Muscle proteins

Myofilaments
Perusal of figure E ( which is a longitudinal section) explains there are two types of myofilaments labelled
as Actin and Myosin. Myosin filaments are the thick filaments and the actin is also known as thin filament.
Here we can see that the myosin filament s are aligned parallel to each other and this is true across the entire
myofibril. Similarly the thin filaments are aligned across the myofibril. The arrangement shows an alternate
overlapping in certain regions along their longitudinal axes. It is this arrangement that gives the skeletal muscle
a striated appearance. This takes a form of alternating light and dark areas. If we look at fig– D we see that
areas of different densities are visible within light & dark bands of myofibrils. The reason for appearance for
these light and dark bands of myofibrils are– Light band– It is singly refractive when viewed under polarized
light , it is therefore isotropic and derives name I - band or isotropic band. Dark band – It is doubly refractive or
anisotropic thus it is known as A –band.

Myosin ( I– band )

Actin ( A– band )

1. Length is around 1.5 um
2. Diameter is around 14– 16 nm.
3.constitute A band of the sarcomere
4. Known as thick filaments.
5. Held in transverse and longitudinal register by thin cross bands located periodically along their length and by cross connections. These are aligned across the center of A band.
6.They constitute primarily of protein myosin.
7.Myosin constitutes around 50– 55% of myofibrillar protein.
8.Molecular wt. Is ~480,000

1. Length is 1 um
2.Diameter is 6- 8 nm
3.Constitute I band of the sarcomere.
4.Known as thin filament.
5.They constitute primarily of protein actin.

6.Molecular wt. Is ~ 42000. 7.It has two forms. The actin molecule which is in monomer form is called G– actin . As the G– actin contains high amount of proline thus by virtue of its imino group ( N– H leads to the folding among the polypeptide chains resulting into globular ( spherical ) shaped molecules having diameter 5.5 nm. The G– actin monomers get each of the myosin heads

Structure of Myosin— The Fig – Given below shows that structure of myosin molecule is elongated , rod
shaped ,with a thickened portion at one end. The thickened end of myosin molecule is usually referred to as
the head region and the long rod like portion forms the tail portion. The portion of molecule between the head and
the tail region is called the neck region. The head region of the molecule is doubly headed and it projects laterally
from the long axis of the filament. The intact myosin molecule is susceptible to proteolytic cleavage at several points.
Neck is the normal cleavage site. Digestion with trypsin cleaves the long tail of the molecule producing two fragments
light meromyosin and heavy meromyosin.Treatment with papain produces a long tail like fragments known as the
myosin rod, while trypsin cleavage of the myosin rod produces light meromyosin and two sub fragments.

Tropomyosin–

Length is ~ 44 A and rod shaped protein nearly 100% alpha– helix.
Composed of two subunits A and B chain each with a molecular wt. of 34000. They twist around one
another to form a coiled structure.
Can be easily crystallized .
It is involved in regulating skeletal muscle contraction in conjunction with troponin
When calcium concentrations are elevated in muscle , tropomyosin may move so that F– actin can bind
to myosin, resulting in muscle contraction.
Constitutes 8- 10% of myofibrillar protein.
It is highly charged molecule having high content of acidic and basic amino acids.
Isoelectric point pH is 5.1
It has low proline content and hence fibrous in nature.
Consist of two coiled peptide chains which attach end to end and form long , thin, filamentous strands.
The tropomyosin strands lie on the surface of each two coiled chains of F– Actin.
Single tropomyosin molecule extends the length of 7 G– Actin molecules in the actin filament.

Troponin

Thin filament protein
Molecular wt. is 76,000.
Composed of three non identical subunits
T n C = Calcium binding troponin
T n T = Tropomyosin binding troponin
Tn I = Troponin inhibitory subunit.
It is a contractile protein which binds calcium and in concert with tropomyosin regulate the interaction of
actin and myosin.
It is globular protein with high proline content.
It is present in the grooves of actin filaments where it lies astride the tropomyosin strands.
They occur in periodic repetitive manner along the length of actin filament and there is one molecule of
troponin for every 7 or 8 G– actin molecules along the actin filaments.

Tn C

Tn I

Tn T

1.Molecular wt.18000 2.Binds Tn T & Tn I

1.Molecular wt. 21000

2.Inhibits actin-myosin interaction when in higher concentrations.

3.Binds Tn C in presence of calcium.

4.Insoluble at low ionic strengths.

1.Molecular wt. 37000 to 45,000.

2. Binds to actin-tropomyosin complex.

3. Connects the Tn I– Tn C complex to thin filament.

4.Isoelectric point~ 8.8

Pseudo H– Zone

In the center of the A band on either sides of the M line , the myosin filament contains the tail portion of
the myosin molecules without any of the globular heads. This region within the H Zone, on either side
of M – line is called the pseudo H - Zone.

C – protein

Found in myosin filament.
Constitutes about 2– 2.5 % of the myofibrillar proteins.
It encircles the myosin molecules together into the bundle that forms the thick filaments.
18 C– protein bands placed 43.2 nm apart encircle each myosin filament. There are nine bands on each
side of H – Zone.

Alpha actinin

It has proline content comparable to that of actin.
It is globular molecule.
It is present in Z – line and constitutes about 2 – 2.5 5 of myofibrillar protein.
It is the major protein substance in the Z – filaments, holding two thin filaments in adjacent sarcomeres
together.

B - actinin

It is a globular protein located on ends of actin filaments and regulates their length by maintaining a
constant length of about 1 um in each half of the sarcomere.
In absence of B – actinin , actin filaments in vitro attain lengths of 3 – 4 um or more.

M – proteins

Constitute the M–line which binds the myosin tails together.
Comprise about 4% of myofibrillar proteins.
One of the M-proteins is creatine phosphokinase which helps make energy available for contraction.

Sarcoplasmic reticulum and T– tubules

Are membranous systems of tubules and cisternae which are flattened reservoirs of Ca 2+.
Sarcoplasmic Reticulum (SR) & T- tubules are separate membrane systems.
T-tubules are continuous with sarcolemma
T– tubules are open to the extracellular space. They conduct the membrane depolarization down into
the muscle cell.
The reticulum membranes of SR are storage site for Ca2+ while a muscle is in resting phase.
Within a single sarcomere the SR consists of several elements which are –
a) Longitudinal tubules – thin tubules , oriented in the direction of myofibrillar axis.
b) Fenestrated collar – a perforated sheet formed by convergence of longitudinal tubules in the
H zone region.
c) Terminal cisternae – Transversely oriented tubular element which is formed by convergence of
longitudinal tubules at the junction of A and I bands .
d)Triad – It is a structure formed collectively by the central T– tubule and two tubular elements has
two triads.
SR is extensive and runs around the myofibrils.
SR and T- tubules control contraction of myofilaments.
Neither SR or T– tubules are visible under a microscope.