A nucleoside is produced when a pentose sugar is added to a base through an N-glycosidic bond. The resulting nucleoside is called a ribonucleoside if the sugar is ribose, and a deoxyribonucleoside if the sugar is 2-deoxyribose. The ribonucleosides of A, G, C, and U are named adenosine, guanosine, cytidine, and uridine, respectively. The deoxyribonucleosides of A, G, C, and T have the prefix "deoxy-" added to their names. The carbon and nitrogen atoms in the base and sugar rings are numbered separately. The pentose sugar atoms are numbered from 1′ to 5′. Adding one or more phosphate groups to a nucleoside produces a nucleotide. The first phosphate group is attached to the 5′-OH of the pentose through an ester linkage, forming a nucleoside 5′-phosphate or a 5′-nucleotide. The type of pentose is indicated by the prefix in the names 5′-ribonucleotide and 5′-deoxyribonucleotide. A nucleoside monophosphate is formed when one phosphate group is attached to the 5′carbon of the pentose. Adding a second or third phosphate group to the nucleoside results in a nucleoside diphosphate or triphosphate. The second and third phosphates are each connected to the nucleotide by a high-energy bond. The negative charges associated with nucleotides are due to the phosphate groups, and this property is what classifies DNA and RNA as nucleic acids. The atoms of the purine ring are derived from various compounds including amino acids, carbon dioxide, and N10-formyltetrahydrofolate. The liver is primarily responsible for constructing the purine ring through a series of reactions that involve adding the donated carbons and nitrogens to a preformed ribose 5-phosphate.
The addition of a pentose sugar to a base through an N-glycosidic bond (see p. 86) produces a nucleoside. If the sugar is ribose, a ribonucleoside is produced, and if the sugar is 2-deoxyribose, a deoxyribonucleoside is produced (Fig. 22.3A). The ribonucleosides of A, G, C, and U are named adenosine, guanosine, cytidine, and uridine, respectively. The deoxyribonucleosides of A, G, C, and T have the added prefix deoxy-(for example, deoxyadenosine). [Note: The compound deoxythymidine is often simply called thymidine, with the deoxy-prefix being understood, because it is incorporated into DNA only.] The carbon and nitrogen atoms in the rings of the base and the sugar are numbered separately (see Fig. 22.3B). [Note: Carbons in the pentose are numbered 1′ to 5′. Thus, when the 5′carbon of a nucleoside (or nucleotide) is referred to, a carbon atom in the pentose, rather than an atom in the base, is being specified.]
C. Nucleotides
The addition of one or more phosphate groups to a nucleoside produces a nucleotide. The first phosphate group is attached by an ester linkage to the 5′-OH of the pentose, forming a nucleoside 5′-phosphate or a 5′-nucleotide. The type of pentose is denoted by the prefix in the names 5′-ribonucleotide and 5′-deoxyribonucleotide. If one phosphate group is attached to the 5′carbon of the pentose, the structure is a nucleoside monophosphate, like adenosine monophosphate (AMP, or adenylate). If a second or third phosphate is added to the nucleoside, a nucleoside diphosphate (for example, adenosine diphosphate [ADP] or triphosphate, for example, ATP) results (Fig. 22.4). The second and third phosphates are each connected to the nucleotide by a “high-energy bond” (a bond with a large, negative change in free energy [–∆G, see p. 70] of hydrolysis). [Note: The phosphate groups are responsible for the negative charges associated with nucleotides and cause DNA and RNA to be referred to as nucleic acids.]
III. PURINE NUCLEOTIDE SYNTHESIS
The atoms of the purine ring are contributed by a number of compounds, including amino acids (aspartate, glycine, and glutamine), carbon dioxide (CO2), and N10-formyltetrahydrofolate (N10-formyl-THF), as shown in Figure 22.5. The purine ring is constructed primarily in the liver by a series of reactions that add the donated carbons and nitrogens to a preformed ribose 5-phosphate. [Note: Synthesis of ribose 5-phosphate from glucose 6-phosphate by the pentose phosphate pathway is discussed on p. 147.]