ADP and ATP
It is possible for the nucleotides (which are composed of a base, a sugar unit, and a phosphate unit) to combine with additional phosphate units (or phosphoric acid units) to make diphosphates and triphosphates.
As indicated, this reaction is usually further abbreviated with these symbols: AMP for adenosine monophosphate, Pi indicates an inorganic phosphate unit, and ADP for or adenosine diphosphate. Quite often, in the equation for this reaction, the water molecule is left out. This is unfortunate because it tends to imply that this is just a combination reaction, instead of a dehydration reaction. On the other hand, it is much simpler to write and does show conversion of the adenosine monophosphate plus an inorganic phosphate unit to form adenosine diphosphate. However, it is a dehydration reaction whether the water molecule is shown or not.
NAD and FAD
I think it's worthwhile to note that the adenosine diphosphate can combine with things other than inorganic phosphates to form very useful biological compounds.
The NAD that we talked about in an earlier lesson as being one of the very important chemicals in transporting or transferring hydrogens and electrons in respiration contains an adenosine diphosphate group bonded to some other molecular portions. (The structure for this is shown in your textbook.)
FAD is another hydrogen and electron transporter in the respiration process. It also is a derivative adenosine diphosphate. (The structure for this is shown in your textbook.)
Another important derivative of ADP is coenzyme A, which was also very important in the citric acid cycle and elsewhere. (The structure for this is not shown in your textbook, but the assempbly of parts is shown there.)
Other Di- and Tri-Phosphates
These reactions are endothermic, energy must be supplied to make these bonding rearrangements take place. This bonded arrangement of atoms in the reactants is more stable and has less energy than this bonded arrangement of atoms in the products.
The tendency for these reactions to occur and provide or release energy is used by a number of enzymes to catalyze reactions which require energy. Muscle contraction is an important and common example.
This enzymatic use of an exothermic reaction, like the hydrolysis of ATP to drive an endothermic reaction, one that requires energy, is called coupling. Coupling with the hydrolysis of ATP is a very typical way in which enzymes can cause endothermic reactions to occur.
E-mail instructor: Sue Eggling
Clackamas Community College