Nucleotide Structure and Function Quiz

Nucleotide Structure

• Nucleotides consist of a nitrogenous base, a pentose monosaccharide, and one to three phosphate groups.
• Nitrogenous bases are categorized as purines or pyrimidines.
• Pentose monosaccharides are formed from glucose through the hexose phosphate shunt (HMP shunt).

Nucleotide Function

• Nucleotides are crucial for all cellular processes.
• They serve as building blocks for RNA and DNA.
• Nucleotides act as carriers of activated intermediates in the synthesis of carbohydrates, lipids, and conjugated proteins (e.g., UDP-glucose, CDP-choline).
• They are structural components of essential coenzymes (e.g., coenzyme A, FAD, NAD+, NADP+).
• Some nucleotides act as second messengers in signal transduction pathways (e.g., cAMP, cGMP).
• Nucleotides are the cell's "energy currency" (e.g., ATP).
• They contribute to the regulation of various metabolic pathways by activating or inhibiting key enzymes.

Nitrogenous Bases

• Purines: Adenine (A), Guanine (G)
• Pyrimidines:
  • DNA: Cytosine (C), Thymine (T)
  • RNA: Cytosine (C), Uracil (U)

Nucleosides

• A pentose sugar added to a base forms a nucleoside.
• Ribonucleoside: Contains ribose sugar.
• Deoxyribonucleoside: Contains 2-deoxyribose sugar.
• Ribonucleosides for A, G, C, and U are: adenosine, guanosine, cytidine, and uridine, respectively.
• Deoxyribonucleosides for A, G, C, and T have the prefix "deoxy-" (e.g., deoxyadenosine).
• Deoxythymidine is often called thymidine.

Nucleotides

• One or more phosphate groups attached to a nucleoside form a nucleotide.
• The first phosphate group is linked to the 5'-OH of the pentose through an ester linkage.
• This forms a nucleoside 5'-phosphate (5'-nucleotide).
• The type of pentose is denoted by the prefix: "5'-ribonucleotide" or "5'-deoxyribonucleotide."
• One phosphate group forms a nucleoside monophosphate (e.g., AMP).
• Two phosphate groups form a nucleoside diphosphate (e.g., ADP).
• Three phosphate groups form a nucleoside triphosphate (e.g., ATP).
• The second and third phosphate groups are connected by "high-energy" bonds.
• Phosphate groups are responsible for the negative charges of nucleotides, leading to DNA and RNA being referred to as "nucleic acids."

Purine Metabolism

• Purine synthesis does not occur as a ring but as a nucleotide.
• Two pathways contribute to purine synthesis:
  • De novo synthesis pathway
  • Salvage pathway

De novo Synthesis Pathway

• The purine ring is constructed from various compounds including amino acids (aspartic acid, glycine, glutamine), CO2, and N10-formyltetrahydrofolate (derived from folic acid, a B vitamin).
• The purine ring is primarily assembled in the liver.

Synthesis of 5-phosphoribosyl-1-pyrophosphate (PRPP)

• PRPP is an activated pentose involved in purine and pyrimidine synthesis and salvage.
• PRPP synthetase (ribose phosphate pyrophosphokinase) catalyzes the synthesis of PRPP from ATP and ribose 5-phosphate.
• PRPP synthetase is an X-linked enzyme activated by inorganic phosphate.
• It is inhibited by purine nucleotides (GMP, AMP, IMP, allopurinol, 6-mercaptopurine): an example of end-product inhibition.