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Introduction

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High-Yield Terms

  • Nucleoside: refers to the complex of nonphosphorylate ribose sugar and a nucleobase such as purine or pyrimidine

  • Nucleotide: refers to the complex of phosphorylated ribose sugar and a nucleobase such as purine or pyrimidine

  • Phosphodiester bond: the bond formed when the phosphate of 1 nucleotide is esterified to the hydroxyls of 2 ribose sugars typical in polynucleotides

  • Phosphoanhydride bond: an anhydride is a bond formed between 2 acids, a phosphoanhydride is the bond formed from 2 phosphoric acids such as with the β and γ phosphates of di- and triphosphate nucleosides, respectively

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High-Yield Concept

Chemically the nucleotides are basic compounds, which is the reason for the common term “bases” in the context of DNA and RNA. There are 5 major bases found in cells. The derivatives of purine are called adenine and guanine, and the derivatives of pyrimidine are called thymine, cytosine, and uracil. The common abbreviations used for these 5 bases are, A, G, T, C, and U, respectively.

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As a class, the nucleotides may be considered one of the most important metabolites of the cell. Nucleotides are found primarily as the monomeric units comprising the major nucleic acids of the cell, RNA, and DNA. However, they also are required for numerous other important functions within the cell.

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  1. They serve as energy stores for future use in phosphate transfer reactions. ATP predominantly carries out these reactions.

  2. They form a portion of several important coenzymes such as NAD+, NADP+, FAD, and coenzyme A.

  3. They serve as mediators of numerous important cellular processes such as second messengers in signal transduction events. The predominant second messenger is cyclic-AMP (cAMP), a cyclic derivative of AMP formed from ATP.

  4. They control numerous enzymatic reactions through allosteric effects on enzyme activity.

  5. They serve as activated intermediates in numerous biosynthetic reactions. These activated intermediates include S-adenosylmethionine (S-AdoMet or SAM) involved in methyl transfer reactions as well as the many nucleotide-activated sugars involved in glycogen and glycoprotein synthesis.

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Nucleoside and Nucleotide Structure and Nomenclature

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The nucleotides found in cells are derivatives of the heterocyclic highly basic compounds, purine, and pyrimidine (Figure 4-1).

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FIGURE 4-1:

Purine and pyrimidine. The atoms are numbered according to the international system. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper's Illustrated Biochemistry, 29th Edition, Copyright 2012, New York: McGraw-Hill.

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The purine and pyrimidine bases (Table 4-1) in cells are linked to carbohydrate and in this form are termed nucleosides. The nucleosides are coupled to d-ribose or 2′-deoxy-d-ribose through a β-N-glycosidic bond between the anomeric carbon of the ribose and the N9 of a purine or N1 of a pyrimidine. The carbon atoms of the ribose present in nucleotides are ...

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