Nucleotide Metabolism II Flashcards
There is a key interaction between purine metabolism and the TCA cycle via
Fumarate
The interconversion of IMP to adenylosuccinate to AMP and then back to IMP with the release of fumarate
Purine nucleotide cycle
Any reaction that serves to replenish the intermediates of another metabolic process is called an
Anapleurotic reaction
In muscle cells, allows for increased energy production via the TCA cycle by replenishing supplies of fumarate
Purine nucleotide cycle
The parent base is synthesized in its entirety and then added to the PRPP backbone in
De novo pyridine synthesis
What is step 1, and also the regulated step, of pyrimidine synthesis?
Synthesis of Carbamoyl phosphate
Two molecules of ATP, 1 CO2, and glutamine are converted to carbamoyl phosphate by the enzyme
Carbamoyl synthase II (CPS-II)
The enzyme carbamoyl synthase II is activated by
ATP and PRPP
Carbamoyl synthetase II is inhibited by the eventual end product of its pathway, which is
-can then be converted to other pyrimidines
UTP
Note that carbamoyl phosphate is also synthesized in the
Urea cycle
However, in the urea cycle, the key enzyme involved is called
Carbamoyl synthetase I (CPS-I)
Located in the mitochondria
CPS-I
Located in the cytosol
CPS-II
Contributes only two of the atoms that comprise the six-membered pyrimidine ring
Carbamoyl phosphate
The remaining 4 atoms of the pyrimidine ring are supplied by
Aspartate
Carbamoyl phosphate synthetase and teo other enzymes are catalytic domains of one large polypeptide called
CAD
Serves to coordinated the synthesis of pyrimidines in both time and space, minimizing side reactions and ensuring greater efficiency
CAD
After one additional reaction that takes place on the outer face of the inner mitochondrial membrane, we end up with a ringed molecule called
Orotate
Once the pyrimidine base is formed, we attach it to the
Ribophosphate backbone
Orotate is attached to PRPP by the enzyme
Orotate Phosphoribosyl Transferase (OMP)
A precursor to the parent pyrimidine UMP
OMP
OMP is converted to Uridylate (UMP) by the enzyme
Orotidylate decarboxylase
Decreased activity of either orotate phosphoribosyl transferase or orotidylate decarboxylase results in a disease called
Orotic Aciduria
Characterized by megaloblastic anemia as well as large amounts of orotate in the urine
Orotic aciduria
Results from a deficiency of nucleotides in rapidly diviing marrow cells, leading to fewer, larger immature red cells
Megaloblastic anemia
Unlike most causes of megaloblastic anemia, is not corrected by supplementation with folate or vitamin B12
Orotic aciduria
We can treat orotic aciduria and bypass the metabolic block with supplementation of
CMP, UMP, or the base Uridine
UMP is phosphorylated by a nucleoside monophosphate kinase to produce
UDP
UDP is then further phosphorylated by nucleoside diphosphate kinase to form
UTP
The only way to synthesize cytosine nucleotides de novo is to
Convert UTP to CTP
The conversion of UTP to CTP requires the enzyme
CTP synthetase
The conversion of UTP to CTP by CTP synthetase requires
1 ATP and glutamine
Serves to negatively inhibit CTP synthetase via a negative feedback mechanism
CTP
Can be created from a precursor molecule called deoxyuridine monophosphate (dUMP)
Thymine
Thymine can also be formed from the deamination of
Deoxycitidine monophosphate (dCMP)
Required for DNA synthesis
Deoxyribonucleotides
Deoxyribonucleotides are synthesized from ribonucleotides via the enzyme
Ribonucleotide Reductase
Highly expressed in proliferating cells that enter the S-phase of the cell cycle
Ribonucleotide Reductase
In order to catalyze the reaction, ribonucleotide reductase requires
NADPH
Ribonucleotide reductase is inhibited by
dATP
Ribonucleotide reductase is activated by
ATP
Ribonucleotide reductase is inhibited by dATP, which prevents the enzyme from reducing
ADP, GDP, and UDP
Determination of which substrate preferentially binds the enzymes catalytic site at any given time is regulated by the binding of the NTP to an auxillary site on the enzyme known as the
Substrate specificity site
Binding of dATP is only inhibitory if it occupies to the
-not inhibitory when bound to the substrate specificity site
Active site
Patients with ADA have high levels of
dATP
This means that in ADA we see the inhibition of
Ribonucleotide reductase
This blocks the production of all deoxyribonucleotides, which prevents the bone marrow from producing
B and T cells
Synthesis of thymine nucleotides requires the molecule
dUMP
dUMP is formed from the deamintion of
dCMP
dUMP is also formed by the action of a phosphatase on
dUTP
The main method of synthesizing dUMP
Deamination of dCMP
Important in getting rid of excess dUTP to prevent it from getting incorporated into DNA
Phosphorylase activity on dUTP
dUMP is ocnverted to thymidylate by the enzyme
Thymidylate synthase
This reaction catalyzed by thymidylate synthase requires
N,N-methyleneTHF
Covalently inhibits thymidylate synthase
The antitumor agent 5-fluorouracil
To block thymidylate synthase, 5-fluorouracil is converted to
5-FdUMP
Methotrexate and other inhibitors of DHFR prevents the recycling of THF, which blocks
Purine and TMP synthesis
Requires 2 steps. The first step couples the base to a sugar and the second adds a phosphate to the nucleoside
Pyrimidine salvage
In step 1, a pyrimidine base and ribose-1-phosphate are converted to a pyrimidine ribonucleoside and inorganic phosphate by
Nucleoside phosphorylase
These phosphorylases can act on both
Ribose and deoxyribose substrates
A pyrimidine nucleoside is phosphorylated to a pyrimidine nucleoside monophosphate by a
Nucleoside kinase
Salvges the nucleoside thymidine to TMP
Thymidine kinase
Interestingly, the herpes simplex virus encodes its own
Thymidine kinase
This virus uses its own thymidine kinase to augment nucleotide supplies to promote its own replication
Herpes simplex virus
A guanosine analog that interferes with viral replication
Acyclovir
Gets phorphorylated by the viral thymidine kinase and gets preferentially incorporated into viral DNA, resulting in premature DNA chain termination
Acyclovir
The catabolism of pyrimidines leads to
Highly soluble products
CMP and UMP are degraded to
β-alanine
TMP is degraded to
β-‐aminoisobutyrate
The degredation of pyrimidines results in the release of
Ammonia and CO2
The single most important risk factor for developing gout
Serum Urate (SU)
A sustained elevation of SU is essential to developing gout, but is it sufficient to cause the disease?
No
Do the majority of patients with hyperuricemia develop gout?
No
Uric acid crystalizes at a level of
6.8 mg/dL
Abrupt and rapid onset causing severe pain. Often at night/early A.M.
Acute gout
An inflammatory granuloma
The Tophus
Deposits of monosodium urate crystals (MSU) crystals are surrounded by
Foreign body ganulomas
All gout is
Tophaceous
