Containing high energy phosphate linkages, nucleotides have high internal energy. Many biochemical processes are coupled with cleavage of ATP, sometimes GTP, imparting negative free energy to the process. Furthermore, nucleotides are thermodynamically activated precursors of not only DNA and RNA but other compounds as well. UDP-glucose and CDP-diacylglycerol are precursors of glycogen and phosphoglycerides, for example. Nucleotides are important building blocks of the coenzymes NAD+, FAD, and CoA. Finally, the nucleotide cyclic AMP is an important second messenger.