Chapter 13, part I


The hydrolysis of ATP is associated with a large negative standard free energy change(-30.5 kJ/mol).  Give three reasons for this--be able to draw the reactions and structures that  contribute to this. (Clues: resonance, relief of charge repulsion and ionization/mass action)

Why is delta G' different from delta G for the hydrolysis of ATP?  Box 14-2 calculates delta G for this reaction under physiological conditions.  Given the values in table 14-5, you should be able to calculate delta G for the hydrolysis of ATP for a given cell type.(problem 12)

Thioesters etc.

Be able to explain (and draw the relevant structures for) why the following compounds have large negative free energy changes: thioesters, phosphoenolpyruvate, phosphocreatine.   Why does the hydrolysis of methyl acetate have a smaller negative delta G than the value for the hydrolysis of acetyl CoA.

Four reasons for more stable products

  1. electrostatic repulsion is relieved
  2. products are stabilized by ionization (releasing H+ into a medium with a low H+ concentration)
  3. products are stabilized by tautomerization or isomerization
  4. products are stabilized by resonance structures (delocalization of charges)

Be able to give a reaction as an example for each of the above reasons.

Energy due to group transfers

Most processes do not involve the direct hydrolysis of ATP to provide energy for a process to proceed.  Most processes involve the transfer of a phosphate from a high energy compound to a lower energy compound.  The coupled reaction is thus exergonic.   However, there are several examples of processes that involve the direct hydrolysis of ATP--be able to give one of these examples.

What is a high energy compound?  Is the energy released when the bond is broken?

Reactions are coupled to transfer phosphates from high energy compounds to lower energy compounds.  For example, the phosphorylation of glucose to make glucose-6-phosphate is endergonic.  This reaction is coupled to the hydrolysis of ATP.  The net reaction is

glucose + ATP-------> Glucose-6-P  + ADP

It can be dissected into:

ATP     -------->  ADP  +  Pi

Glucose + Pi ------>  Glucose-6-P

Given the values for  delta G' for each of the above reactions, see page 495, calculate the  delta G' for the net reaction. 

Figure 13-9 shows that transfers of phosphates from PEP (phosphoenolpyruvate) or Cr-P (phosphocreatine) to ADP are exergonic.  The text also goes through the numbers to show this on page 501.  

Remember that while the hydrolysis of ATP is thermodynamically spontaneous, it does not hydrolyze spontaneously.  ATP is stable in water and will not hydrolyze unless the kinetic barrier (activation energy) is circumvented by an enzyme.  ATP is kinetically stable.

ATP as a donor

ATP transfers phosphates, pyrophosphates and adenylyl groups via what type of mechanism?  Alcohols that are enriched with O18 were used to determine what oxygen formed the linkage between the substrate and the phosphate from ATP.   These experiments determined that P03-2 was transferred, not P04-2.  Based on this information, what is the nucleophile in these reactions?  What is the electrophile? 

What gives the energetic "push" for adenylylation reactions?  (I always drop one of the "yl"s from that word.)

ATP molecules provide energy for assembling macromolecules, they counteract negative entropy! 

How is ATP involved in making proteins and DNA?  Be able to give a specific reaction as an example.

ATP moves muscles and other stuff.

How is ATP involved in transporting stuff across a membrane.  Give an example.   How does ATP move muscles? 

How does ATP make the fireflies light up?  See box 13-2.


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