Nucleotide Variations
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Nucleotide Variations

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.

The formulas have been abbreviated here and the phosphate is on the other side, but this is intended to show that the adenosine bonded to one phosphate can combine with another phosphate (or phosphoric acid) to form an adenosine that's bonded to two phosphates. In the process, a water molecule is lost, so this is a dehydration reaction.

Equation for the reaction of AMP with P to form ATP. [69017.jpg]

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.

The adenosine with two phosphate can in turn combine with a third phosphate group to form adenosine with three phosphates and give off a water molecule, so again, this is a dehydration reaction. This reaction is often abbreviated as adenosine diphosphate (ADP) combining with and inorganic phosphate (Pi) group to form adenosine triphosphate (ATP). Again, this is a dehydration reaction whether or not the H2O is shown.

Equation for the reaction of ADP with P to form ATP. [69018.jpg ]

 

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

The other nucleotides, as well as adenosine, can take on additional phosphate units to form not only the monophosphate but also the diphosphate and the triphosphate.
AMP CMP GMP TMP UMP
ADP CDP GDP TDP UDP
ATP CTP GTP TTP UTP

Phosphorylation Reactions

These dehydration reactions, or condensation reactions as they are sometimes called, can also be viewed as phosphorylation reactions because they involve attaching a phosphate unit to an existing molecule.

Equation for the reaction of ADP with P to form ATP. [69018.jpg]

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.

Consequently, the reverse reactions are exothermic. The hydrolysis or dephosphorylation reactions of these multiple phosphate nucleotides releases energy.

Equation for the reaction of ADP with P to form ATP also showing the reverse hydrolysis reaction. [69023.jpg]

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.

 

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