Organic Reaction Mechanisms: We can categorize all common biological reactions into four groups:
Look at ATP. In the figure the bolded region is the "recognition" part of the molecule, while the polyphosphate is the chemically active portion. Each of the phosphoric acid anhydride bonds is unstable. That is hydrolyzing either will release a lot of energy. [Figure 10.12]
So why ATP? First, we want a compound with intermediate hydrolysis energy so it can pick up energy from some reactions and deliver to others. Second we want a kinetically stable molecule which is thermodynamically unstable. Thus acetic acid anhydride would not work: it is thermodynamically unstable to hydrolysis, but it is also kinetically unstable, with the carbonyl carbons wide open to water attack. Phosphoric acid anhydride is equally unstable, but is is sterically protected from water attack - in order to react quickly we need a catalyst - perfect.
ATP is sometimes referred to as a "Hi Energy" compound. High energy in this case does not refer to total energy in compound, rather just to energy of hydrolysis. Thus ATP is unstable to hydrolysis, or has a large negative deltaG for hydrolysis. For biochemistry High Energy is defined in terms of ATP: if a compound's free energy for hydrolysis is equal to or greater than ATP's then it is "High Energy," if its free energy of hydrolysis is less than ATP's then it is not a "hi energy" compound. Note that ATP has two hi energy anhydride bonds.[Table10.4]
You should memorize the structures for ATP, ADP, & AMP.
Glycolysis is going to be our first pathway, and it is arguably the most important and universal of the metabolic pathways. Thus we will spend extra time on it, exploring it in some detail from a variety of perspectives. But before we begin glycolysis let's take a brief look at how glucose (and carbohydrate generally) gets to the tissue from food intake.
© R. A. Paselk 2010;
Last modified 11 March 2013