Exercise biochemistry
Skeletal
muscle
• Muscle
– One of 4 principle tissue of
the body
• Muscle, nerve, connective,
epithelial
– 3 forms
• Skeletal, smooth, cardiac
– Only skeletal is voluntary
– Function
• Exerts force (for
locomotion)
– Energy is chemical
• Other functions
–
–
–
–
Heat production
Posture
Protection
Shape
Muscle structure
• Muscle layers
– Epimysium
• Surrounds whole muscle
– Perimysium (fascia)
• Surrounds (bundles of fibers)
– Endomysium
• Surrounds individual muscle
fibers
• Each muscle fiber
– One nerve ending
• Motor endplate
– Connection between muscle fiber and
nerve
• Neurotransmitter
– Acetylcholine (Ach)
• Muscle blood flow
– Capillaries surround the
muscle fibers
• Travel in 3 dimensions
• Capillaries are very
“tortuous”
• Capillaries are fed by an
arteriole
– Great ability to adapt to
changes in work
– Skeletal muscle blood flow
• Can increase markedly from
rest to maximal exercise
• This is dependent upon the
muscle type
– Slow twitch
» Large capacity to
increase flow
– Fast twitch
» Oxidative; moderate
ability to increase
flow
» Glycolytic; poor
ability to increase
flow
Muscle structure
Muscle fiber structure
Myocytes
• Multinucleated
• Thin
•10-100 µm
•Length varies
•3 mm to 30 cm
•Interior
•Sarcoplasm
•Mitochondria, myoglobin
(gives muscle it’s “red”
appearance) and myofibrils
•Sarcoplasmic reticulum
•Surrounds myofibrils
•Growth, repair, maintenance
•T-tubules
•Involved in nerve
transmission and calcium
release
Muscle fiber structure
• Skeletal muscle has a unique
“banded” appearance
– Due to “Dark” A bands and
“Light” I bands
• A bands: Myosin and actin
– Anisotropic: different properties in
different directions
• I bands: just actin
– Isotropic: Same properties in all
directions
– H zone
• Light zone (“helle”) that contains
M line (middle)
• No overlap between actin and
myosin
– Z disc
• Separate sarcomeres
• When actin connects
• Also connects adjacent
myofibrils
Molecular composition of the
myofilaments
• Myosin filament
– Rod-like tail with two heads
• ATPase located here
– Interacts with actin
• Globular (G actin) and
fibrous (F actin)
• Tropomyosin
– Stiffens the actin
filament
• Troponin
– Troponin I: Binds to actin
– Troponin T: Binds to
tropomyosin
– Troponin C: binds
calcium
• Force development
– Sarcomeres shorten
• Actin and myosin slide over
one another
– Calcium allows tropomyosin
to move “Unblocking” actin
• Ca2+ binds to troponin C
– Myosin interacts with actin
(1)
– Myosin head moves actin
• ADP + Pi are released (2)
– ATP binds to myosin head
• Allows myosin to disengage
from actin (3)
– ATP is broken down
• This allows for myosin head
movement (4)
– Cycle starts again
Force
development
Control of force
development
2
1) Neurotransmitter release
–
Acetylcholine
1
2) Action potential propagation
–
Sarcolemma to T-tubules
3) Calcium release
–
6
Terminal cisternae of SR
4) Calcium concentration rises in
sarcoplasm
–
Binds to troponin C
•
Causes tropomyosin shift
5) Contraction occurs
6) Calcium resequestered
–
–
3
When neural stimuli ceases
Tropomyosin blockage
restored
4
5
Motor units
• A motor neuron and the
fibers it innervates
– Number of fibers varies
• Muscle that perform very fine
movements (eye, hands)
– Small motor units
• Muscle involved in larger
movements
– Large motor units
• All or none
– A stimulus will either activate
the entire motor unit, or none
of it
Muscle fiber types
• Type 1
– Slow twitch, very high fatigue resistance, used for prolonged activity,
red
• Type 2
– Two types
• Type 2a, fast-twitch oxidative, intermediate fatigue resistance, used for longer
high intensity exercise
• Type 2b, fast-twitch glycolytic, fast to fatigue, used for very brief high intensity
contractions
Muscle fiber types
• Size principle
– Larger fiber types recruited as
intensity increases
• Type 1, type 2a, type 2b
• Human muscle contains a
mixture of fiber types
– Postural muscle
• High proportion of type 1
– Rapid movement muscle
• Hand and eye
– High proportion of type 2
– Many are mixed fiber muscle
• Quadriceps
• This seems to be determined
primarily at birth
– Distance runners have a high
percentage of type 1 fibers
– Sprinters, type 2
Types of muscle action
• Concentric contraction
– Muscle shortens while developing
force
• Eccentric “contraction”
– Muscle lengthens while generating
force
• Isometric contraction
– Muscle generates force with no
change in length
Plasticity of muscle
• Plasticity
– Ability to adapt
• Alterations in
– Size
– Fiber composition
(small)
– Metabolic capacity
– Capillary density
Sources of energy for muscular
contraction
• Adenosine triphosphate
– Provides the energy for
muscular contraction
1) ATP↔ADP
+Pi + energy
– ATPase
2) PCr
+ ADP ↔ ATP + Cr
– Reactions 1 and 2 work together
3) Anaerobic metabolism
of
glucose
– Yields lactate and small amt
of ATP
4) Aerobic metabolism
of CHO,
fast and proteins
– Aerobic metabolism yields
lots of ATP, but it is slow
Phosphagen system
Energy systems
• Immediate energy system
– Phosphagen system
• Relies on Phosphocreatine (PCr) to quickly resynthesize ATP
• PCr + ADP ↔ ATP + Cr
– Creatine kinase
• Resynthesizes ATP very fast
• Very small amounts of ATP and PCr stored in the muscle
– Enough for about 10s of activity
Glycolytic system
• Since most activity lasts
longer than 10s or so,
need another system
– Glycolytic system
• Uses glucose or glycogen
• Glycolysis
– Breaks down glucose to 2
pyruvate molecules
• Anaerobic glycolysis
– Pyruvate is converted to lactate
– Small amount of ATP
• Allows work to continue for
about 2-5 minutes
– Dependent upon pain tolerance
Aerobic metabolism
• Requires
– Mitochondria, oxygen
– Can use
• Carbohydrates, fats or
proteins
– Huge capacity
• Virtually unlimited
– Slow ATP delivery
• Sustains long-term, lowintensity work
Tricarboxylic acid cycle
• AKA
– Kreb’s cycle and Citric acid
cycle
• Function
– Break down Acetyl-CoA to CO2
and H+
– Also forms small amount of
ATP
– H+ carried by
• NADH and FADH2
• To ETC
• Net production
– 1 ATP (GTP transfers terminal
P to ADP; nucleotide
diphosphokinase)
– 3 NADH
– 1 FADH2
Tricarboxylic acid cycle
• 1st step
– Formation of citrate
– Citrate synthase
• 2cd step
– Formation of isocitrate
– Isomer of citrate
• 3rd step
– Formation of alphaketoglutarate
– Isocitrate dehydrogenase
– 1 NADH
• 4th step
– Formation of Succinyl CoA
– Alpha-ketoglutarate
dehydrogenase
– 1 NADH
Tricarboxylic acid cycle
• 5th step
– Formation of succinate
– Succinyl-CoA synthetase
– 1 ATP
• 6th step
– Formation of fumarate
– Succinate dehydrogenase
– 1 FADH2
• 7th step
– Formation of malate
• 8th step
– Formation of oxaloacetate
– Malate dehydrogenase
– 1 NADH
Tricarboxylic acid cycle
• Summary
– Acetyl-CoA + ADP + 3NAD+ + FAD ↔ 2CO2 + ATP + 3NADH + 3H+ + FADH2 + CoA
– Citrate synthase
• Key regulatory enzyme
– Inhibited by
» ATP, NADH,
Succinyl-CoA
» Thus, when cellular
energy state is HIGH,
TCA cycle is
inhibited
– Stimulated by
» ADP, NAD+,
Acetyl-CoA
• Basically
– Stimulated by substrate,
inhibited by products
Electron transport chain
• Linked proteins which
remove the electrons from
H+
• This separates the charge
and creates, in essence,
a molecular battery
• This charge difference
powers the formation of
ATP
• The electrons (and H+)
then combine with
molecular O2 to form H2O
Electron transport chain
• NAD and FAD
– Electron (and H+)
carriers
– So much of
metabolism is
providing an
electro-chemical
charge difference
across the inner
membrane of the
mitochondria to
drive ATP formation
Electron Transport Chain
• Step 1
– NADH dehydrogenase
complex
– 2 e- transferred to FMN to
form FMNH2
• 1 ATP formed
• Step 2
– Coenzyme Q
– Accepts e- from FADH2
• Step 1 and 2
– Dropping off H+ and e-
Electron transport chain
• Step 3
– Cytochrome chain
– Electrons are transferred
from CoQ to each
cytochrome
• Cytochrome b
• Cytochrome c
– 1 ATP
• Cytochrome oxidase
– 1 ATP
• Step 3 summary
– Electron transport
and ATP formation
(oxidative
phosphorylation)
• Glycogen:
– Storage form of glucose
• Muscle (~350g) , liver (~80g)
and blood (~5g of glucose)
– Total (70 kg man; 15%
bodyfat)
» ~435g
• Lipids
– Stored primarily in adipocytes
• Also in muscle and around
organs as well circulating in
blood (Fatty acids)
– ~10,500g
• Marathon
– Requires ~2900-3000 kcal of
energy
– Requires about 750g of CHO
– Only requires ~ 330g of fat
Energy storage
Energy storage