540
Biomolecular Structure and Interactions
• Myosin motors operate skeletal muscles. Myosin molecules have swivel heads at
their end to ‘walk’ along actin molecular fibers, contracting muscles.
• Large protein complexes within cells are often too large to diffuse from their
point of creation to the locations where they are needed. In these cases, small
molecular motor proteins called kinesins attach to the complex at one end of the
kinesin, and then literally walk with two ‘feet’ at the other end of the kinesin
along a cellular microtubule, using ATP to power conformational changes to the
kinesin’s feet, causing each foot to sequentially attach, drag the kinesin’s cargo
forward, and then detach from the tubule.
For us, the majority of the energy for our molecular motors derives from that
stored in a small molecule, adenosine triphosphate, ATP, which can easily release
that energy by hydrolyzing to ADP. We also use guanosine triphosphate, GTP, for
microtubule polymerization. Even though there is only about 250 g of ATP in all our
cells, we recycle our own body weight of ATP in 1 day.
The energy we use from ATP is released from the bond that holds the third
phosphate group to its adjacent phosphate in the reaction ATP→ADP +HPO 3 . ATP
has a relatively low activation energy, so release of its dissociation energy is not
difficult. In the reaction, 30.5 kJ/mol = 0.315 eV/molecule is made available. (This
is the change in Gibbs free energy.) Some biological molecular motors can have
high efficiency, and so utilize a good fraction of the released energy.
Bacterial motors that drive the rotation of flagella by the flow of a current of
protons through the motor can be of high efficiency in the power used to pull charges
through the motor compared to energy delivered to the flagellum from ATP-charge
pumping. The motor power is expended against viscous drag on the body of the
bacterium to propel it through fluids. 18
Problems
11.1 Give an example of a molecule involving a valence d-orbital contributing
to binding. How are these d-orbitals like hydrogenic d-orbitals, and how are they
different?
11.2 Suppose an electron in a valence orbital in methane is excited by a photon.
What makes this excited state unstable, and why is the state not a steady solution of
the wave equation?
11.3 Show that
ψ 1s (r, θ, ϕ) = N exp (−r/a 0 )
18 Edward M. Purcell, The efficiency of propulsion of rotating flagellum, PNAS 94:21, 11307–
11311 (1997).
Biomolecular Structure and Interactions
• Myosin motors operate skeletal muscles. Myosin molecules have swivel heads at
their end to ‘walk’ along actin molecular fibers, contracting muscles.
• Large protein complexes within cells are often too large to diffuse from their
point of creation to the locations where they are needed. In these cases, small
molecular motor proteins called kinesins attach to the complex at one end of the
kinesin, and then literally walk with two ‘feet’ at the other end of the kinesin
along a cellular microtubule, using ATP to power conformational changes to the
kinesin’s feet, causing each foot to sequentially attach, drag the kinesin’s cargo
forward, and then detach from the tubule.
For us, the majority of the energy for our molecular motors derives from that
stored in a small molecule, adenosine triphosphate, ATP, which can easily release
that energy by hydrolyzing to ADP. We also use guanosine triphosphate, GTP, for
microtubule polymerization. Even though there is only about 250 g of ATP in all our
cells, we recycle our own body weight of ATP in 1 day.
The energy we use from ATP is released from the bond that holds the third
phosphate group to its adjacent phosphate in the reaction ATP→ADP +HPO 3 . ATP
has a relatively low activation energy, so release of its dissociation energy is not
difficult. In the reaction, 30.5 kJ/mol = 0.315 eV/molecule is made available. (This
is the change in Gibbs free energy.) Some biological molecular motors can have
high efficiency, and so utilize a good fraction of the released energy.
Bacterial motors that drive the rotation of flagella by the flow of a current of
protons through the motor can be of high efficiency in the power used to pull charges
through the motor compared to energy delivered to the flagellum from ATP-charge
pumping. The motor power is expended against viscous drag on the body of the
bacterium to propel it through fluids. 18
Problems
11.1 Give an example of a molecule involving a valence d-orbital contributing
to binding. How are these d-orbitals like hydrogenic d-orbitals, and how are they
different?
11.2 Suppose an electron in a valence orbital in methane is excited by a photon.
What makes this excited state unstable, and why is the state not a steady solution of
the wave equation?
11.3 Show that
ψ 1s (r, θ, ϕ) = N exp (−r/a 0 )
18 Edward M. Purcell, The efficiency of propulsion of rotating flagellum, PNAS 94:21, 11307–
11311 (1997).