Final: Purine Metabolism (Ben) Flashcards
Which 5 molecules contribute to the formation of the purine ring structure?
How?
- Glycine - C, C, N
- Glutamine - N, N
- Aspartate - N
- CO2 - C
- Formyl-H4F - C, C
(Gly, Glu, Asp, CO, Fo)
What is the origin of the ribose-5-P used in nucleotide synth?
How is it further “activated” for use?
(via what rxn and how is it regulated)
R5P comes from the pentose phosphate pathway…
-
PRPP Synthase further activates
- R-5-P + ATP –> Phosphoribosyl Pyrophosphate + AMP
- inhibited by AMP, GMP
- activated by Pi (all allosteric)
What is the committed step in de novo purine synthesis?
(plus regulation)
Gln:PRPP Amidotransferase
- PRPP + Gln + ATP –> Phosphoribosyl-amine + Glu + ADP + Pi
- inhibited by AMP/GMP/IMP or ADP/GDP
- stimulated by PRPP
De novo purine synthesis is awful…
List the steps and any important co-factors of them.
(Obviously you won’t be able to recite all of this, it’s just a review card… the main regulatory/committed step is probably the only one where detailed knowledge is necessary.)
-
Gln:PRPP Amidotransferase (amidophosphoribosyltransferase)
- PRPP + Gln + H20 –> Phosphoribosyl-amine + Glu + PPi
- AMP, GMP, IMP inhibit ; PRPP activates (COMMITTED STEP)
-
GAR Synthase
- PRA + Gly + ATP –> Glycinamide ribotide + ADP + Pi
-
GAR Transformylase
- GAR + Formyl-H4F –> Formylglycinamide ribotide (FGAR) + H4F
-
FGAM Synthetase
- FGAR + Gln + ATP + H2O > Formylglycinamidine Ribotide + Glu/ADP/Pi
-
AIR Synthetase
- FGAM + ATP –> Aminoimidazole Ribotide + ADP + Pi
-
AIR Carboxylase
- AIR + CO2 –> Carboxyaminoimidazole ribotide
-
SACAIR Synthetase
- CAIR + Asp + ATP –> SACAIR + ADP + Pi
-
Adenilosuccinase
- SACAIR –> ACAIR + fumarate
-
ACAIR Transformylase
- ACAIR + Formyl-H4F –> FACAIR + H4F
-
IMP Cyclohydrolase
- FACAIR + H2O –> Inosine 5’-Monophosphate (IMP) ( = parent purine!!! )
What is the order in which the nitrogen and carbon contributor molecules are added onto the growing purine ring?
(Which additions use ATP? … And which atoms are added by each?)
- Glutamine……………. ( + N )
- Glycine (ATP) ……….. ( + C, C, N )
- Formyl-H4F…………. ( + C )
- Glutamine (ATP) ….. ( + N )
- CO2………………………. ( + C )
- Aspartate (ATP) …… ( + N )
- Formyl-H4F…………. ( + C )
(Glu, Gly, For, Glu, CO, Asp, For)
Glycine adds CCN, all other AAs only add N, others only add C
How many ATP equivalents are used in the formation of a new purine ring?
During the addition of atoms from which molecules?
(What is the formed “parent purine” called?)
- 6 ATP equivalents (5 molecules, one broken down to AMP + PPi)
- PRPP Synthetase (ATP –> AMP + PPi)
- Glycine (GAR synthase)
- Glutamine (2nd one, FGAM Synthase)
- ATP only (cleaved for energy only, no atom addition, AIR Synthetase)
- Aspartate (SACAIR Synthetase)
- forms Inositol 5’-Monophosphate (IMP) parent purine
- ( notice: it’s all the synthase/synthetase enyzmes that are using ATP!!! )
From the parent nucleotide IMP…
How can AMP be formed?
(2 steps + regulation + what is happening to the purine ring structure?)
-
Adenylosuccinate Synthase
- IMP + Asp** + **GTP –> Adenylosuccinate + GDP + Pi
-
Adenylosuccinase
- Adenylosuccinase –> AMP + fumarate
- Regulation: AMP inhibits adenylosuccinate synthase
- The =O is replaced with -NH2
How is AMP broken down/recycled to IMP?
AMP Deaminase
- AMP + H2O –> IMP + NH4+
How can GMP be made from IMP?
(2 steps + regulation + what is happening to the puring ring structure?)
-
IMP Dehydrogenase
- IMP + NAD+ + H2O –> Xanthosine MP + NADH + H+
-
GMP Synthase
- XMP + Gln** + **ATP –> GMP + Glu + AMP + PPi
- Regulation: GMP inhibits IMP Dehydrogenase
- The 6 carbon ring is gaining an -NH2 group (bottom left, via =O intermediate)
How is GMP broken down / recycled to IMP?
GMP Reductase
- uses NAPH to reduce GMP to IMP and yield NH3
Briefly, how do AMP + GMP regulate their own synthesis?
-
If one is in excess… it will inhibit its own synthesis to spare IMP for synthesis of the other
- ( AMP inhib adenylosuccinate lyase + GMP inhib IMP dehydrogenase )
- If both are in excess … they will inhibit Gln:PRPP Amidotransferase and thus inhibit overall purine synthesis
In summary, what co-factors + substrates are required for synthesis of…
AMP from IMP?
GMP from IMP?
- AMP needs Aspartate and GTP
- GMP needs Glutamine and ATP… (plus NAD+ for dehydrogenase)
What tissue is high in a purine recycling enyzme that sends its products to the liver for processing?
What are the products and how are they processed?
What about deficiency of the enyzme?
- Muscle is high in AMP deaminase to break down high amts of AMP made during exercise.
- It gives off NH3 and IMP
- NH3 goes to liver for urea cycle
- IMP is broken down to inosine then sent to the liver to become urate for excretion
- AMP DA deficiency results in cramps but no NH3/urate elevation
Once the purine monophosphates are made…
how can phosphates be added to make di and triphosphates ?
-
AMP Kinase
- AMP + ATP 2 ADP
-
Guanylate Kinase
- GMP + ATP GDP + ADP
- note: these are reversible and the monophosphates can be converted back to IMP and then broken down to urate
How do purine salvage reactions work?
Generally and via which specific enyzmes?
(What about the energetic favorability of the reactions?)
Phosphoribosyl transferases add R-5-P back to purines…
-
Adenine P-R Transferase (APRT)
- Adenine + PRPP –> AMP + PPi
-
Hypoxanthine-Guanine P-R Transferase (HGPRT)
- Guanine + PRPP –> GMP + PPi
- deficiency = Lesh-Nyhan Syndrome (retardation, hyperuricemia, X-linked)
- reactions are made irreversible by the PPi-cleavage action of pyrophosphatase
How can purine nucleotides AMP/GMP be broken down into simple bases?
(As in have their ribose + P groups removed.)
- Generally…
-
5’-nucleotidases
- remove the Pi to yield nucleosides (base + ribose) adenosine and guanosine
-
Purine nucleotide phosphorylases
- use Pi to removed the ribose (as ribose-1-P) leaving only the base adenine and guanine
-
5’-nucleotidases
- (actually with AMP there is a middle step of adenosine deaminase removing ammonia to make inosine nucleoside which loses its ribose to become hypoxanthine base… but those are the general steps)
Once purine nucleotides are catabolized down to their bases…
How are they further metabolized for excretion?
-
Guanase
- Guanine + H2O –> Xanthine + NH3
-
Xanthine Oxidase (2 rxns)
- Hypoxanthine + H2O + O2 –> Xanthine + H2O2
- Xanthine + H2O + O2 –> URATE + H2O2
- note the production of ROS by xanthine oxidase
- Urate goes on to be excreted
