4 - Muscle Metabolism and the Exercise State Flashcards
How is muscular hexokinase regulated?
Inhibition by glucose 6-P.
How does the rate of glucose concentration change in the muscle during the fed state?
Greatly, as insulin and muscle contraction stimulate the GLUT4 transported to take up the extra glucose.
How does the rate of glycolysis change in the muscle during the fed state?
Only marginally, most of the glucose taken up is used in glycogenolysis.
How do resting myocytes in the fed state ensure they perform only enough glycolysis to support themselves?
Glucose 6-P product inhibition of hexokinase.
What is the key point of muscular glycolysis regulation?
The PFK-1/FBPase-1 complex.
How is muscular PFK-1 regulated?
+ Fructose 2,6-BisP
+ AMP
- ATP
- Citrate (CAC saturation)
Explain the effects of ATP/AMP and citrate on muscular PFK-1.
ATP lowers the affinity for the potent activator F26BP. AMP competes with ATP for this event without causing the same effect.
Citrate amplifies the effect of ATP.
Describe the regulation of the final glycolytic enzyme, pyruvate kinase.
\+ AMP \+ Fructose 1,6-BisP (feedforward activation) - ATP - Citrate (CAC saturation) - Acetyl CoA (Beta oxidation)
Briefly describe the beta oxidation inhibition mechanism.
Increased glycolysis saturates the CAC. Citrate leaves via the M-A shuttle, and produces Acetyl CoA in the cytosol. Acetyl CoA is converted to Malonyl CoA by Acetyl CoA Carboxylase (stimulated by the citrate). Malonyl CoA inhibits the CPTI transporter.
Describe the rate of muscular glycogenolysis in the fasting state.
Low, the enzymes that control this are far more sensitive to exercise signalling.
Relate the mechanism of resting state muscular beta oxidation stimulation.
It is identical to that of the liver, largely being controlled by substrate abundance and lack of CPTI inhibition.
What is the Randle Cycle?
The mutual inhibition and competition for oxidation between glucose and fatty acids in muscle and adipose tissue.
What two inhibition events are key in the Randle Cycle?
Acetyl CoA inhibition of Pyruvate Kinase
Malonyl CoA inhibition of CPTI
When muscular glycolysis in inhibited in the fasting state, what occurs to the small amounts of pyruvate that is produced?
Its gluconeogenic potential is preserved as Acetyl CoA in the mitochondrial matrix from beta oxidation inhibits pyruvate dehydrogenase (product inhibition).
What is aerobic exercise?
Low-moderate intensity exercise that can be sustained for long periods of time.
What is anaerobic exercise?
High intensity exercise such as sprinting or weightlifting.
What type of exercise are Type I muscle fibres suited to?
Aerobic exercise.
What five adaptations do Type I muscle fibres possess that makes them fit for purpose?
Moderate glycolytic ability. High oxidative capacity/Mitochondria rich. Good blood supply. More TAG stored compared to glycogen. Slower myosin ATPases.
What type of exercise are Type II muscle fibres suited to?
Anaerobic exercise.
What are Type II muscle fibres also known as?
Fast-twitch fibres or White Skeletal muscle fibres.
How are Type II muscle fibres adapted for their role?
High glycolytic capacity. More glycogen storage, less TAG. Low oxidative capacity/few mitochondria. Poor blood supply. Fast myosin ATPases.
Describe the action of Adenylate Kinase.
2x ADP -> 1x ATP + 1x AMP
Why is Adenylate Kinase used?
Because ADP cannot be used by myosin ATPases for muscle contraction, so conversion to ATP + AMP preserves energetic potential.
What are the effects of Adenylate Kinase action on the relative concentrations of ATP, ADP and AMP?
ATP decreases only by 10%.
ADP is constant and v. small.
AMP increases by the same amount ATP decreases, but due to lower initial concentration this is a 600% increase.
What is the most potent signalling molecule for exercise?
AMP.
Why is AMP an sensitive indicator and thus common allosteric effector for stimulating exercise metabolism?
Because the action of adenylate kinase means that its concentration increases by 600% very quickly as exercise starts.
What two enzymes allow for regeneration of ATP from ADP at the onset of exercise?
Adenylate Kinase and Creatine Kinase
Describe the mechanism of phosphocreatine ATP regeneration.
Phosphocreatine + ADP + H+
->
ATP + Creatine
What is phosphocreatine used for?
Storage of phosphate groups on largely inert creatine molecules as a reservoir that can replenish ATP at the onset of exercise.
Why is it necessary to have fast and short term ATP regeneration methods for the onset of exercise?
To allow the muscles to continue functioning while their metabolism adjusts for the exercise.
How long do stores of phosphocreatine generally last when exercise begins?
Five seconds.
Which type of muscle fibre contains a larger phosphocreatine supply? Why is this?
Type II, for anaerobic exercise when the ATP supply is depleted faster.
What two factors increase the rate of Creatine Kinase?
Increased ADP concentration as ATP is depleted.
Decreased pH due to anaerobic gycolysis lactate production.
What is the concentration of phosphocreatine in resting Type I muscle fibres?
7mM
What is the concentration of phosphocreatine in resting Type II muscle fibres?
30mM
Which two ATP regeneration methods rely on O2?
The Citric Acid Cycle and Fatty Acid Oxidation.
Which ATP regeneration methods do not require O2?
Adenylate Kinase, Creatine Kinase, Glycolysis.
What stimulates increased muscle glucose uptake during exercise? How?
AMP and Adrenaline
Increased GLUT4 receptor population.
How is the blood glucose concentration increased during exercise?
Adrenaline stimulated glycogenolysis in the liver.
What myocyte receptors recognise adrenaline?
Beta-Adrenergic receptors.
What effect does adrenaline binding have on the intracellular domains of beta-adrenergic receptors?
Activation of Adenylate Cyclase.
What reaction is catalysed by adenylate cyclase?
ATP -> cAMP
What effect does adrenaline-stimulated cAMP have in myocytes? Outline the mechanism of this.
PKA activation though cAMP binding to the inhibitory PKA regulatory subunits, allowing the catalytic subunits to dissociate.
What effect on glycogenolysis does adrenaline stimulated PKA have in myocytes? Outline the mechanism.
Increased glycogenolysis through phosphoactivation of a PP1 inhibitor.
What enzyme is the key point of control for muscular glycogenesis? How is it regulate?
Glycogen Synthase, inhibited by phosphorylation.
What three exercise-stimulated enzymes are responsible for inhibiting glycogen synthase? How are they activated?
PKA - adrenergic stimulation
AMP Kinase - Increased [AMP]
CaCMK - Increased [Ca2+]
Why are calcium ions used as an effector for entering the exercise state?
They are released in the myocyte when it is nervously stimulated to contract.
What phosphoinhibits muscular glycogen synthase in the fed state?
GSK3
What four exercise signalling events are used to stimulate muscular glycogenolysis?
[AMP] increase
Adrenergic stimulation
Calcium ion release
[Pi] increase from PCr hydrolysis
How is adrenergic stimulation used to increase glycogenolysis?
By activating PKA, which phosphoactivates phosphorylase kinase which phosphoactivates glycogen phosphorylase.
What enzymes are capable of phosphoactivating Glycogen Phosphorylase?
Phosphorylase Kinase only.
How does AMP activate glycogen phosphorylase?
Allosteric effect.
What is the structure of Phosphorylase Kinase?
Heterotetrameric, (aByd)4.
Which glycogen phosphorylase subunit interacts with calcium ions?
The delta subunit, which binds it directly to activate the enzyme.
Which glycogen phosphorylase subunit is phosphorylated by PKA or dephosphorylated by PP1?
The Beta subunit.
What residues on glycogen phosphorylase are phosphorylated to activate it?
A pair of serines.
How is muscular PFK-1 regulated?
\+ AMP (ATP competition) \+ F26BP (potent) - ATP (lowers F26BP affinity) - Citrate (enhances ATP effect) - H+ (enhances ATP effect) - Phosphocreatine (present only when resting)
What PFK-1 regulatory mechanisms are specific to exercise?
H+ - only present due to anaerobic glycolysis lactate production.
Phosphocreatine - only present when muscle is resting.
What effect does ATP have on the activity of PFK-1?
It is both a substrate of the enzyme and an inhibitor of it (by decreasing affinity for F26BP). Hence it is required at low-medium concentrations for high activity.
Describe the graph of PFK-1 activity against increasing ATP concentration.
A bell curve.
What is used to produce lactate? What enzyme catalyses this reaction?
Pyruvate
Lactate Dehydrogenase
When does lactate production occur?
When there is too little O2 to continue the CAC, so pyruvate builds up as pyruvate dehydrogenase is inhibited.
Describe the regulation of muscular pyruvate dehydrogenase.
+ AMP
+ Pyruvate (substrate activation)
+ Ca++ (exercise)
- ATP
- NADH (indicates lack of O2/respiratory chain function)
- Acetyl CoA (product inhibition/CAC saturation)
How do low oxygen levels lead to anaerobic glycolysis?
Lack of O2 causes NADH buildup, which inhibits pyruvate dehydrogenase.
How does CAC saturation fix itself?
Increased Acetyl CoA levels activate (are required for) pyruvate carboxylase, which produces oxaloacetate, thus increasing [CAC intermediates].
This mechanism is identical to other tissues.
Describe and explain the relative sensitivity of pyruvate dehydrogenase to its activators and inhibitors.
Highly sensitive enzyme, much more so to its inhibitors. Require all stimulators and no inhibitors to be active.
This prevents loss of gluconeogenic potential.
Which fuel is the most energy dense?
Fatty acids.
Why are fatty acids less suitable than glucose for short bursts of intense exercise?
They are in a more reduced state than glucose, so more O2 is required for their oxidation. During intense exercise oxygen is likely to be low, so the rate of FA oxidation is limited.
What limits the rate of fatty acid oxidation during exercise?
The rate of oxygen diffusion through the muscle.
What is the best fuel for short and intense exercise?
Glucose, as less O2 is required for its total oxidation and it can produce energy anaerobically by glycolysis and NADH production through lactate synthesis.
What is the advantage of possessing muscular lactate dehydrogenase?
It allows production of NADH by conversion of pyruvate to lactate when there is not O2, thus allowing glycolysis to continue.
What kind of muscle fibre stores fatty acids? How?
Type I muscle, in small droplets throughout the tissue.
What are fatty acids stored as in the muscle?
Intra-Muscular Triacyl Glycerol (IMTG)
How do muscles produce a store of fatty acids?
By expression of LPL, which allows them to take up blood-borne FAs released by adipocytes.
How do adipocytes respond to exercise and due to what stimulant?
Adrenaline stimulates lipolysis and FA secretion for muscular use.
Will muscles use blood-borne fatty acids more during the fed or fasting state? Why?
Fasting, as in the fed state insulin inhibits lipolysis in the adipocytes so there are fewer fatty acids in the blood (except in case of a high fat meal).
How do fatty acids enter myocytes?
By diffusion or through transporters.
What transporters do myocytes use to take up fatty acids from the blood?
CD36 and Fatty Acid Translocase
What happens to fatty acids when they are taken up into myocytes? What enzyme catalyses this?
They are esterified with CoASH to make Fatty Acyl CoA.
This is done by the membrane bound Fatty Acyl CoA Synthetase.
How is the muscular synthesis of fatty acyl CoA made favourable?
By coupling it to removal of a diphosphate from ATP to form AMP and Pi2.
The diphosphate is further hydrolysed by pyrophosphatase.
What is pyrophosphatase used for?
Breaking down diphosphates hydrolysed from ATP. Being a highly favourable reaction this is often coupled to others to drag them forward.
What is the Carnitine Cycle responsible for?
Transporting fatty acyl CoA into the mitochondrial matrix..
Why is the Carnitine Cycle necessary?
Because fatty acyl CoA cannot cross the mitochondrial membranes.
What three enzymes are involved in the Carnitine Cycle?
CPTI, CACT and CPTII.
Briefly describe the Carnitine Cycle.
1 - CPTI on the cytosolic side of the outer mitochondrial membrane transesterifies fatty acyl CoA with carnitine to form Acyl Carnitine.
2 - Acyl Carnitine is transported into the matrix through CACT.
3 - IMM embedded CPTII undoes the first reaction, producing a fatty acyl CoA and allowing the carnitine to diffuse back to the cytosol.
How do muscles regulate fatty acid oxidation and synthesis due to exercise?
High [AMP] stimulates AMPK, which phosphorylates Acetyl CoA Carboxylase (in either its inactive dimer or active polymer forms), deactivating it. This prevents Malonyl CoA from being produced.
AMPK also phosphoactivates Malonyl CoA decarboxylase, causing [Malonyl CoA] to decrease further.
What changes occur in muscle when it is endurance trained?
Larger IMTG stores and increased AMPK expression.
What effects does the increased [AMPK] in endurance trained muscle cause?
Faster entry into fatty acid oxidation as Malonyl CoA synthesis is inhibited and its breakdown activated, preventing its CPTI inhibition.
Through what product does beta oxidation inhibit glycolysis and glycogenolysis?
Acetyl CoA.
What effect does Acetyl CoA have on muscular carbohydrate metabolism?
Inhibition of glycolysis and glycogenolysis.
How does Acetyl CoA inhibit glycolysis?
Transport into the cytosol via the C-M shuttle, followed by inhibition of both Pyruvate Kinase and Pyruvate Dehydrogenase.
How does Acetyl CoA glycolysis inhibition lead to glycogenolysis?
Inhibition of pyruvate synthesis and the link reaction backs up the glycolytic pathway causing a glucose 6-P buildup which inhibits glycogen phosphorylase.
How do muscles regulate the CAC in response to exercise?
Ca++ stimulates Isocitrate Dehydrogenase and a-Ketoglutarate Dehydrogenase.
What change leads to the return of myocyte metabolism to the resting state?
Increased concentrations of ATP and citrate caused by lack of ATP hydrolysis.
How does ATP turn off exercise signalling?
By reversing the action of Adenylate Kinase, causing AMP and ATP to be converted to ADP to allow for oxidative phosphorylation, thus turning off AMPK and removing AMP as a signal.
What three occurrences help the muscle to recover after exercise?
Increased insulin sensitivity, so more GLUT4 to replenish glycogen stores.
ATP used to regenerate Phosphocreatine stores.
LPL expression remains high for 24hrs to replenish IMTG.
Give a qualitative description of the respiratory quotient. What does this tell us?
The ratio of CO2 exhaled to O2 inhaled, indicating the ratio of carbohydrate respiration to fat respiration.
What is the RQ of a person who is oxidising only carbohydrate?
1
What is the RQ of a person who is oxidising only fat?
0.707
What is the equation for calculating the ratio of carbohydrate to fat respiration?
(RQ-0.707)/(1-0.707) x 100
What assumption does the calculation of carbohydrate:fat respiration ratio make?
That use of proteins as a fuel is negligible, which is true except during starvation.
What would the respiratory quotient be of a person oxidising only protein products?
0.809
Explain muscle fuel choice at low intensity exercise.
Blood based fatty acids, as O2 is not limiting and it will not stimulate enough adrenaline to cause IMTG breakdown.
Explain muscle fuel choice at moderate intensity exercise.
Adrenaline increase leads to IMTG breakdown and use over plasma FAs, as well as increased glucose uptake. Glycogen must be used due to increased energy demand.
Explain muscle fuel choice at high intensity exercise.
Glycogen used to supply most energy as O2 is limiting so use of all fats decreases. Blood glucose use increases to compensate, more present due to adrenergic liver glycogenolysis stimulation.
Explain how fuel use changes over time during moderate intensity exercise.
Muscle glycogen used initially to respond to sudden exercise. Muscle TAG used as much as possible but soon depleted. Plasma FA use increases steadily.
How does trained muscle differ in fuel choice from untrained muscle?
Increased oxidative capacity so far more mitochondria rich, allowing for greater fat use.
