates far from equilibrium
- very low enzyme concentration
- post-translational modification (ie. phosphorylation/dephosphorylation) transitions enzyme from on and off state
- ie. pyruvate dehydrogenase
What are the two main steps in ATP pathways that are controlled by flux generating enzymes?
- glycogen breakdown/synthesis
- pyruvate to acetyl-CoA
What does glycogen synthase do?
What is it stimulated and inhibited by?
converts glucose 1-P into glycogen
- stimulated by dephosphorylation – insulin activates protein phosphatase, which dephosphorylates glycogen synthase and stimulates glycogen synthesis
- inhibited by phosphorylation – glucagon and epinephrine increase the cAMP levels, which activates protein kinase A, which phosphorylates glycogen synthase and inhibits glycogen synthesis
What does glycogen phosphorylase do?
What is it stimulated and inhibited by?
converts glycogen into glucose 1-P
- stimulated by glucagon and epinephrine
- inhibited by insulin
What does pyruvate dehydrogenase do?
forms acetyl-CoA from pyruvate
What regulates pyruvate dehydrogenase?
two other enzymes
- PDH phosphatase: removes phosphate, results in high PDH enzyme activity
- PDH kinase: adds phosphate, results in low PDH enzyme activity
What is the main factor that causes activation of pyruvate dehydrogenase upon initiation of muscle contraction?
Ca2+
- Ca2+ is released from SR, high concentration in muscle cells
- Ca2+ activates PDH phosphatase
High-Intensity Exercise – Rainbow Trout
What muscles power intense exercise in rainbow trout?
white, type IIb muscle (fast twitch, fast contraction)
High-Intensity Exercise – Rainbow Trout
What does very intense exercise result in?
- decreases in white muscle [ATP], [creatine phosphate], and glycogen
- increases in lactate to high levels, and small increases in other glycolytic intermediates (pyruvate)
High-Intensity Exercise – Rainbow Trout
What is their exercise fuel?
- first 10 seconds of intense exercise is fueled around 50:50 with CrP hydrolysis and glycolysis
- glycolysis contributes to ATP production throughout the bout of intense exercise
- oxidative phosphorylation also contributes to a small degree, and peaks later
High-Intensity Exercise – Rainbow Trout
What is glycogen phosphorylase (PHOSa)?
enzyme that breaks down glycogen to glucose 1-P
- activated very quickly during intense exercise
High-Intensity Exercise – Rainbow Trout
What is pyruvate dehydrogenase (PDHa)?
enzyme that converts pyruvate to acetyl-CoA in mitochondria
- slow to activate during intense exercise
High-Intensity Exercise – Rainbow Trout
There is a difference in rate/regulation between glycogen phosphorylase (PHOSa) and pyruvate dehydrogenase (PDHa). What does this cause?
results in lactate accumulation in white muscle
- PDH is slow to activate during intense exercise
- mitochondrial content, and therefore PDH capacity, are low in white muscle
- white muscle (~2% mitochondria)
- red muscle (~20% mitochondria)
High-Intensity Exercise – Rainbow Trout
Is white muscle limited in O2 during intense ‘so-called anaerobic’ exercise?
- mitochondria from white muscle becomes progressively oxidized (higher NAD+) over the course of exhaustive exercise
- NADH can only be oxidized by the mitochondria via functioning electron transfer system, with the terminal electron acceptor O2
O2 is available for mitochondria during intense exercise
- indicated by the fact that NAD+/NADH ratio is increasing over time consistently – more NAD means there is no limit of oxygen
- limitation of oxygen is NOT an issue
High-Intensity Exercise – Fish vs. Humans
In both trout and humans, what is high-intensity exercise fueled by?
- initially by PCr hydrolysis and glycolysis (yielding lactate)
- glycolysis supports ATP production throughout
- contributions from oxidative phosphorylation increase over time
High-Intensity Exercise – Rainbow Trout
What causes lactate accumulation in white muscle during intense exercise?
due to metabolic inertia
- PHOS is fast to activate allowing glucose to enter glycolysis
- PDH is slow to activate, but does fully activate and supply acetyl-CoA to an oxygenated mitochondria
- mitochondrial content is low in white muscle
- the only option to allow continued functioning of glycolysis is for lactate to be produced
High-Intensity Exercise – Fish vs. Humans
What generates ATP the fastest?
PCr can generate ATP the fastest at nearly 2.5 mmol/kg/s
- want this in fast twitch muscle
High-Intensity Exercise – Fish vs. Humans
What generates ATP the slowest?
fat oxidation generates ATP the slowest
- explains why fat is not really a source of ATP here
High-Intensity Exercise
How does cellular pH change?
white muscle becomes acidotic during intense exercise – many studies have demonstrated an association between lactate accumulation and acidosis
High-Intensity Exercise
What might be the cause of acidosis in muscle?
lactic acid is produced when lactate reacts with H+, which is very acidic – can explain acidosis in muscle
source of H+:
- X generation of ATP via creatine phosphate consumes H+
- X glycolysis produces H+, but they are balanced throughout the reaction (not available)
- ✓ ATP hydrolysis
High-Intensity Exercise
How is ATP hydrolysis the source of H+ contributing to acidosis in muscle?
- ATP hydrolysis (which supports muscle during high-intensity exercise) is the main source of metabolic acid
- during high-intensity exercise, rate of ATP hydrolysis to support cellular work exceeds the capacity of mitochondrial oxidative phosphorylation
- produced much faster than can be used
- ADP and Pi from ATP hydrolysis are recycled during ATP production via glycolysis but the accumulation of H+ is not, resulting in acidosis
High-Intensity Exercise
Why doesn’t the cell become acidotic during low-intensity exercise?
because ATP is hydrolyzed at appreciable rates during ‘aerobic’ exercise
- no accumulation of H+ or Pi
Recovery from High-Intensity Exercise
What occurs during recovery?
- replenish energy stores – ATP, phosphocreatine, glycogen
- re-establish ion gradients, Ca2+ stores, and pH
- these aspects of recovery are achieved using ATP produced by oxidative phosphorylation
Recovery from High-Intensity Exercise
Describe how the rate of oxygen consumption changes during and after exercise.
- exercise causes a rapid increase in the rate of oxygen consumption
- once exercise stops, respiration declines but remains elevated above the resting rate for extended periods
- duration of this elevated post-exercise oxygen consumption (oxygen debt) depends on intensity of exercise, and varies among species
Recovery from High-Intensity Exercise – Rainbow Trout
What happens during recovery in white muscle?
- creatine phosphate (CrP) recovers very quickly
- ATP takes 2-4 hrs to recover
- glycogen is very slow to recover
- lactate recovers between 2-4 hrs
Recovery from High-Intensity Exercise – Rainbow Trout
What are the two possible options of where ATP comes from to support recovery from intense exercise?
- X lactate → pyruvate → oxidative phosphorylation
- ✓ lipids
Recovery from High-Intensity Exercise – Rainbow Trout
Explain why ATP does NOT come from lactate → pyruvate → oxidative to support recovery.
- immediately upon completing high-intensity exercise, glycogen synthase is 80% activated and remains activated for 8 hrs post-exercise
- pyruvate dehydrogenase is high immediately post-exercise, but is inactivated within 15 mins of recovery
- intramuscular lactate is converted into pyruvate and shuttled toward glycogen synthesis by inactivation of PDH – this means that the source of ATP cannot come from lactate
Recovery from High-Intensity Exercise – Rainbow Trout
Explain why ATP comes from lipids to support recovery.
- during recovery there is an increase in white muscle acetyl-CoA
- rapid inactivation of white muscle PDH during recovery shuttles lactate toward glycogen synthesis – lactate is therefore not oxidized during recovery to generate ATP, meaning that the source of acetyl-CoA cannot be lactate or pyruvate
immediately post-exercise, the increase in white muscle acetyl-CoA comes from digestion of lipids and fatty acids
- free carnitine in the white muscle decreases during the early part of recovery, suggesting that they may be used for fatty acid transport
- long-chain fatty acyl carnitine (and acetyl-carnitine) accumulate, suggesting higher fat transport into the mitochondria during recovery
- could explain the level of acetyl-CoA and PDH being inactivated
Recovery from High-Intensity Exercise – Rainbow Trout
How is lactate removed?
- some muscles can convert lactate to glucose to replenish glycogen stores
- other muscles release lactate into the blood which transports it to the liver or oxidative tissues such as the heart
- glucose taken up from the blood is used to replenish muscle glycogen stores
Recovery from High-Intensity Exercise – Rainbow Trout
How does the liver remove lactate?
converts lactate to glucose (gluconeogenesis) – some of this glucose can be shuttled back towards to Cori cycle for further generation of glycogen
Recovery from High-Intensity Exercise – Rainbow Trout
How do oxidative tissues (such as the heart) remove lactate?
oxidize lactate to produce ATP
What is graded exercise?
exercise that starts slowly and gradually increases over time
Graded Exercise – Fish Treadmill (Swim Tunnel)
Describe speed.
- Ucrit = maximal ‘sustained’ swimming speed
- speed increases over time, but there are periods of fatigue in between each increase in speed
- electric potential being generated in muscle tissue for both red (sigmoidal) and white (exponential) muscle increases as Ucrit increases
Graded Exercise – Fish Treadmill (Swim Tunnel)
At what speeds could swimming be sustained for long periods of time? How?
swimming under control conditions and at 30 and 60% Ucrit are sustainable for long periods of time
- supported by O2 based metabolism – oxygen consumption increased over time
Graded Exercise – Fish Treadmill (Swim Tunnel)
At what speeds were fish unable to sustain swimming for long periods of time?
swimming at 90% Ucrit only lasted for between 50 and 80 min, and then they were exhausted
Graded Exercise – Fish Treadmill (Swim Tunnel)
What can ‘aerobic’ ATP turnover be calculated from?
can be calculated from O2 update rate
- O2 consumption increases with increase in Ucrit
- ADP:O2 ratio of 6
Graded Exercise – Fish Treadmill (Swim Tunnel)
Describe white muscle changes.
- good for short bursts of fast swimming
- at 90% Ucrit, white muscle ATP, CrP, and glycogen are depleted, and lactate accumulates
- very limited changes in white muscle metabolites during swimming at 30 and 60% Ucrit
Graded Exercise – Fish Treadmill (Swim Tunnel)
Describe red muscle changes.
- red muscle [ATP] does not change during submaximal exercise
- CrP is depleted in red muscle at 90% Ucrit initially, but stabilized during swimming
- remember steady state functioning of metabolism: lack of change in [ATP] and [CrP] during ongoing swimming does not mean there is no change in flux through pathways associated with ATP and CrP production and consumption – being used at the same rate as it is being replenished
- red muscle glycogen depleted in fish at 90% Ucrit, with lactate accumulation
- almost no changes in red muscle glycogen and lactate over 240 min swimming at 30 and 60% Ucrit
Graded Exercise – Fish Treadmill (Swim Tunnel)
Red Muscle – When is pyruvate dehydrogenase activated?
activated early at all swimming speeds
- within 15 min swimming at 30 and 60% Ucrit, PDH is inactivated
- PDH remains active throughout 45 min of swimming at 90% Ucrit
- explains glycogen is being used as a source, and is being shuttled towards acetyl-CoA and TCA cycle, especially at higher speeds
Graded Exercise – Fish Treadmill (Swim Tunnel)
Red Muscle – What accumulates in red muscle? What is the source?
acetyl groups accumulate in red muscle in trout swimming at 60 and 90% Ucrit
- amount of free carnitine is decreasing, while the amount of carnitine-conjugated fatty acids is increasing, suggesting that more lipids are being transported to the mitochondria, which could explain the increased level of acetyl-CoA
Graded Exercise – Fish Treadmill (Swim Tunnel)
Red Muscle – How can the beta-oxidation pathway be kept ‘on’ during swimming to provide acetyl-CoA?
malonyl-CoA: allosteric inhibitor of carnitine palmitoyl transferase (CPT1)
- concentration decreases in red muscle during swimming at all speeds
- if you inhibit and inhibitor, the pathway continues – CPT1 remains activated, therefore more lipids can be transported to the mitochondria via carnitine to be used during beta-oxidation to provide more acetyl-CoA
Graded Exercise – Fish Treadmill (Swim Tunnel)
What fuels red muscle?
- swimming at 30 and 60% Ucrit is initially fueled by carbohydrate oxidation (2 min), but then by lipids – (source of lipids (plasma fatty acid/muscle TAG) is unknown)
- as swimming speed increases (ie. 90% Ucrit), muscle glycogen contributes more substantially to support muscle contraction
- at different speeds, different fuels are functioning
- similar effects of running speed on human oxidation
Hummingbirds
What do hummingbirds require to support their metabolic rate?
high caloric intake
Hummingbirds
What is the source of food mainly used during hovering flight?
carbohydrates/sugar
- ingested sugar from nectar is used to support flight muscle metabolism
Hummingbirds
What is the source of food mainly used during rest periods?
lipids
What is a respiratory quotient (ratio)?
volume of CO2 released over the volume of O2 absorbed during respiration
- amount of O2 consumed by an animal compared with the amount of CO2 produced can tell us something about the fuels that are used
- amount of O2 used and CO2 produced is different when using carbohydrates vs. lipids
- around 0.7 = lipid
- around 1.0 = carbohydrate
