Metabolism of Nucleotides

Vocabulary flashcards covering major terms and enzymes involved in nucleotide structure, biosynthesis, salvage, regulation, and degradation.

Nucleotide

A molecule consisting of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and one or more phosphate groups—basic unit of nucleic acids.

Nitrogenous Base

Heterocyclic purine or pyrimidine ring that provides the informational component of a nucleotide.

Purine

Two-ring nitrogenous base (adenine or guanine); attaches to sugar through N-9.

Pyrimidine

Single-ring nitrogenous base (cytosine, thymine, uracil); attaches to sugar through N-1.

Adenine (A)

Purine base with an amino group at C-6; pairs with thymine or uracil.

Guanine (G)

Purine base with a ketone at C-6 and amino group at C-2; pairs with cytosine.

Cytosine (C)

Pyrimidine base containing an amino group at C-4; pairs with guanine.

Thymine (T)

5-methyluracil; pyrimidine found in DNA that pairs with adenine.

Uracil (U)

Pyrimidine found in RNA that pairs with adenine.

D-Ribose

Pentose sugar with a 2'-OH group; component of RNA nucleotides.

2'-Deoxyribose

Pentose sugar lacking a 2'-OH; component of DNA nucleotides.

Nucleoside

A nitrogenous base linked to a sugar without phosphate (e.g., adenosine).

N-Glycosidic Linkage

C1'–N bond connecting the sugar to N-9 of purines or N-1 of pyrimidines.

Phosphoribosyl Pyrophosphate (PRPP)

Activated ribose donor (ribose-5-phosphate + ATP) used in de novo and salvage nucleotide synthesis.

Inosine Monophosphate (IMP)

First fully formed purine nucleotide; branch point precursor for AMP and GMP.

De Novo Purine Synthesis

Formation of purine nucleotides from amphibolic intermediates rather than recycled bases.

Salvage Pathway

Re-conversion of free purine bases or nucleosides into mononucleotides, conserving energy and bases.

PRPP Synthetase

Enzyme that forms PRPP from ribose-5-phosphate and ATP; activated by Pi, inhibited by AMP, GMP, IMP.

PRPP Glutamyl Amidotransferase

First committed step of purine biosynthesis; transfers glutamine amide nitrogen to PRPP; feedback-inhibited by AMP and GMP.

Adenylosuccinate Synthase

Catalyzes IMP → adenylosuccinate in AMP pathway; inhibited by AMP; requires GTP.

IMP Dehydrogenase

Catalyzes IMP → xanthosine monophosphate (XMP) in GMP pathway; inhibited by GMP; requires NAD⁺.

Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)

Salvage enzyme that converts hypoxanthine or guanine plus PRPP to IMP or GMP; inhibited by IMP/GMP.

Adenine Phosphoribosyltransferase (APRT)

Salvage enzyme that converts adenine + PRPP to AMP.

Adenosine Kinase

Enzyme that directly phosphorylates adenosine to AMP using ATP.

Ribonucleotide Reductase

Catalyzes reduction of ribonucleoside diphosphates to deoxyribonucleoside diphosphates for DNA synthesis.

Thioredoxin

Small protein that donates reducing equivalents to ribonucleotide reductase; regenerated by thioredoxin reductase and NADPH.

Xanthine Oxidase

Liver molybdoflavoprotein that converts hypoxanthine → xanthine → uric acid, producing H₂O₂.

Uric Acid

End product of human purine catabolism; excreted in urine.

Uricotelic

Organisms (birds, reptiles, amphibians) that excrete nitrogen mainly as uric acid.

Ureotelic

Organisms (e.g., humans) that excrete most nitrogen as urea rather than uric acid.

Purine Catabolism

Degradation pathway where AMP/GMP → hypoxanthine/guanine → xanthine → uric acid; converges at xanthine.