The unusually high hydrolysis energy that makes ATP useful as an energy-storage molecule also is found in simple polyphosphates, without the ribose ring and adenine. Some bacteria store energy in the form of polyphosphates. Then why bother with the complication of the adenosine "handle" on the triphosphate? The most probable answer is that these reactions are controlled by being carried out at the surface of enzyme molecules, and the adenosine is indeed a handle by which the enzyme molecule recognizes and binds an ATP molecule so it can undergo reaction.

Nucleoside diphosphates also are important carriers of chemical free energy in the form of reducing power in oxidation-reduction reactions, as we shall see in the next chapter. The standard pattern is a combination of a nucleoside diphosphate with a molecule capable of being oxidized and reduced. In nicotinamide adenine dinucleotide, NAD⁺ (first diagram), the reducible group is an amide of nicotinic acid; and in flavin adenine dinucleotide, FAD (2nd diagram), it is a molecule of riboflavin. These shuttle molecules are needed only in minute amounts because they are reduced at one place and reoxidized elsewhere.