Lecture 3: Thermodynamics of Phosphate compounds Flashcards
What is the long name for Pi?
Orthophosphate (PO4^3-)
What is the long name for PPi?
Pyrophosphate (P2O7^4-)
Give an examples of another type of high energy compound.
acyl phosphates
What is a high energy bond?
A bond that is more negative than -25 kJmol-1
What are the phosphoanhydride bonds in ATP high energy?
1) Resonance stabilisation less in phosphoanhydride bond than in hydrolysis product
2) Greater electrostatic repulsion in phosphoanhydride compared to hydrolysis product
3) Smaller solvation energy of a phosphoanhydride bond compared to hydrolysis product.
Mostly it is thought that reasons 1 and 2 are most important, but some scientists argue that reason 3 is most important.
Give 4 examples of free energy sources/shuttles.
- phosphate anhydrides
- enoyl phosphates
- some thioesters
- compounds containing N-P bonds
What does PEP stand for?
Phosphoenolpyruvate
What is the Gibbs free energy of PEP –> Pyruvate + Pi?
-62 kJmol-1 (favourable)
What is the Gibbs free energy of ATP synthesis?
+31 kJmol-1 (unfavourable)
Is PEP or ATP a high energy compound? Explain using coupled reactions involving the two.
Phosphoenolpyruvate is a higher energy compound than ATP, so transfer of the phosphate from PEP can synthesise ATP.
The coupled reaction is
PEP + ADP –> pyruvate + ATP delta G = -31 kJmol-1 (favourable)
What is the phosphoryl transfer potential measured against (as a reference)?
The hydrolysis reaction with water (phosphoryl transfer to water), which is always a thermodynamically favourable process (delta G = negative), no matter which molecule provides the phosphate group.
Relate the phosphoryl transfer potential to the delta G of the reference hydrolysis reaction for that molecule.
The greater the phosphoryl transfer potential of a compound with a phosphate group, the more energetically favourable is its hydrolysis reaction (more negative delta G).
Where on the scale is the phosphoryl transfer potential of ATP and how is this important?
Intermediate, in the middle of the scale. This is important because this means that ATP can donate phosphate to lower energy compounds and can accept phosphate from higher energy compounds. The highly exergonic phosphoryl transfer reactions of nutrient degradation are coupled to formation of ATP.
Give the scale of phosphoryl transfer potentials.
- 62 PEP
- 49 1,3-bisphosphoglycerate
- 43 Creatine phosphate
- 30 ATP
- 33 PPi
- 4 AMP
- 10 Glycerol-1-phosphate
Is the hydrolysis of acetyl CoA favourable or unfavourable?
Favourable. Delta G = -32 kJmol-1
Give the ways the ATP is consumed.
- early stages of sugar breakdown producing low energy phosphate compounds (glycolysis, glucose + ATP –> Glucose-6-phosphate)
- interconversion of nucleoside triphosphate:
ATP + NDP ADP + NTP (where N is a nucleoside) catalysed by nucleoside diphosphate kinase (no base specificity - can be A, G, T, C, U) - physiological processes such as muscle contraction and active transport against concentration gradient
- additional phosphoanhydride cleavage:
ATP —> AMP + PPi, then PPi –> 2 Pi. This provides extra energy for fatty acid acetyl CoA synthesis and nucleic acid synthesis
What is the adenylate kinase (myokinase) reaction?
Normally ATP ADP + Pi, but there is a notable exception called the adenylate kinase reaction. In this reaction 2 ADP ATP + AMP.
This means that when muscles are contracting rapidly and using lots of ATP and producing lots of ADP, the adenylate kinase reaction goes to the right, using up ADP and replenishing half the ATP. The reaction also produces AMP, dramatically increasing the concentration of AMP in the cell. AMP is an activator of energy producing pathways, by allosteric activation and by activating cAMP-dependent protein kinase, which regulates many metabolic pathways. AMP acts as signal telling the cell that energy levels are very low and prompts nutrient oxidation to replenish ATP supplies.
When ATP is replaced, myokinase reaction moves to the left, AMP concentration falls and the signal is turned off.
Comment on the rate of ATP turnover.
It is very rapid. ATP is a free energy transmitter not a reservoir. The ATP pool can supply energy for about 1 minute.
What is the function of phosphocreatine?
Phosphocreatine provides a high energy reservoir for ATP formation.
ATP + creatine —> Phosphocreatine + ADP (Delta G = +12.6 kJmol-1)
We store phosphocreatine and hydrolyse it to produce ATP when ATP levels are low.
