Proteins regulating muscle contraction Classification. And mechanism of action of enzymes

1. Muscle Contraction and
Movement

2. Molecular Characteristics of the Contractile Filaments
Myosin is a contractile molecule composed of
6 polypeptide chains.
Thick filaments of myosin (about 15 nm in
diameter
 2 heavy chains
• Molecular weight of 200,000 each
• Double helix
• Tail
– Ends: head
 4 light chains
• Molecular weight of 20,000 each
• Part of myosin head (2 at each head)
• Help control the function of the head
during muscle contraction.
Cross
bridge
Body
Hinges
1200 rotation

3. Skeletal Muscle
Body- bundled tails of myosin
molecules.
• Arm- extends the head outward from
the body.
• Head- globular polypeptide structure.
• Cross-bridges- heads and arms
together.
• Hinges- flexible point of a cross-bridge

5. Actin
Polymerization takes place to form long
helical filaments followed by hydrolyzed
of ATP by ATPase.
ATPase activity of the Myosin Head
– The myosin head functions as an ATPase
enzyme.
• Actin Filaments are composed of
Actin, Tropomyosin, and Troponin.
–The double stranded Filamentous
actin (F-actin) protein molecule
• backbone of the actin filament
• wounded in a helix
• (each strand) is composed of
polymerized G-actin molecules.
Actin, protein that is an important
contributor to the contractile property
of muscle and other cells.
Size: about 7 nm in diameter
It exists in two forms: G-
actin (monomeric globular actin)
and F-actin (polymeric
fibrous actin), the form involved in
muscle contraction.
G-actin is the soluble monomer
while F-actin is the actin filament
G-actin is globular while F-actin is
filamentous

6. Actin chain (Thin Filament)
G-actin molecule has a molecular weight of
about 42,000.
• One molecule of ADP is attached to each
of it.
• Each actin filament is about 1 micrometer
long.
• The bases of the actin filaments is
inserted strongly into the Z-discs.
Titin-27,000 amino acids-stabilize the thick
filament and reduction on overstretching of
sarcomere

7. Each molecule of tropomyosin found in the actin filament has a molecular
weight of 70,000 and a length of 40 nanometers.
These molecules are wrapped spirally around the sides of the F-actin helix.
In the resting stage, the tropomyosin molecules lie on top of the active sites of
the actin strands
Attached intermittently along the sides of the tropomyosin molecule is
the troponin molecule.
• There are three subunits:
Troponin 1: has strong affinity for actin
Troponin T: for tropomyosin
Troponin C: for calcium ions

8. Types of muscle contractions
There are three types of muscle contraction:
Isometric
Concentric,
Eccentric.
• Isometric: A muscular contraction in which
the length of the muscle does not change.
• Isotonic: A muscular contraction in which
the length of the muscle changes.
• Eccentric: An isotonic contraction where
the muscle lengthens.
• Concentric: An isotonic
contraction where the muscle shortens.

9. Sliding filament theory
• The basic unit controlling changes in muscle length, scientists
proposed the sliding filament theory to explain the molecular
mechanisms behind muscle contraction. Within the sarcomere,
myosin slides along actin to contract the muscle fiber in a process
that requires ATP.
ATP binds to a myosin head, which is released from an actin
filament
Hydrolysis of ATP cocks the myosin head
The myosin head attaches to an actin binding site with the help of
Calcium
The power stroke slides the thin filament when ADP and Pi are
released from it

10. Process of muscular contraction
The process of muscular contraction occurs over a number of key
steps, including:
• Depolarization and calcium ion release.
• Actin and myosin cross-bridge formation.
• Sliding mechanism of actin and myosin filaments.
• Sarcomere shortening (muscle contraction)

11. Muscles are attached to bones by tendons.
Muscles work in antagonistic pairs
Ex. Biceps and triceps, whereby One muscle contracts while the other relaxes

12. Contractile apparatus
Skeletal muscle
Muscle cell = muscle fiber (a single cell
with one nucleus)
Muscle fibers are made of myofibrils
(striated)
Myofibrils are made of units called
sarcomeres
Sarcomeres are made of thick and thin
filaments
Z line is the end of the sarcomere
Thick and thin filaments slide over one
another to shorten the muscle during
contraction

13. 1. Depolarisation and Calcium Ion Release
• An action potential from a motor neuron triggers the release
of acetylcholine into the motor end plate
• Acetylcholine initiates depolarisation within the sarcolemma,
which is spread through the muscle fibre via T tubules
• Depolarisation causes the sarcoplasmic reticulum to release
stores of calcium ions (Ca2+)
• Calcium ions play a pivotal role in initiating muscular
contractions

14. 2. Actin and Myosin Cross-Bridge Formation
• On actin, the binding sites for the myosin heads are covered by a blocking
complex (troponin and tropomyosin)
• Calcium ions bind to troponin and reconfigure the complex, exposing the
binding sites for the myosin heads
• The myosin heads then form a cross-bridge with the actin filaments

15. 3. Sliding Mechanism of Actin and Myosin
• ATP binds to the myosin head, breaking the cross-bridge between
actin and myosin
• ATP hydrolysis causes the myosin heads to change position and
swivel, moving them towards the next actin binding site
• The myosin heads bind to the new actin sites and return to their
original conformation
• This reorientation drags the actin along the myosin in a sliding
mechanism
• The myosin heads move the actin filaments in a similar fashion to the
way in which an oar propels a row boat

17. 4. Sarcomere Shortening
• The repeated reorientation of the
myosin heads drags the actin
filaments along the length of the
myosin
• As actin filaments are anchored to Z
lines, the dragging of actin pulls the Z
lines closer together, shortening the
sarcomere
• As the individual sarcomeres become
shorter in length, the muscle fibres as
a whole contracts

18. Summary of Muscle Contractions
• Action potential in a motor neuron triggers the release of Ca2+ ions from the
sarcoplasmic reticulum
• Calcium ions bind to troponin (on actin) and cause tropomyosin to move,
exposing binding sites for the myosin heads
• The actin filaments and myosin heads form a cross-bridge that is broken by
ATP
• ATP hydrolysis causes the myosin heads to swivel and change orientation
• Swiveled myosin heads bind to the actin filament before returning to their
original conformation (releasing ADP + Pi)
• The repositioning of the myosin heads move the actin filaments towards the
centre of the sarcomere
• The sliding of actin along myosin therefore shortens the sarcomere, causing
muscle contraction

19. Motor neurons and muscle contraction
• Motor neurons stimulate muscle contraction
• Motor neurons are branched and can stimulate more than one muscle
fiber
• Motor unit = motor unit and all the muscle fibers it controls
• Neuromuscular junctions = the synapse between a motor neuron and a
muscle fiber
• The strength of a muscular contraction is controlled by the number of
motor units activated. More motor units = stronger contractions
• Muscles requiring precise control have one motor neuron per muscle
fiber

20. Myofibril
of
muscle
fibre
Mechanism of stimulation/relaxation

21. Motor neurons and muscle contraction
• Mechanism of stimulation:
• An action potential releases
acetylcholine into the neuromuscular
junction
• Acetylcholine depolarizes the muscle
cell channels inside on the
sacroplasmic reticulum (SR) release Ca
so it can reach the contractile apparatus
• Mechanism of relaxation
• Motor neuron stops firing
• Ca pumped back into the SR

22. Contraction of Smooth Muscle

23. QUIZZES
• Differentiate between fast and slow muscle fibers
• Compare between Isometric and isotonic muscle contraction
• Describe the skeletal and cardiac muscle contraction
• Discuss on causes and management of muscular dystrophy.
• Discuss on rigor mortis.

24. Enzymes
What is an enzyme
Active s
ite
Globular protein which functions as a
biological catalyst, speeding up reaction rate
by lowering activation energy without being
affected by the reaction it catalyse
Ribozymes are RNA molecule with enzymatic
activity.
 Catalytic behaviour of any enzyme depends
upon its primary, secondary, tertiary or
quaternary structure.
 Enzymes of digestive tract and those found in
blood are present in inactive form called
zymogen or proezymes

25. Enzymes are composed of long
chains of amino acids that have
folded into a very specific three-
dimensional shape which contains
an active site.
An active site is a region on the
surface of an enzyme to which
substrates will bind and catalyses a
chemical reaction.

26. Mechanism of enzyme action
The enzymatic reactions takes
place by binding of the
substrate with the active site of
the enzyme molecule by
several weak bonds.
E + S ‹--------› ES --------› E + P
Formation of ES complex is the
first step in theenzyme
catalyzed reaction then ES
complex is subsequently
converted to product and free
"Lock and key" or Template model

27. Induced-fit model

29. Nomenclature / enzyme classification
According to the IUBMB system of enzyme nomenclature
enzymes are grouped into 6 major classes
EC 1 OXIDOREDUCTASES
EC 2 TRANSFERASES
EC 3 HYDROLASES
EC 4 LYASES
EC 5 ISOMERASES
EC 6 LIGASES

30. Factors affecting reaction velocity
Temperature
Hydrogen ion concentration (pH)
Substrate concentration
Enzyme concentration
Products of the reaction
Presence of activator/inhibitor
Allosteric effects
Time

31. Michaelis- Menten Kinetics
The model involves one substrate molecule,
k1 k2
E + S ‹-------------› ES ------------ › E + P
k-1
Where
• S is the substrate
• E is the enzyme
• K1, k-1 and k2 are the rate constants

32. • The mathematical equation that defines the quantitative
relationship between the rate of an enzyme reaction and the
substrate concentration is the Michaelis-Menten equation:
Vmax [S]
V₀ = -------------
Km + [S]
V₀ is the observed velocity at the given [S]
Km is the Michaelis-Menten constant
Km = (K-1 + K2) / K1
Vmax is the maximum velocity at saturating [S] conc

33. Lineweaver-Burk (double reciprocal) plot
A linear representation is more accurate and convinient for
determining Vmax and Km.
This equation is obtained by taking reciprocal of both the side of
Michelis-Menton equation.
• 1/[S] vs. 1/Vo

34. Enzyme Inhibiton
Any substance that can diminish the velocity of an enzyme
catalyzed
These include drugs, antibiotics, poisons, and anti-metabolites.
Useful in understanding the sequence of enzyme catalyzed
reactions, metabolic regulation, studying the mechanism of cell
toxicity produced by toxicants.
Forms the basis of drug designing.

35. Types of Enzyme Inihibiton
Reversible inhibitors
 Irreversible inhibitors
Reversible inhibitors can be classified into :
 Competitive
 Non-competitive
 Un-competitive

36. Competitive Inhibition

37. Non-Competitive Inhibition

38. Un-competitive Inhibiton
Binds only to the enzyme-substrate
complex.
Does not have the capacity to bind
to the
free enzyme.
Not overcome by increasing
substrate concentration.
Both the Km and Vmax are reduced.

39. Enzyme Inhibition (Plots)