purine nucleotides and phosphocreatine. free energy Δg = Δh – tΔs –g=free energy –h=...
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Purine nucleotides and phosphocreatine
Free energy• ΔG = ΔH – TΔS
– G=free energy– H= enthalpy (heat energy)– T= temperature– S=entropy
• ΔG– Difference in free energy between the
reactants and the products– When direction of Rx is as follows
• ATP→ ADP + Pi• ΔG is negative
• Because we are losing usuable energy
ATP and PCr
• Free energy released from the breakdown of CHO and fats– Stored in “high-energy”
phosphates• ATP, PCr• Enzymes break down both
these compounds to:– ATP: perform cellular work
» ATP↔ADP + Pi» ATPase (ATP kinase)
– PCr: resynthesize ATP» PCr +ADP↔ATP+Cr» Creatine kinase
– Can also use the ADP» ADP + ADP ↔ ATP + AMP» Adenylate kinase
ATP• ATP
– Adenine, ribose and 3 phosphate groups
– Used for almost all energy requiring reactions in the body
– Large change in free energy when Pi is cleaved by ATPase
– Enough stored to fuel ~2s of maximal effort
– Why don’t ATP levels fall?• PCr
– Acts as temporal (filling the time until mitochondria comes fully online) buffer of ATP concentrations
ATP & PCr
• PCr– ~3-4 times as much as
ATP• So, now we have ~8s of
maximal activity• PCr + ADP ↔ ATP + Cr
• This reaction occurs faster than the ATPase reaction
– Thus, ATP does not fall
– Cr and PCr are more mobile within the cell than ATP
• Thus, may also act as a spatial buffer of ATP concentrations
•Note how PCr falls and recovers with about the same rate
•Time constant: ~30-60s
•PCr recovery is dependent upon
•Oxygen delivery•pH
•ATP+Cr ↔ ADP + PCr + H+
•Note how resynthesis of PCr acidifies the cell
ATP and PCr in recovery from work
Muscle adenylate pool
• Energy charge=
• Indication of the energy status of the cell– Ability to perform work
• Energy charge at rest– Close to 1
• Typically, 0.9-0.95
• Energy charge at complete exhaustion– Close to 0.75
Adenylate pool• Changes in the energy
charge– Dictate how fast ATP
resynthesis occurs• Lower energy charge, faster ATP
resynthesis (1)• Accelerates all ATP providing Rx• If energy charge gets low enough
(2)– Fatigue
• Rate of ATP production and utilization
– Same where lines intersect– Note that energy charge is usu above
this level
– Increased ADP, AMP and Pi stimulate ATP production
– Increased ATP inhibits these Rx
1
2
Adenine nucleotide loss• Adenine nucleotide
– ATP, ADP, AMP– Adenylate kinase Rx
• ATP + AMP ↔ ADP + ADP
• Maintains [ATP]
• Build up of AMP limits this Rx
– Convert AMP• IMP or adenosine
• IMP and Inosine– Can be re-converted to AMP or leaves the body as uric
acid– Mostly seen in type II muscle fibers– Allows AK Rx to occur, maintaining high ATP
concentrations
5-nucleotidase
Purine nucleotide cycle• Occurs in cytoplasm
– AMP deaminase• AMP to IMP• Ammonia formation• Activated by decrease in energy
charge
– Adenylosuccinate synthetase• IMP to adenylosuccinate• Loss of Pi
– Adenylosuccinate lyase• Adenylosuccinate to AMP• Fumarate produced
• Purpose of cycle– Maintenance of cellular energy
charge– Recycling of adenylate pool
Reamination in the PNC• Adenylosuccinate
synthetase and lyase– Reamination occurs in
recovery– Fumarate is produced
• Helps maintain the Kreb’s cycle (6th step)
– Ammonia is produced• Excreted in urine
• Deamination: – Helps maintain energy
charge during contractions
• Reamination:– Replenished muscle
adenylate pool
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