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*:

1) M + ATP → M·ATP

2) M·ATP → M·ADP·P_i

3) M·ADP·P_i → M·ADP + P_i (aq)

4) 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 6.02x10²³ myosin molecules.

e) Are the results from parts c and d similar?  If not, what are some possible ways to account for the discrepancy?

Problem by Ben Dreyfus