Synthesis of Nucleotides Flashcards
What are the roles of nucleotides?
o Precursors of nucleic acids
o Carriers of energy (ATP and GTP)
o Components of cofactors (NAD, FAD, CoA)
o Initiators of glycogenesis: before glucose can be put in a glycogen chain, it must be
modified by UDP-Glucose (nucleotide)
o Secondary messengers (cyclic AMP)
Who contributed to the synthesis of purines?
John Buchanan
What is a nucleoside?
nitrogenous base + pentose sugar
What is a nucleotide?
nitrogenous base + pentose sugar + phosphate group
What is a nucleic acid?
a series of nucleotides attached in a chain (DNA and RNA)
How are nucleotides linked together?
Two pentose sugars of nucleotides form a phosphodiester bond between the 5’ and 3’ carbon
Differentiate the 5’ and 3’ end of a nucleotide. Which one is available for bonding?
o One end of a nucleotide contains a free phosphate group on the 5’ (5’ end)
o One end of the nucleotide on the 3’ is available for bonding (3’ end)
The phosphate group is added to which carbon of the pentose sugar?
5’ carbon
What are purines? What are the two?
- Two carbon-nitrogen rings
- Guanine and Adenine
What are pyrimidines? What are the three?
- One carbon-nitrogen ring
- Cytosine, Thymine, Uracil
What are the two types of nucleotide synthesis pathways?
- De novo pathway
- Salvage pathway
What is the de novo pathway?
Synthesis of nucleotides from metabolic precursors, such as amino acids, ribose 5-phosphate, CO2, and NH3
What is the salvage pathway?
- Purine and pyrimidine bases are released from the degradation of nucleotides
- Metabolic precursors are not needed for this type of synthesis
Are nucleotides synthesized as dependent or independent molecules?
Dependent (always found as part of an entity), unlike glucose and fatty acids
What are purine rings built upon?
Ribose phosphate
What are pyrimidine rings built upon?
- First synthesized as “orotate”
- Then, attached to ribose pohsphate
Which precursor of nucleotide synthesis is synthesized from the pentose phosphate pathway?
5-phosphoribosyl-1-pyrophosphate (PRPP), which is synthesized from ribose-5-phosphate
Which amino acid precursors provide the carbon backbone in purines? What about pyrimidines?
- Purine: glycine
- Pyrimidine: aspartate
What is the amino group donor in nucleotide synthesis?
Glutamine
How are the levels of the nucleotide pools?
Kept low
What is the first step in the de novo synthesis of purines? Which enzyme catalyzes this step?
- Enzyme: Glutamine-PRPP amidotransferase
- PRPP is combined with Glutamine
In the de novo synthesis of purines, how does the conversion of AIR to CAIR differ between Eukaryotes and Prokaryotes? What causes the difference?
- Eukaryotes: single-step
- Prokaryotes: two-step
- Eukaryotes possess the enzyme AIR carboxylase
What is the end-product of the de novo synthesis of purines? What must it be converted to?
- Inosinate (IMP), which is a monophosphate
- Must be converted to either an Adenylate (AMP) or a Guanylate (GMP)
What provides the amino group when IMP is synthesized into AMP? Where is the amino group added? What provides the energy?
- Amino: Aspartate
- 6th carbon of IMP
- Energy: GTP
Which enzymes are used in the conversion of IMP to AMP?
- Adenylosuccinate synthetase
- Adenylosuccinate lyase, which removes the fumarate carbon backbone
What provides the amino group when IMP is synthesized into GMP? Where is the amino group added? What provides the energy?
- Amino: Glutamine
- 2nd carbon of IMP
- Energy: ATP
Which enzymes are used in the conversion of IMP to GMP?
- IMP dehydrogenase
- XMP-glutamine amidotransferase
How is the de novo synthesis of purines regulated? What causes overall inhibition?
- Negative regulation: end-products inhibit
- Overall inhibition: IMP inhibition
What is the reciprocal inhibition in the regulation of the de novo synthesis of purines?
- GTP limits the synthesis of AMP
- ATP limits the synthesis of GMP
What are the three precursors to the de novo synthesis of pyrimidines?
- Aspartate
- Carbamoyl phosphate
- PRPP
Where is carbamoyl phosphate also located?
In the urea cycle
What is the first step to the de novo synthesis of pyrimidines? What is the enzyme? What does it produce?
- Aspartate reacts with carbamoyl phosphate
- Enzyme: aspartate trans-carbamoylase
- Produces orotate
What synthesizes the carbamoyl phosphate used for the de novo synthesis of pyrimidines?
Cytoplasmic carbamoyl phosphate synthetase II (similar to urea cycle)
What does orotate require to be converted to a nucleotide?
It NEEDS to be build upon PRPP
What is produced once orotate is build upon PRPP?
- UMP –> UTP (uses 2 ATPs)
UTP receives the amino group from what? At which carbon?
- 4th carbon
- Glutamine
Once UTP has received its amino group, what is it converted to? What does it require?
- Requires energy from ATP
- Creates CTP
What is required for the reduction of ribose nucleotides to deoxyribonucleotides? Who donates it?
- Hydrogen atoms
- Donated by NADPH
What are the two ways to donate hydrogen atoms from NADPH to ribonucleotides?
- Glutaredoxin
- Thioredoxin
What does glutaredoxin require?
Glutathione
What does thioredoxin require?
FAD
What is the difference between a ribonucleotide and a deoxyribonucleotide?
OH- is replaced by H at the 2nd carbon of the ribose sugar
What is dTMP derived from?
dCDP and dUMP
Which phase of the pentose phosphate pathway generates PRPP? What else does the phase generate?
- Oxidative phase
- NADPH and ribose-5-phosphate
What does the non-oxidative phase of the pentose phosphate pathway generate?
Regenerates G-6-P from R-5-P
What is the non-oxidative phase of the pentose phosphate pathway used for?
o Used in tissues requiring more NADPH than R-5-P, such as the liver and adipose tissue
o More metabolism means more oxidative stress, for which you need NADPH
What regulates the partitioning of glucose into glycolysis over the pentose phosphate pathway?
NADPH: high NADPH inhibits the pentose phosphate pathway
