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NUCLEOTIDES / 287NH 2NH 2OONNNHNNHNN9NO1NH 2 NN9NO1NHOOHOOHOOHOOOHOHOHOHAdenosine Cytidine GuanosineUridineFigure 33–3.Ribonucleosides, drawn as the syn conformers.OHOHOHOHNNH 2NNNAdenineMononucleotides are nucleosides with a phosphorylgroup esterified to a hydroxyl group of the sugar. The3′- and 5′-nucleotides are nucleosides with a phosphorylgroup on the 3′- or 5′-hydroxyl group of the sugar,respectively. Since most nucleotides are 5′-, the prefix“5′-” is usually omitted when naming them. UMP anddAMP thus represent nucleotides with a phosphorylgroup on C-5 of the pentose. Additional phosphorylgroups linked by acid anhydride bonds to the phosphorylgroup of a mononucleotide form nucleosidediphosphates and triphosphates (Figure 33–4).Steric hindrance by the base restricts rotation aboutthe β-N-glycosidic bond of nucleosides and nucleotides.Both therefore exist as syn or anti conformers(Figure 33–5). While both conformers occur in nature,anti conformers predominate. Table 33–1 lists themajor purines and pyrimidines and their nucleosideand nucleotide derivatives. Single-letter abbreviationsare used to identify adenine (A), guanine (G), cytosine(C), thymine (T), and uracil (U), whether free or presentin nucleosides or nucleotides. The prefix “d”(deoxy) indicates that the sugar is 2′-deoxy-D-ribose(eg, dGTP) (Figure 33–6).Nucleic Acids Also ContainAdditional BasesSmall quantities of additional purines and pyrimidinesoccur in DNA and RNAs. Examples include 5-methylcytosineof bacterial and human DNA, 5-hydroxymethylcytosineof bacterial and viral nucleic acids, andmono- and di-N-methylated adenine and guanine ofHOO –POOOPO –OOO –POCH 2OHO OHRiboseNNH 2NNNNNNH 2NNAdenosine 5′-monophosphate (AMP)HOOHOOAdenosine 5′-diphosphate (ADP)Adenosine 5′-triphosphate (ATP)Figure 33–4. ATP, its diphosphate, and itsmonophosphate.SynOHOHAntiFigure 33–5. The syn and anti conformers of adenosinediffer with respect to orientation about the N-glycosidicbond.OHOH