BIOL4510 / KINE 4510
Contractile Proteins 1
Peter Backx 2012
There are two properties of muscle you need to fully understand:
1) The force-length relationship
T20sion (% Max)
1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8
Sarcomere Length (m)
2) The dependence of force generation on the intracellular Ca 2+ levels. Ca 2+
acts as a catalyst for activating the ATPase in muscle for the purposes of
conversion of chemical energy into force.
1 Heavy Filament Myosin
Myosin is the molecular motor of the contractile protein machinery. It is the protein that
converts chemical energy from ATP to mechanical energy, thus generating force and
There 17 different classes of myosin; myosins
are involved in force generation and movement
in muscle (cardiac, skeletal and smooth muscle
are comprised of Type II myosins.
Type II dimerizes…two myosin molecule are
connected by coil-coil structures in the tail
Type I is the most primitive myosin: moves
molecular cargos like vesicles within cells; they
are Ca -dependent (i.e. activated by a rise in
intracellular Ca ) via Calcium/calmodulin
binding in neck region
Myosins are generally large proteins with a total molecular mass/weight of ~500,000 Da.
Each myosin pair (the molecular unit of the thick filament) is comprised of:
a tail, a neck and a head region
The head region is the site of ATPase activity…ATP binding and hydrolysis is
associated with changes in molecular structure that can be harnessed via the
organizational structure of the sarcomere and myofibrils to generate force and
Myosin molecules have a neck with an -helix structure with amino acid sequences that
bind light chains. The light chains contribute to the stability of the helix in the neck
region and modulate (increase) ATPase activity.
Myosin heavy chain tails form an -helical coiled-coil tail domain (tail) that holds the two
2 myosins (in a dimer) together. This is also the region that allows for self-assembly into
Individual long tails of myosin interact with each other allowing myosin molecules to
assemble into thick filaments. About 150 myosin dimers molecules polymerize in a
thick filament (about 300 myosin heads/thick filament). The myosin is arranged in the
thick filament in a nearly crystalline-like structure: every 14.5nanometers there are three
cross bridges (each with 2 myosins) projecting from the thick filament (at 120 relative
to one another). Each triplicate is rotated relative to the adjacent triplicate by 40
degrees. Therefore the repeat structure along the filament is 43.5 nanometers.
3 Myosin Heavy Chain (MHC)
There are two types of cardiac myosin heavy chains (MHC and MHC isoforms)
MHC = fast
MHC = slow
myosin ATPase = 5 * myosin ATPase
myosin ATPase is thought to be more “energetically efficient”
and -myosin heavy chains are separate gene products. A common theme in
biology (and biochemistry) is that reactions that take place quickly are often less
efficient than slow reactions: the price you pay for speed is a reduced efficiency.
These two isoforms can co-assemble in 3 possible ways:
V = (slow)
In the human heart, the myosin heavy chains, MHC and MHC, are expressed in a
tissue-specific manner. The MHC is preferentially expressed in human atria. ~90% of
the myosin in the human ventricles is MHC (rest is MHC ). In human fetal and
neonatal hearts, MHC is the predominant myosin expressed in the ventricles and atria.
There are species differences in MHC expression. The rat ventricle predominantly
expresses MHC rabbit ventricles express MHC and MHC
MHC is found primarily in large mammals, which correlates with speed of contraction
and heart rate; large mammals have slower heart rates and slower contraction.
Human MHC contains 1939 amino acids residues while MHC contains 1935 amino
acids. They only differ by 131 amino acids, mainly confined to regions of biological
significance in the head domain such as the N-terminus, the ATP binding pocket, the
actin binding cleft, the light chain binding domain and in the
two hinge regions further down in the rod (tail) domain.
ATPase of MHC is modulated by actin and Ca 2+
Ca 2+ (1-3 µM) stimulates ATPase in presence of actin
Ca influences the rate of movement
Increase of Ca 2+to >~10 µM decreases velocity
immediately followed by a gradual decay
- human heart disease levels of MHC goes up and MHC goes down,.
- speed of contraction slows in heart disease
- mutations in myosin genes were the first gene mutations to be associated with
inherited hypertrophic cardiomyopathy
4 - heart on the left (above) is hypertrophic cardiomyopathy, heart on right is normal.
- Mutations of contractile proteins can cause massive hypertrophy. Why would the
heart do this? Answer compensation
In developing skeletal muscle, MHC-emb (embryonic) and MHC-neo (neonatal) are
In post-natal skeletal muscle there are 4 MHC isoforms
• MHC (also referred to as MHC slow, same as cardiac MHC), IIA, IIB, and IIX
• MHC/slow, IIA, and IIX (humans although IIB gene is present, it does not seem
to be expressed)
These isoforms >80% identical (i.e., their amino acid sequences are < 20% different)
MHC IIa MHC IIB MHC
Most muscle have a combination of fibers (fast, intermediate and slow). The exact
combination depends on the muscle and previous training. Each motor unit will
innervate only one type of fiber. Muscle contractions in most muscles show a temporal
sequence: slow followed by fast followed by intermediate.
5 Myosin Light Chain
The type of myosin matches up with type of myosin light
chains: different myosin light chains for MHC versus
MHC myosin in the human heart.
There are two types of myosin light chains (MLC), essential
and regulatory, which are associated with the neck region
of MHC myosin. The essential MLC is also referred to as MLC-1 or alkali MLC. It is
called essential because myosin head will not work well without this chain. The
regulatory MLC is also known as MLC-2 or phosphorylatable MLC. It is phosphorylated
by myosin light chain kinase. Phosphorylation modulates myosin ATPase rate and
increases Ca 2+ sensitivity (although most of Ca 2+ regulation resides on thin filament
Crystal structure of the myosin head and neck
domain (S1) showing the different binding domains
and the MLC1and MLC2 binding to the neck region
In the human heart, there are two essential MLC isoforms, the MLC1sb (ventricle) and
MLC1sa atrium). Both are regarded as slow.
- the developing human embryo expresses MLCemb/atria in the whole heart and
skeletal muscle. MLC-1emb/atrial protein levels decrease in ventricles to undetectable
levels during early postnatal development (due to thyroid hormone) but persistent
expression occurs in the atrium throughout adult life.
Skeletal Muscle Isoforms
Fast skeletal muscle (MHC IIA, IIB, IIX): MLC1f or MLC3f as well as MLC2f
Slow skeletal muscle (MHCslow): MLC1sa or MLC1sb (cardiac: atrial or ventricular)
As well as MLC2s
Embryonic skeletal muscle (MHCemb/a)
MLC1emb or MLC1f as well as MLC2f
Heart disease induces the expression of MLC1sb in the atrium and MLC1sa in the
ventricle. The changes in gene expression th