BIOL 207 Lecture Notes - Lecture 27: Sarcomere, Phosphate, Cellular Respiration
Molecular Mechanism of Muscle Contraction - SFM: Sliding Filament Mechanism (pgs. 287, 288. figure
9.12, 9.13 on pgs. 292-295)
• the cross bridges of the myosin has 2 binding sites
• one is known as the actin binding site - the cross bridge will attach to actin. at the very end of
cross bridges
• the second is known as the ATP binding site. as soon as ATP lands on its binding site it splits
automatically, and gives rise to APP+PI.
• the reason it splits immediately is because that cross bridge has a special kind of enzymatic
capability that is called ATPase
• the energy released by the splitting of ATP causes the cross bridge to become cocked. they are
cocked, energized, ready to do something
Sliding filament mechanism theory (SFM) (cont.)
• striated muscle consists of series of proteins (myosin and actin), on the actin there are troponin
and tropomyosin (TT Complex)
• these proteins are arranged in a manner they have a basic contractile unit known as the
sarcomere
• the sarcomeres themselves are lined up in tandem to form a long string of sarcomeres known as
myofibrils
• muscle cells have many myofibrils within them
• myosin cross bridges go through myosin myofilaments
• there are 2 binding sites on the myosin cross bridge
• the first is known as the actin binding site. this means that somehow, somewhere, the cross
bridge will attach to actin (these will attach to the myosin binding site (on actin) at some point). they are
on the ends of the myosin cross bridge.
• the second is the ATP binding site. this is below the actin binding site. at some point in time, the
ATP is going to bind to that cross bridge. At some point in time an ATP is going to bind to a cross bridge,
but the cross bridge itself has a capacity, a special capability. the cross bridge itself at that spot (where
the ATP binds) has an ATPase capability. This is an enzymatic capability that splits ATP. When ATP is split,
it splits into ADP+PI. When ATP is split by ATPase it gives rise to ADP+PI. PI is inorganic phosphate.
Because of the splitting of this high energy bond that energy is released. A lot of body temperature/heat
is the result of ATP splitting. Some of the energy is captured, some of it is lost as heat. Whenever ATP
lands on the cross bridge, immediately ATP splits into ADP+PI. In due time the ADP+PI will be recycled in
cell respiration to form new ATPs. When that ATP attaches to the cross bridge it is going to cock it, it is
going to cause it to be at a certain angle. Energy that was released by the splitting of the ATP
reofigures the agle of the ross ridge, ad o that ross ridge is i hat e all the eergized
state, the oked state. The ross ridge has a apait to do soethig, apait for stored eerg i
cross bridge will only be released at some point in time.
• *** the function of the TT complex is that it masks, covers, the myosin binding site on that actin
filament.
• wherever you have a TT complex, there is a myosin binding site underneath it. this suggests
there is a place where the cross bridge can attach. so until this TT complex is pushed aside or removed
find more resources at oneclass.com
find more resources at oneclass.com
