Nucleosides, Nucleotides, and Nucleic Acids

• Nucleosides and Nucleotides are major derivatives of Purine and Pyrimidine bases
• Nucleosides and Nucleotides are the building blocks of nucleic acids
• Two main types of Nucleic Acids exist; Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA)
• A nucleotide is the building block for nucleic acids
• Nucleotides comprise a Nitrogenous base, Pentose sugar and Phosphate group

Nucleic Acid (polynucleotide) Components

• A nucleotide is made of a nucleoside and a phosphate
• A nucleoside is made out of a nitrogenous base and a pentose sugar

Nitrogenous Bases

• Nitrogenous bases are heterocyclic ring compounds made of carbon and nitrogen
• Two classes of nitrogenous bases exist, based on abundance in cells

Major Bases

• Major bases exist in large amounts
• Major bases include purines, which are adenine and guanine
• Adenine and guanine are found in both DNA and RNA
• Major bases include pyrimidines, which are cytosine, thymine, and uracil
• Cytosine is found in both DNA and RNA
• Thymine occurs only in DNA
• Uracil occurs only in RNA

Minor Bases

• Minor bases are present in trace amounts and are always methylated
• Examples of minor bases include 5-methyl cytosine and N7-methylguanine
• Minor bases also serve important functions
  • Oligonucleotide recognition
  • Regulating the half-life of RNAs
  • Protecting host DNA from digestion by DNAases
• Some bases are present in free state (e.g. xanthine, hypoxanthine, and uric acid)
  • Methylated xanthine can be found in caffeine, theophylline, and theobromine

Nucleosides

• Nucleosides are made of a base and a sugar (ribose or deoxyribose)
• Sugars and bases bind via a covalent, B-N-glycosidic bond
  • This links the C1 of sugar with N-9 of a purine or N-1 of a pyrimidine
• Numbering sugar atoms uses a prime (e.g. 3' or 5') to differentiate sugar atoms from those of the base

Nucleotides

• Nucleotides are composed of a base, sugar, and phosphate group
• The phosphate group attaches to the C-5 of the sugar

Tautomerism

• Purines and pyrimidines exist in two isoforms
  • Keto (or lactam) or enol (lactim) form
  • Amino (-NH2) or imino (=NH) form
• Tautomerism is the change from one form to the other
• At physiological pH the keto and amino forms are common

Syn and Anti-Conformers

• Nucleotides exist as stable non-interconvertible syn and anti conformers
• The B-N-glycosidic bond is a very firm bond
  • Because of this, there is no freedom of rotation about this bond
  • Interconversion from one form to another only occurs by rupturing and reforming the glycosidic bond
• The anti-conformer predominates

Bases, Nucleosides, and Nucleotides Nomenclature

• Adenine (A) becomes adenosine, and adenosine monophosphate (AMP)
• Guanine (G) becomes guanosine, and guanosine monophosphate (GMP)
• Xanthine (X) becomes xanthosine, and xanthosine monophosphate (XMP)
• Hypoxanthine (I) becomes inosine, and inosine monophosphate (IMP)
• Cytosine (C) becomes cytidine, and cytidine monophosphate (CMP)
• Uracil (U) becomes uridine, and uridine monophosphate (UMP)
• Thymine (T) becomes thymidine, and thymidine monophosphate (TMP)

Functions of Nucleotides

• Purine and pyrimidine nucleotides enter the structure of nucleic acids such as DNA and RNA
• Purine nucleotides enter:
  • High energy stores like ATP and GTP
  • Intracellular signals (cAMP and cGMP act as 2nd messengers for many hormones)
  • Structures of many coenzymes like NAD, NADP, FAD, and FMN (hydrogen carriers), and COASH (acid carrier)
  • Active methyl donor (S-adenosyl methionine, SAM)
  • Active sulfate donor (3'phosphoadenosine-5'-phosphosulfate, PAPS)
  • Regulate the rate of oxidative phosphorylation in mitochondria (e.g., ADP levels)
• Pyrimidine nucleotides:
  • Enter the structure of high energy intermediates (Uridine diphosphate glucose (UDP-glucose) used in glycogenesis and uronic acid pathway)
  • High energy stores like CTP and UTP

Learn about nucleosides, nucleotides, and nucleic acids, the building blocks of DNA and RNA. A nucleotide comprises a nitrogenous base, pentose sugar, and phosphate group. Nitrogenous bases are classified as purines (adenine and guanine) or pyrimidines (cytosine, thymine, and uracil).