Transcript The Use of the Energy in ATP for Muscle Contractions

The Use of the Energy in ATP for
Muscle Contractions
Nitya Anand
Melissa Donaldson
Leigh McDonald
Chris Smyre
What is ATP?
ATP: Adenosine Tri-phosphate
Adenine
Ribose
3 phosphate groups
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.
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)
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.mcgrawhill.com/sites/0072495855/student_view0/chapter10/
animation__breakdown_of_atp_and_crossbridge_movement_during_muscle_contraction.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.
Mechanism of ATP Hydrolysis
ATP + H2O ↔ ADP + Pi
Thermodynamics of ATP Hydrolysis
Theoretically, based on bond energies:
-A°=ΔG°=-30.5 kJ/mol at 25 °C, 1 atm
To find ΔG 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 ΔG~-50 kJ/mol
Reaction Coupling
Reactions with positive ΔG can be coupled with reactions with negative ΔG
Allows the cell to undergo reactions that are not thermodynamically favorable
Example: formation of glutamine from glutamic acid
Glutamic acid + NH3 → glutamine
ΔG=+14.2 kJ/mol
In the presence of ATP:
Glutamic acid + ATP → glutamyl phosphate + ADP
Glutamyl phosphate + NH3 → glutamine + Pi
Total ΔGrxn= -16.3 kJ/mol
Application of this during muscle contraction: energy from ATP hydrolysis
induces conformation change in myosin
Binding of ATP and Conformational
Change in Myosin
Myosin head can assume two
different structural
conformations
EA for the change from low
energy to high energy
conformation is provided by
ATP hydrolysis
Relative free energy (ΔG)
Pre-powerstroke
EA
Rigor state
Reaction progress/distance
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
Will be > 0
• Spontaneous Reactions are Irreversible
• Entropy production Similar to a Carnot cycle
in ATP hydrolysis process
d eS
diS>0
Irreversibility of ATP cleavage
• 4 forms of energy:
–
–
–
–
Gibb’s Energy
Helmoltz’s Energy
Enthalpy Energy
Total Energy
• Energy is always being minimized
• Gibb’s Energy is the easiest to measure
T & P held constant
• We know that ATPADP drop in Gibb’s energy
Conclusion
• ATP hydrolysis is important in biological
processes such as the actomyosin crossbridging 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.
References
“Animation: Breakdown of
ATP and Cross-Bridge Movement During Muscle Contraction.”
McGraw-Hill Higher Education. The McGraw-Hill Companies.
<http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animatio
n__breakdown_of_atp_and_cross-bridge_movement_during_muscle_contraction.html>
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
<http://www.bms.ed.ac.uk/research/others/smaciver/Myosin%20II.htm>.