Over exposing the myosin binding sites on actin as

over, exposing the
myosin binding sites
on actin. As long as
Ca
2 +
is present,
cross - bridge
formation can occur.
When Ca
2 +
is
removed from the
cytosol, it is also
released from TnC,
causing the whole
complex to return to
its original position.
Clearly, the point of
control is how
quickly and tightly
TnC binds to Ca
2 +
,
thereby allowing the
interactions between
troponin,
tropomyosin, actin,
and myosin to occur.
By having different
isoforms of all these
proteins, organisms
can regulate these
interactions and their
371

binds to the actin
(inhibiting
myosin/actin
interactions), and
TnT binds to the
tropomyosin. It is
the tropomyosin that
physically blocks the
myosin binding site
on actin. When Ca
2 +
binds to TnC, it
causes a
conformational
change so that TnC
and TnI become
more tightly bound
to each other. This,
in turn, decreases the
interaction between
TnI and actin, so that
the whole
troponin/tropomyosi
n complex slides
over, exposing the
myosin binding sites
on actin. As long as
Ca
2 +
is present,
cross - bridge
formation can occur.
When Ca
2 +
is
removed from the
cytosol, it is also
released from TnC,
causing the whole
complex to return to
its original position.
Clearly, the point of
control is how
quickly and tightly
TnC binds to Ca
2
+
,
thereby allowing the
interactions between
troponin,
tropomyosin, actin,
and myosin to occur.
By having different
isoforms of all these
proteins, organisms
can regulate these
interactions and their
sensitivities to
physiological
372

myosin/actin
interactions), and
TnT binds to the
tropomyosin. It is
the tropomyosin that
physically blocks the
myosin binding site
on actin. When Ca
2 +
binds to TnC, it
causes a
conformational
change so that TnC
and TnI become
more tightly bound
to each other. This,
in turn, decreases the
interaction between
TnI and actin, so that
the whole
troponin/tropomyosi
n complex slides
over, exposing the
myosin binding sites
on actin. As long as
Ca
2 +
is present,
cross - bridge
formation can occur.
When Ca
2 +
is
removed from the
cytosol, it is also
released from TnC,
causing the whole
complex to return to
its original position.
Clearly, the point of
control is how
quickly and tightly
TnC binds to Ca
2 +
,
thereby allowing the
interactions between
troponin,
tropomyosin, actin,
and myosin to occur.
By having different
isoforms of all these
proteins, organisms
can regulate these
interactions and their
sensitivities to
physiological
variables (pH,
temperature, etc.).
373

TnT binds to the
tropomyosin. It is
the tropomyosin that
physically blocks the
myosin binding site
on actin. When Ca
2 +
binds to TnC, it
causes a
conformational
change so that TnC
and TnI become
more tightly bound
to each other. This,
in turn, decreases the
interaction between
TnI and actin, so that
the whole
troponin/tropomyosi
n complex slides
over, exposing the
myosin binding sites
on actin. As long as
Ca
2 +
is present,
cross - bridge
formation can occur.