The molecular process that makes muscle contraction possible is
that ATP binds to myosin, which releases the energy needed to make
the muscle contract. In this problem, we will examine the
quantities of energy involved in this process.
Part 1
The reaction of ATP hydrolysis catalyzed by myosin (M) occurs in
several steps*:
M + ATP → M·ATP
M·ATP → M·ADP·Pi
M·ADP·Pi → M·ADP + Pi (aq)
M·ADP → M + ADP
* T. Kodama & R.C. Woledge, "Enthalpy Changes for
Intermediate Steps of the ATP Hydrolysis Catalyzed by Myosin
Subfragment-1", J. Biol. Chem. 254(14), 6382-6836
According to experimental data, the heats of reaction for these
four reactions are -90 kJ/mol, +83 kJ/mol, -88 kJ/mol, and +72
kJ/mol.
a) Based on what you know about chemical bonding (from this class,
from chemistry, and from anywhere else), explain the positive and
negative signs for each of these four reactions. What does a
positive and a negative sign mean, and why is a positive (or
negative) sign expected in each case?
b) Find the total energy released (in kJ/mol) in the overall ATP
hydrolysis reaction.
Part 2
The energy released by this reaction in each muscle fiber,
multiplied by a huge number of muscle fibers, results in the
kinetic energy of your muscles moving.
c) Estimate the kinetic energy of your leg when you are walking at
a normal pace.
d) Using the data from Part 1, estimate the total energy released
by ATP-myosin reactions throughout your entire leg. Explain how you
arrived at this estimate.
Some possibly useful information: Muscle fibers (cells) are
composed of many myofibrils, tubular components measuring 1-2
micrometers in diameter. Each myofibril is divided along its length
into sarcomeres (the basic unit of a muscle, with a single myosin
filament), each of which is about 2 micrometers long. Remember that
"kJ/mol" means kilojoules for every 6.02×1023 myosin
molecules.
e) Are the results from parts c and d similar? If not, what are
some possible ways to account for the discrepancy?
(A) A positive sign means reaction is Endothermic i.e. energy is absorbed during the reaction. As aresult products hve more energy than reactants. A Negative sign means reaction is Exothermic i.e. energy is released during reaction. As a result the products have less energy than reactants.
(B) Total energy change is given by the sum of the energies involved in four reactions
E = (-90 kJ/mol) + (+83 kJ/mol) + (-88 kJ/mol) + (+72 kJ/mol)
E = - 23KJ/mol
NOTE: For (C), (D) and (E) parts more information is needed like number of muscle fibers present in legs.
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