Molecular Biology Flashcards
Nucleotides are composed of three subunits:
- Pentose sugar
- Ribose
- Deoxyribose - Nitrogenous base
- Purine
- Pyrimidine - Phosphate group: Forms the linkages between nucleotides.
How are nucleotides linked?
By a 3′–5′ phosphodiester bond
What are effective carriers?
Effective carriers are molecules that are relatively stable as leaving groups. These include phosphoryl (ATP), electrons (NADH, NADPH, FADH2), sugars (UDP glucose), methyl (SAM), 1 carbon (THF), CO2 (biotin), and acyl (coenzyme A).
Which bond increases the melting temperature?
G-C bonds (3 H-bonds) are stronger than A-T bonds (2 H-bonds). Increased G-C content increases the melting temperature (Tm), which is the
temperature at which half of the DNA base-pair hydrogen bonds are broken.
Purine nucleotide synthesis pathways
De novo or salvage
- De novo synthesis utilizes elemental precursors and is used primarily for rapidly dividing cells.
- The salvage pathway recycles the nucleosides and nitrogenous bases that are released from degraded nucleic acids; it is considered the major route for synthesis in adults.
Purine analogs that inhibit PRPP amidotransferase
Allopurinol and 6-mercaptopurine
PRPP
5-phosphoribosil-1-pyrophosphate
Purine de novo synthesis
- Rate-limiting step by glutamine PRPP amidotransferase
- PRPP amidotransferase is inhibited by downstream products (IMP, GMP, AMP) and purine analogs
- Required cofactors: tetrahydrofolate, glutamine, glycine, aspartate
Reciprocal substrate effects in purine de novo synthesis
GTP and ATP are substrates in AMP and GMP synthesis, respectively. For example, ↓ GTP → ↓ AMP → ↓ ATP.
This allows for balanced synthesis of adenine and guanine nucleotides.
Purine Salvage pathway
-Recycles ~90% of the preformed purines that are released when cells’ nucleases degrade endogenous DNA and RNA and make new purine nucleotides.
-Catalyzed by hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which is inhibited by IMP and GMP.
-Nitrogenous base (guanine, hypoxanthine) + PRPP → GMP/IMP + PPi
.
What is Lesch-Nyhan Syndrome?
X-linked recessive disorder of failed purine salvage due to the absence of HGPRT. HGPRT converts hypoxanthine → IMP and guanine → GMP. The inability to salvage purines leads to excess purine synthesis and consequent excess uric acid production.
Lesch-Nyhan Syndrome Presentation
Retardation, cerebral palsy, self-mutilation, aggression, gout, choreoathetosis, arthritis, nephropathy
Lesch-Nyhan Syndrome Diagnosis
Orange crystals in diaper, difficulty with movement, self-injury, hyperuricemia
Lesch-Nyhan Syndrome Treatment
Allopurinol, which inhibits xantine oxidase. Treatment does not ameliorate neurologic symptoms
Lesch-Nyhan Syndrome Prognosis
Urate nephropathy, death in the first decade, usually as a result of renal failure
What is Gout?
Disorder associated with hyperuricemia, due to either overproduction or underexcretion of uric acid. Uric acid is less soluble than hypoxanthine and xanthine, and, therefore, sodium urate crystals deposit in joints and soft tissues, leading to arthritis.
Primary gout?
Due to hyperuricemia without evident cause. Affected individuals may have a familial disposition. May occur in association with PRPP synthetase hyperactivity or HGPRT deficiency of Lesch-Nyhan syndrome; most common form.
Secondary (acquired) gout?
Uric acid overproduction can be caused by leukemia, myeloproliferative syndrome, multiple myeloma, hemolysis, neoplasia, psoriasis, and alcoholism and is more common in men. Secondary gout due to urate underexcretion can be caused by kidney disease and drugs such as aspirin, diuretics, and alcohol.
