Proteins and Nucleic Acids: Information Carriers
Nucleotides and Nucleic Acids
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.