Nucleotide Metabolism I Flashcards
Synthesizes the parental purine from scratch on a ribophosphate backbone
Purine de novo pathway
Synthesizes the parental pyrimidine from scratch and THEN attaches it to the ribophosphate backbone
Pyrimidine de novo pathway
Recycles an existing ring structure and attaches it to a ribophosphate backbone
The salvage cycle for purines and pyrimidines
All nucleotides are comprised of which three entities?
Sugar, phosphate, and a base
Comprise the backbone structure of the nucleotide
The sugar and phosphate
Composed of a two ring structure
Purines (adenine and guanine)
Composed of a single ring structure
Pyrimidines (cytosine, thymine, and uracil)
When we have just a base and ribose, but no phosphate, we have a
Nucleoside (adenosine, guanosine, cytidine, uridine, and thymidine)
Step 1 of de novo purine synthesis is creating the
Ribose phosphate backbone (PRPP)
The source of the ribose phosphate moiety in all pathways of nucleotide synthesis for both purines and pyrimidines
PRPP
Ribose-5-phosphate is converted to PRPP by the enzyme
PRPP synthetase
How much ATP is required for the conversion of R-5-P to PRPP?
1 ATP
Activated by inorganic phosphate, signifying a need for nucleotides
PRPP synthetase
PRPP synthetase is inhbited by the
Purine ribonucleotides (ADP, ATP, GDP, GTP)
What is the committed step of purine synthesis?
Conversion of PRPP tp PRA
Which enzyme catalyzes the committed step of purine synthesis?
PRPP-amido-transferase
In the conversion of PRPP to PRA, PRPP receives an amie group from
-releases a high energy pyrophosphate
Glutamine
Effectively activated by its substrate, PRPP, because PRPP concentrations are held well below the enzyme’s saturation capacity
PRPP-amido-transferase
PRPP-amido-transferase is inhibited by
AMP, GMP, and IMP
Irreversible inhibitors of PRPP-amido-transferase
-structurally similar to glutamine
Azaserine and DON
Azaserine and DON are potent inhibitors of
Purine nucleotide metabolism
Nine more steps, all of which are unregulated, convert PRA to
IMP
A parental purine because it serves as the precursor to the purines AMP and GMP
IMP
How many molecules of ATP are consumed in the conversion of PRA to IMP
4 ATP
What are the 5 contributors toward making the purine ring of IMP?
CO2, Aspartate, Glycine, Glutamine, and THF
Essential for the synthesis of purines
THF
Humans create THF by modifying
Dietary folic acid
Dietary Folic acid is converted to dihydrofolate (DHF) and then to THF by the enzyme
DHF Reductase (DHFR)
Inhibit DHFR, thereby stifling nucleotide synthesis and killing rapidly growing tumores
Chemotherapeutic agents such as aminopterin and methotrexate
Another DHFR inhibitor. Binds much more strongly to bacterial DHFR, making it an effective antibiotic
Trimethoprim
Unlike humans, who rely on dietary folic acid, bacteria synthesize their own folate from a molecule called
PABA
Antibiotics structurally similar to PABA, which competitively inhibit folic acid synthesis in bacteria
The Sulfonamides
IMP is committed to becoming either
AMP or GMP
The conversion process of IMP to either AMP or GMP requires
2 steps
Required for the conversion of IMP to adenylosuccinate
GTP and aspartate
The enzyme which converts IMP to adenylosuccinate is inhibited by
AMP
In a second reaction, adenylosuccinate is converted to AMP, a process which results in the release of
Fumarate
IMP is converted to the intermediate XMP by the enzyme IMP dehydrogenase. This requires
NAD+
IMP dehydrogenase is inhibited by
GMP
Required for the conversion of XMP to GMP
ATP and glutamine
An antiviral used to treat hepatitis C that also inhibits IMP dehydrogenase
Ribavirin
Depletes intracellular pools of guanine nucleotides
Ribavirin
The synthesis of AMP and GMP are regulated by
Feedback inhibition
AMP and GMP synthesis is also regulated so as to coordinate the relative amounts of AMP and GMP to one another. This process is called
Reciprocity
Required to convert IMP to AMP
GTP
Required to convert IMP to GMP
ATP
In step 5 of purine synthesis, AMP and GMP are phosphorylated by
Nucleoside monophosphate kinases
This phosphorylation produces
ADP and GDP
ADP and GDP are further phosphorylated by nucleoside diphosphate kinase to create
ATP and GTP
Has a broad specificity for all nucleotide diphosphates
Nucleoside diphosphate kinase
Takes advantage of the fact that free-floating bases including adenine, guanine, and hypoxanthine are present in the cell from other metabolic pathways
Purine salvage pathway
The nitrogenous base of IMP
Hypoxanthine
Simply attaches a preformed purine base to the PRPP backbone
Purine salvage pathway
Converts adenine to AMP in the purine salvage pathway
Aenine Phosphoribosyl Transferase (APRT)
Recognizes both guanine and hypoxanthine as substrates to catalyze the formation of GMP and IMP respectively
Hypoxanthine-Guanine Phosphoribosyl Transferase (HGPRT)
A disease caused by the deficiency of HGPRT
Lesch-Nyhan Syndrome
An X-linked congenital defect that results in neurological deficits including spasticity, mental retardation, aggression, and self-mutilation
Lesch-Nyhan Syndrome
The biochemical basis for the neurological symptoms is thought to be due to insufficient cell supplies of
GTP
Relies much more heavily on the salvage pathway for the synthesis of nucleotides
Brain tissue
Consequently, Lesch-Nyhan Syndrome patients have a neuronal deficiency of
GMP
Thought to be involved in a number of cell signaling pathways that may be involved in the developmental process of dopaminergic neurons
GTP
This may explain why patients with Lesch-Nyhan have fewer
Dopaminergic nerves
GTP is also a precursor for tetrahydropterin, which is a cofactor in the biosynthesis of
Dopamine
The HGPRT deficiency results in increased levels of
PRPP
The increased levels of PRPP increases
De novo purine biosynthesis
This is because PRPP is an activator of
PRPP-amido-transferase
One additional problem of patients with Lesch-Nyhan syndrome is that they have excess buildup of
Uric acid (results in gout)
A deficiency of HGPRT leads to increased levels of the enzymes substrates
Hypoxanthine and Guanine
Without salvage, these purine bases are destined to become
Uric acid
What is step 1 of the degradation of purines to uric acid?
The phosphate is removed
Removes the phosphate group from AMP, GMP, and IMP resulting in the formation of adenosine, guanosine, and inosine respectively
A 5’ nucleosidase
There is also a pathway for the direct conversion of AMP to IMP via the enzyme
AMP deaminase
In step two of the degradation of AMP to uric acid, adenosine is converted to
Inosine
Converts adenosine to inosine
Adenosine Deaminase (ADA)
This reaction serves as a point of convergence in the degredation of both AMP and IMP
Conversion of adenosine to inosine
A deficiency of the enzyme ADA results in a subtype of
Severe Combined Immunodeficiency
ADA deficiency is characterized by a severe lack of
B and T lymphocytes
Is often fatal, but can be treated with a bone marrow transplant
ADA deficiency
Removes the ribose group from inosine and guanosine, converting them to their respective nitrogenous bases hypoxanthine and guanosine
Purine Nucleoside Phosphorylase (PNP)
Interestingly, mutations in PNP result in another subtype of SCID known as PNP deficiency, which is characterized by a
T-cell deficiency
Predisposes patients to bacterial, viral, and opportunistic infections and commonly presents in childhood
PNP deficiency
Hypoxanthine and guanine are then degraded to
Xanthine
Hypoxanthine is converted to Xanthine by the enzyme
Xanthine oxidase
Guanine is converted to xanthine by the enzyme
Guanase
These two enzymes serve as the convergence in the degradation of all three purine nucleotides, AMP, GMP, and IMP
Xanthine oxidase and guanase
Then, Xanthene is converted to
-also catalyzed by xanthene oxidase
Uric Acid
Deposit throughout the body’s tissues when uric acid levels rise beyond their solubility limit
Insoluble crystal of sodium urate
This results in a condition known as
Gout
Characterized by monoarticular, excruciating joint pain, swelling, warmth, redness, and tenderness
Gout
Gout is directly caused by
Hyperuricemi
Most commonly affects the first metatarsophalangeal joint, knees, ankles, and wrists
Gout
Caused from an inborn error in uric acid metabolism, resulting in overproduction of uric acid
Primary hyperuricemia
Overactive PRPP synthetase, deregulation of PRPP-amido-transferase, and Lesch-Nyahn syndrome all cause
Hyperuricemia
Prevents salvage, which allows hypoxanthine and guanine to be diverted towards the degrative pathway, increasing uric acid
HGPRT deficiency
Caused from an accumulation of uric acid due to another disease state such as malignancies, chronic renal insufficiency, G6P deficiency, and use of HCTZ
Secondary hyperuricemia
Gout is treated with an analog of hypoxanthine that inhibits xanthine oxidase. This medicine is called
Allopurinol
Serves as a competitive inhibitor of xanthine oxidase
Allopurinol
Xanthine oxidase converts allopurinol into
-a potent inhibitor of xanthine oxidase
Alloxanthine
More soluble than uric acid and can be excreted
Xanthine and hypoxanthine
Plays an important role in the serum s an antioxidant, scavenging highly reactive and harmful oxygen species
-the reason we evolved to excrete uric acid
Urate
Another mechanism by which allopurinol exerts its effects is by leading to the buildup of
Hypoxanthine
This hypoxanthine can then be salvaged to produce
IMP
Serves as an inhibitor of PRPP-amido-transferase, the key enzyme in purine synthesis
IMP
