What is ATP?

1
The Use of the Energy in ATP for Muscle
Contractions
Nitya Anand Melissa Donaldson Leigh
McDonald Chris Smyre
2
What is ATP?
Adenine

• ATP Adenosine Tri-phosphate

Ribose
3 phosphate groups
3
How Do Muscles Contract?

• Muscle contractions require the use of actin
  filaments and myosin molecular motors.
• The movable myosin heads pull along the actin,
  causing muscles to shorten.
• This process is driven by the release of energy
  from an ATP (adenosine triphosphate) molecule.
• As such, ATP is the major energy currency in the
  human body.

4
Actomyosin Mechanism
Myosin II

• A ATP binds to myosin heads, releasing it from
  binding the actin.
• B ATP is hydrolyzed to ADP and Pi. The myosin
  head moves back due to the energy release, but
  does not release the ADP/Pi.
• C Pi leaves the myosin head, so it can bind to
  the actin.
• D The myosin pulls the actin filament forward
  as it releases ADP in what is called the power
  stroke. (Geeves, 1999)

5
The Importance of ATP

• Energy is released when ATP is hydrolyzed to ADP
  and Pi, causing a conformational shift.
• This energy goes into the myosin head, which
  allows it to pull back in order to drive the
  power stroke seen in part D.
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter10/animation__breakdown_of_atp
  _and_cross-bridge_movement_during_muscle_contracti
  on.html
• If there is no ATP available, there will be no
  energy to drive the head backwards.
  Additionally, without ATP, the myosin cannot
  release the actin filament. This is the primary
  cause of rigor mortis.

6
Mechanism of ATP Hydrolysis
ATP H2O ? ADP Pi
7
Thermodynamics of ATP Hydrolysis
Theoretically, based on bond energies -A?G-3
0.5 kJ/mol at 25 C, 1 atm To find ?G under
normal cellular conditions Start with equation
for affinity Thus for the reaction ATP H2O
? ADP Pi H, Cells usually maintain a high
ratio of ATP to ADP, resulting in ?G-50 kJ/mol
8
Reaction Coupling
Reactions with positive ?G can be coupled with
reactions with negative ?G Allows the cell to
undergo reactions that are not thermodynamically
favorable Example formation of glutamine from
glutamic acid Glutamic acid NH3 ? glutamine
?G14.2 kJ/mol In the
presence of ATP Glutamic acid ATP ? glutamyl
phosphate ADP Glutamyl phosphate NH3 ?
glutamine Pi Total ?Grxn -16.3
kJ/mol Application of this during muscle
contraction energy from ATP hydrolysis induces
conformation change in myosin
9
Binding of ATP and Conformational Change in Myosin
Pre-powerstroke
Myosin head can assume two different structural
conformations EA for the change from low energy
to high energy conformation is provided by ATP
hydrolysis
Rigor state
10

Entropy

• Second Law of Thermodynamics
• Entropy is always increasing in the universe
• 2 Components in changes of Entropy (dS deS
  diS)
• deS Entropy change due to exchange of matter
  energy Can be negative or positive
• diS Entropy change due to irreversible
  processes deS
• Will be gt 0
• Spontaneous Reactions are Irreversible
• Entropy production Similar to a Carnot cycle
  diSgt0
• in ATP hydrolysis process

11
Irreversibility of ATP cleavage

• 4 forms of energy
• Gibbs Energy
• Helmoltzs Energy
• Enthalpy Energy
• Total Energy
• Energy is always being minimized
• Gibbs Energy is the easiest to measure
• T P held constant
• We know that ATP?ADP drop in Gibbs energy

12
Conclusion

• ATP hydrolysis is important in biological
  processes such as the actomyosin cross-bridging
  that controls muscle contractions.
• To drive contractions, ATP is used in a coupled
  reaction that results in an exothermic (negative)
  Gibbs Free Energy for the reaction.
• Entropy change in the process is analogous to the
  change in entropy in a Carnot cycle which
  explains the heat generated by muscle
  contractions.

13
References
Animation Breakdown of ATP and Cross-Bridge
Movement During Muscle Contraction. McGraw-Hill
Higher Education. The McGraw-Hill Companies.
lthttp//highered.mcgrawhill.com/sites/0072495855/s
tudent_view0/chapter10/animation__breakdown_of_atp
_and_cross-bridge_movement_during_muscle_contracti
on.htmlgt Burghardt, T.P., Yan Hu, J., Ajtai, K.
(2007). Myosin dynamics on the millisecond time
scale. Biophysical Chemistry, 131 (1-3),
15-28. Geeves, M.A. Holmes, K.C. 1999.
Structural mechanism of muscle contraction.
ANNUAL REVIEW OF BIOCHEMISTRY 68 687-728.
Karp, G.C. (2008). Cell and Molecular Biology
Concepts and Experiments. Atlantic Highlands
John Wiley and Sons, Inc. Kondepudi, D. (2008).
Introduction to Modern Thermodynamics. West
Sussex John Wiley and Sons Ltd. Myosin II.
College of Medicine School of Biomedical
Sciences. The University of Edinburgh. 30 Nov.
2008 lthttp//www.bms.ed.ac.uk/research/others/smac
iver/Myosin20II.htmgt.