nucleotides: synthesis and degradation
TRANSCRIPT
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Nucleotides: Synthesis and Degradation
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Nucleotides: Synthesis and Degradation
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Nitrogenous BasesPlanar, aromatic, and heterocyclicDerived from purine or pyrimidineNumbering of bases is “unprimed”
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Nucleic Acid BasesPurines Pyrimidines
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SugarsPentoses (5-C sugars)Numbering of sugars is “primed”
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Sugars D-Ribose and 2’-Deoxyribose
*Lacks a 2’-OH group
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Nucleosides Result from linking one of the sugars
with a purine or pyrimidine base through an N-glycosidic linkage
Purines bond to the C1’ carbon of the sugar at their N9 atoms
Pyrimidines bond to the C1’ carbon of the sugar at their N1 atoms
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Nucleosides
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Phosphate Groups Mono-, di- or triphosphates
Phosphates can be bonded to either C3 or C5 atoms of the sugar
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NucleotidesResult from linking one or more phosphates
with a nucleoside onto the 5’ end of the molecule through esterification
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Nucleotides RNA (ribonucleic acid) is a polymer of
ribonucleotides DNA (deoxyribonucleic acid) is a
polymer of deoxyribonucleotides Both deoxy- and ribonucleotides contain
Adenine, Guanine and Cytosine Ribonucleotides contain Uracil Deoxyribonucleotides contain Thymine
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Nucleotides Monomers for nucleic acid polymers Nucleoside Triphosphates are important
energy carriers (ATP, GTP) Important components of coenzymes
FAD, NAD+ and Coenzyme A
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Naming Conventions Nucleosides:
Purine nucleosides end in “-sine” Adenosine, Guanosine
Pyrimidine nucleosides end in “-dine” Thymidine, Cytidine, Uridine
Nucleotides: Start with the nucleoside name from above
and add “mono-”, “di-”, or “triphosphate” Adenosine Monophosphate, Cytidine
Triphosphate, Deoxythymidine Diphosphate
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In-Class Activities Look at the Nucleotide Structures
Take the Nucleotide Identification Quiz
Be prepared to identify some of these structures on an exam. Learn some “tricks” that help you to distinguish among the different structures
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Nucleotide MetabolismPURINE RIBONUCLEOTIDES: formed de novo
i.e., purines are not initially synthesized as free bases
First purine derivative formed is Inosine Mono-phosphate (IMP) The purine base is hypoxanthine AMP and GMP are formed from IMP
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Purine NucleotidesGet broken down into Uric Acid (a purine)
Buchanan (mid 1900s) showed where purine ring components came from:
N1: Aspartate AmineC2, C8: FormateN3, N9: GlutamineC4, C5, N7: GlycineC6: Bicarbonate Ion
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Purine Nucleotide Synthesis
OH
H
H
CH2
OH OH
H HO
O2-O3P
-D-Ribose-5-Phosphate (R5P)
O
H
H
CH2
OH OH
H HO
O2-O3P
5-Phosphoribosyl--pyrophosphate (PRPP)
P
O
O
O P
O
O
O
ATP
AMP
RibosePhosphatePyrophosphokinase
H
NH2
H
CH2
OH OH
H HO
O2-O3P
-5-Phosphoribosylamine (PRA)
AmidophosphoribosylTransferase
Glutamine + H2O
Glutamate + PPi
H
NH
H
CH2
OH OH
H HO
O2-O3P
CO
H2C NH2
Glycinamide Ribotide (GAR)
GAR Synthetase
Glycine + ATP
ADP+ Pi
H2C
CNH
O
CH
HN
O
Ribose-5-Phosphate
Formylglycinamide ribotide (FGAR)
H2C
CNH
O
CH
HN
HN
Ribose-5-Phosphate
Formylglycinamidine ribotide (FGAM)
THFN10-Formyl-THF
GAR Transformylase
ATP +Glutamine +H2O
ADP +Glutamate + Pi
FGAM Synthetase
HC
CN
CH
N
H2N
Ribose-5-Phosphate
4
5
5-Aminoimidazole Ribotide (AIR)
ATP
ADP + Pi
AIR Synthetase
C
CN
CH
N
H2N
OOC
Ribose-5-Phosphate
4
5
Carboxyamidoimidazole Ribotide (CAIR)
ATP+HCO3
ADP + PiAIR Car boxylase
Aspartate+ ATP
ADP+ Pi
SAICAR Synthetase
AdenylosuccinateLyase
Fumarate
C
CN
CH
N
NH
Ribose-5-Phosphate
4
5
5-Formaminoimidazole-4-carboxamideribotide (FAICAR)
CH2N
O
CH
O
C
CN
CH
N
H2N
Ribose-5-Phosphate
4
5
5-Aminoimidazole-4-carboxamideribotide (AICAR)
CH2N
O
C
CN
CH
N
H2N
CNH
O
HC
COO
CH2
COO
Ribose-5-Phosphate
4
5
5-Aminoimidazole-4-(N-succinylocarboxamide)ribotide (SAICAR)
THF
AICAR Transformylase
N10-Formyl-
THF
Inosine Monophosphate (IMP)
HN
HCN
C
CC
N
CH
N
O
4
5
HH
CH2
OH OH
H HOO2-O3P
IMPCyclohydrolase
H2O
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Purine Nucleotide Synthesis at a Glance ATP is involved in 6 steps
PRPP in the first step of Purine synthesis is also a precursor for Pyrimidine Synthesis, His and Trp synthesis
Role of ATP in first step is unique– group transfer rather than coupling
In second step, C1 notation changes from to (anomers specifying OH positioning on C1 with respect to C4 group)
In step 2, PPi is hydrolyzed to 2Pi (irreversible, “committing” step)
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Coupling of Reactions
Hydrolyzing a phosphate from ATP is relatively easy G°’= -30.5 kJ/mol
If endergonic reaction released energy into cell as heat energy, wouldn’t be useful
Must be coupled to an exergonic reaction When ATP is a reactant:
Part of the ATP can be transferred to an acceptor: Pi, PPi, adenyl, or adenosinyl group
ATP hydrolysis can drive an otherwise unfavorable reaction
(synthetase; “energase”)
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Purine Biosynthetic Pathway Channeling of some reactions on pathway
organizes and controls processing of substrates to products in each step Increases overall rate of pathway and protects
intermediates from degradation In animals, IMP synthesis pathway shows
channeling at: Reactions 3, 4, 6 Reactions 7, 8 Reactions 10, 11
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In Class Activity***
Calculate how many ATP equivalents are needed for the de novo synthesize IMP. Assume that all of the substrates (R5P, glutamine, etc)
are available
Note: You should be able to do this calculation for the synthesis of any of the nucleoside monophosphates
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IMP Conversion to AMP
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IMP Conversion to GMP
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Regulatory Control of Purine Nucleotide Biosynthesis
GTP is involved in AMP synthesis and ATP is involved in GMP synthesis (reciprocal control of production)
PRPP is a biosynthetically “central” molecule (why?) ADP/GDP levels – negative feedback on Ribose Phosphate
Pyrophosphokinase Amidophosphoribosyl transferase is activated by PRPP levels APRT activity has negative feedback at two sites
ATP, ADP, AMP bound at one site GTP,GDP AND GMP bound at the other site
Rate of AMP production increases with increasing concentrations of GTP; rate of GMP production increases with increasing concentrations of ATP
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Regulatory Control of Purine Biosynthesis Above the level of IMP production:
Independent control Synergistic control Feedforward activation by PRPP
Below level of IMP production Reciprocal control
Total amounts of purine nucleotides controlled
Relative amounts of ATP, GTP controlled
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Purine Catabolism and Salvage All purine degradation leads to uric acid (but it
might not stop there) Ingested nucleic acids are degraded to
nucleotides by pancreatic nucleases, and intestinal phosphodiesterases in the intestine
Group-specific nucleotidases and non-specific phosphatases degrade nucleotides into nucleosides Direct absorption of nucleosides Further degradation
Nucleoside + H2O base + ribose (nucleosidase) Nucleoside + Pi base + r-1-phosphate (n. phosphorylase)
NOTE: MOST INGESTED NUCLEIC ACIDS ARE DEGRADED AND EXCRETED.
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Intracellular Purine Catabolism Nucleotides broken into nucleosides by
action of 5’-nucleotidase (hydrolysis reactions)
Purine nucleoside phosphorylase (PNP) Inosine Hypoxanthine Xanthosine Xanthine Guanosine Guanine Ribose-1-phosphate splits off
Can be isomerized to ribose-5-phosphate Adenosine is deaminated to Inosine
(ADA)
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Intracellular Purine Catabolism Xanthine is the point of convergence
for the metabolism of the purine bases
Xanthine Uric acid Xanthine oxidase catalyzes two reactions
Purine ribonucleotide degradation pathway is same for purine deoxyribonucleotides
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Adenosine Degradation
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Xanthosine Degradation
• Ribose sugar gets recycled (Ribose-1-Phosphate R-5-P ) – can be incorporated into PRPP (efficiency)• Hypoxanthine is converted to Xanthine by Xanthine Oxidase• Guanine is converted to Xanthine by Guanine Deaminase• Xanthine gets converted to Uric Acid by Xanthine Oxidase
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Xanthine Oxidase A homodimeric protein Contains electron transfer proteins
FAD Mo-pterin complex in +4 or +6 state Two 2Fe-2S clusters
Transfers electrons to O2 H2O2 H2O2 is toxic Disproportionated to H2O and O2 by
catalase
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THE PURINE NUCLEOTIDE CYCLEAMP + H2O IMP + NH4
+ (AMP Deaminase)
IMP + Aspartate + GTP AMP + Fumarate + GDP + Pi (Adenylosuccinate Synthetase)
COMBINE THE TWO REACTIONS:
Aspartate + H2O + GTP Fumarate + GDP + Pi + NH4
+
The overall result of combining reactions is deamination of Aspartate to Fumarate at the expense of a GTP
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Purine Nucleotide CyclePurine Nucleotide Cycle******
In-Class Question: Why is the purine nucleotide In-Class Question: Why is the purine nucleotide cycle important in muscle metabolism during a cycle important in muscle metabolism during a burst of activity?burst of activity?
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Uric Acid Excretion Humans – excreted into urine as insoluble
crystals Birds, terrestrial reptiles, some insects –
excrete insoluble crystals in paste form Excess amino N converted to uric acid
(conserves water) Others – further modification :
Uric Acid Allantoin Allantoic Acid Urea Ammonia
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Purine Salvage Adenine phosphoribosyl transferase
(APRT)Adenine + PRPP AMP + PPi
Hypoxanthine-Guanine phosphoribosyl transferase (HGPRT)
Hypoxanthine + PRPP IMP + PPi
Guanine + PRPP GMP + PPi
(NOTE: THESE ARE ALL REVERSIBLE REACTIONS)
AMP,IMP,GMP do not need to be resynthesized de novo !
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A CASE STUDY : GOUT A 45 YEAR OLD MAN AWOKE FROM SLEEP WITH A
PAINFUL AND SWOLLEN RIGHT GREAT TOE. ON THE PREVIOUS NIGHT HE HAD EATEN A MEAL OF FRIED LIVER AND ONIONS, AFTER WHICH HE MET WITH HIS POKER GROUP AND DRANK A NUMBER OF BEERS.
HE SAW HIS DOCTOR THAT MORNING, “GOUTY ARTHRITIS” WAS DIAGNOSED, AND SOME TESTS WERE ORDERED. HIS SERUM URIC ACID LEVEL WAS ELEVATED AT 8.0 mg/dL (NL < 7.0 mg/dL).
THE MAN RECALLED THAT HIS FATHER AND HIS GRANDFATHER, BOTH OF WHOM WERE ALCOHOLICS, OFTEN COMPLAINED OF JOINT PAIN AND SWELLING IN THEIR FEET.
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A CASE STUDY : GOUT THE DOCTOR RECOMMENDED THAT THE
MAN USE NSAIDS FOR PAIN AND SWELLING, INCREASE HIS FLUID INTAKE (BUT NOT WITH ALCOHOL) AND REST AND ELEVATE HIS FOOT. HE ALSO PRESCRIBED ALLOPURINOL.
A FEW DAYS LATER THE CONDITION HAD RESOLVED AND ALLOPURINOL HAD BEEN STOPPED. A REPEAT URIC ACID LEVEL WAS OBTAINED (7.1 mg/dL). THE DOCTOR GAVE THE MAN SOME ADVICE REGARDING LIFE STYLE CHANGES.
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GoutGout Impaired excretion or overproduction of uric Impaired excretion or overproduction of uric
acidacid Uric acid crystals precipitate into joints Uric acid crystals precipitate into joints
(Gouty Arthritis), kidneys, ureters (stones)(Gouty Arthritis), kidneys, ureters (stones) Lead impairs uric acid excretion – lead Lead impairs uric acid excretion – lead
poisoning from pewter drinking gobletspoisoning from pewter drinking goblets Fall of Roman Empire?Fall of Roman Empire?
Xanthine oxidase inhibitors inhibit Xanthine oxidase inhibitors inhibit production of uric acid, and treat goutproduction of uric acid, and treat gout
Allopurinol treatment – hypoxanthine Allopurinol treatment – hypoxanthine analog that binds to Xanthine Oxidase to analog that binds to Xanthine Oxidase to decrease uric acid productiondecrease uric acid production
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ALLOPURINOL IS A XANTHINE OXIDASE ALLOPURINOL IS A XANTHINE OXIDASE INHIBITORINHIBITOR
A SUBSTRATE ANALOG IS CONVERTED TO AN A SUBSTRATE ANALOG IS CONVERTED TO AN INHIBITOR, IN THIS CASE A “SUICIDE-INHIBITOR”INHIBITOR, IN THIS CASE A “SUICIDE-INHIBITOR”
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Choi HK, Atkinson K, Karlson EW et al. . 2004. “Alcohol intake and risk of incident gout in men:a prospective study”. Lancet 363: 1277-1281
ALCOHOL CONSUMPTION AND GOUT
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Lesch-Nyhan SyndromeLesch-Nyhan Syndrome
A defect in production or activity ofA defect in production or activity of HGPRT HGPRT
Causes increased level of Hypoxanthine and Causes increased level of Hypoxanthine and Guanine (Guanine ( in degradation to uric acid) in degradation to uric acid)
Also,PRPP accumulatesAlso,PRPP accumulates stimulates production of purine nucleotides stimulates production of purine nucleotides
(and thereby increases their degradation)(and thereby increases their degradation) Causes gout-like symptoms, but also Causes gout-like symptoms, but also
neurological symptoms neurological symptoms spasticity, spasticity, aggressiveness, self-mutilationaggressiveness, self-mutilation
First neuropsychiatric abnormality that First neuropsychiatric abnormality that was attributed to a single enzymewas attributed to a single enzyme
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Purine AutismPurine Autism
25% of autistic patients may 25% of autistic patients may overproduce purinesoverproduce purines
To diagnose, must test urine over To diagnose, must test urine over 24 hours24 hours Biochemical findings from this test Biochemical findings from this test
disappear in adolescencedisappear in adolescence Must obtain urine specimen in Must obtain urine specimen in
infancy, but it’s difficult to do!infancy, but it’s difficult to do!• Pink urine due to uric acid crystals may Pink urine due to uric acid crystals may
be seen in diapersbe seen in diapers
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IN-CLASS QUESTION*** IN von GIERKE’S DISEASE,
OVERPRO- DUCTION OF URIC ACID OCCURS. THIS DISEASE IS CAUSED BY A DEFICIENCY OF GLUCOSE-6-PHOSPHATASE.
EXPLAIN THE BIOCHEMICAL EVENTS THAT LEAD TO INCREASED URIC ACID PRODUCTION?
WHY DOES HYPOGLYCEMIA OCCUR IN THIS DISEASE?
WHY IS THE LIVER ENLARGED?
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Pyrimidine Ribonucleotide Synthesis Uridine Monophosphate (UMP) is synthesized first
CTP is synthesized from UMP Pyrimidine ring synthesis completed first; then
attached to ribose-5-phosphate
N1, C4, C5, C6 : AspartateC2 : HCO3
-
N3 : Glutamine amide Nitrogen
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2 ATP + HCO3- + Glutamine + H2O
CO
O PO3-2
NH2
Carbamoyl Phosphate
NH2
CNH
CH
CH2
C
COOO
HO
O
Carbamoyl Aspartate
HN
CNH
CH
CH2
C
COOO
O
Dihydroorotate
HN
CNH
C
CHC
COOO
O
Orotate
HN
CN
C
CHC
COOO
O
HH
CH2
OH OH
H HO
O2-O3P
Orotidine-5'-monophosphate(OMP)
HN
CN
CH
CHC
O
O
HH
CH2
OH OH
H HO
O2-O3P
Uridine Monophosphate(UMP)
2 ADP +Glutamate + Pi
CarbamoylPhosphateSynthetase II
AspartateTranscarbamoylase(ATCase)
Aspartate
Pi
H2O
Dihydroorotase
Quinone
ReducedQuinone
DihydroorotateDehydrogenase
PRPP PPi
Orotate PhosphoribosylTransferase
CO2
OMP Decarboxylase
Pyrimidine Synthesis
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UMP Synthesis Overview 2 ATPs needed: both used in first step
One transfers phosphate, the other is hydrolyzed to ADP and Pi
2 condensation rxns: form carbamoyl aspartate and dihydroorotate (intramolecular)
Dihydroorotate dehydrogenase is an intra-mitochondrial enzyme; oxidizing power comes from quinone reduction
Attachment of base to ribose ring is catalyzed by OPRT; PRPP provides ribose-5-P PPi splits off PRPP – irreversible
Channeling: enzymes 1, 2, and 3 on same chain; 5 and 6 on same chain
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OMP DECARBOXYLASE : THE MOST CATALYTICALLY PROFICIENT ENZYME FINAL REACTION OF PYRIMIDINE PATHWAY ANOTHER MECHANISM FOR
DECARBOXYLATION A HIGH ENERGY CARBANION INTERMEDIATE
NOT NEEDED NO COFACTORS NEEDED ! SOME OF THE BINDING ENERGY BETWEEN
OMP AND THE ACTIVE SITE IS USED TO STABILIZE THE TRANSITION STATE “PREFERENTIAL TRANSITION STATE BINDING”
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UMP UTP and CTP Nucleoside monophosphate kinase
catalyzes transfer of Pi to UMP to form UDP; nucleoside diphosphate kinase catalyzes transfer of Pi from ATP to UDP to form UTP
CTP formed from UTP via CTP Synthetase driven by ATP hydrolysis Glutamine provides amide nitrogen for C4 in
animals
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Regulatory Control of Pyrimidine SynthesisDiffers between bacteria and animals
Bacteria – regulation at ATCase rxnAnimals – regulation at carbamoyl phosphate
synthetase II UDP and UTP inhibit enzyme; ATP and PRPP
activate it UMP and CMP competitively inhibit OMP
Decarboxylase
*Purine synthesis inhibited by ADP and GDP at ribose phosphate pyrophosphokinase step, controlling level of PRPP also regulates pyrimidines
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Orotic Aciduria Caused by defect in protein chain with enzyme activities of last two steps of pyrimidine synthesis
Increased excretion of orotic acid in urine
Symptoms: retarded growth; severe anemia
Only known inherited defect in this pathway (all others would be lethal to fetus)
Treat with uridine/cytidine IN-CLASS QUESTION: HOW DOES URIDINE AND
CYTIDINE ADMINISTRATION WORK TO TREAT OROTIC ACIDURIA?
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Degradation of Pyrimidines CMP and UMP degraded to bases
similarly to purines Dephosphorylation Deamination Glycosidic bond cleavage
Uracil reduced in liver, forming -alanine Converted to malonyl-CoA fatty acid
synthesis for energy metabolism
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Deoxyribonucleotide Formation Purine/Pyrimidine degradation are
the same for ribonucleotides and deoxyribonucleotides
Biosynthetic pathways are only for ribonucleotide production
Deoxyribonucleotides are synthesized from corresponding ribonucleotides
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DNA vs. RNA: REVIEW DNA composed of
deoxyribonucleotides
Ribose sugar in DNA lacks hydroxyl group at 2’ Carbon
Uracil doesn’t (normally) appear in DNA Thymine (5-methyluracil) appears instead
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Formation of Deoxyribonucleotides Reduction of 2’ carbon done via a free
radical mechanism catalyzed by “Ribonucleotide Reductases”
E. coli RNR reduces ribonucleoside diphosphates (NDPs) to deoxyribonucleoside diphosphates (dNDPs) Two subunits: R1 and R2
A Heterotetramer: (R1)2 and (R2)2 in vitro
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RIBONUCLEOTIDE REDUCTASE R1 SUBUNIT
Three allosteric sites Specificity Site Hexamerization site Activity Site
Five redox-active –SH groups from cysteines
R2 SUBUNIT Tyr 122 radical Binuclear Fe(III) complex
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Ribonucleotide Reductase R2 Subunit
Fe prosthetic group– binuclear, with each Fe octahedrally coordinated Fe’s are bridged by O-2 and carboxyl gp of
Glu 115 Tyr 122 is close to the Fe(III) complex
stabilization of a tyrosyl free-radical During the overall process, a pair of –
SH groups provides the reducing equivalents A protein disulfide group is formed Gets reduced by two other sulfhydryl gps
of Cys residues in R1
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Chime ExerciseE. coli Ribonucleotide Reductase:
3R1R and 4R1R: R1 subunit1RIB and 1AV8: R2 subunit
• Explore 1AV8: Ribonucleotide Reductase in detail.This is the R2
subunit of E. coli Ribonucleotide Reductase. The biological molecule consists of a heterotetramer of 2 R1 and two R2 chains.
• Identify the following structures:
– 8 long -helices in one unit of R2– Tyr 122 residue– The binuclear Fe (III) complex– The ligands of the Fe (III) complex
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Mechanism of Ribonucleotide Reductase Reaction Free Radical Involvement of multiple –SH groups RR is left with a disulfide group that
must be reduced to return to the original enzyme
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RIBONUCLEOTIDE REDUCTASE ACTIVITY IS RESPONSIVE TO LEVEL OF
CELLULAR NUCLEOTIDES: ATP ACTIVATES REDUCTION OF
CDP UDP
dTTP INDUCES GDP REDUCTION INHIBITS REDUCTION OF CDP. UDP
dATP INHIBITS REDUCTION OF ALL NUCLEOTIDES dGTP
STIMULATES ADP REDUCTION INHIBITS CDP,UDP,GDP REDUCTION
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RIBONUCLEOTIDE REDUCTASE CATALYTIC ACTIVITY VARIES WITH STATE OF
OLIGOMERIZATION: WHEN ATP, dATP, dGTP, dTTP BIND TO
SPECIFICITY SITE OF R1 (CATALYTICALLY INACTIVE MONOMER) CATALYTICALLY ACTIVE (R1)2
WHEN dATP OR ATP BIND TO ACTIVITY SITE OF DIMERS TETRAMER FORMATION (R1)4a (ACTIVE STATE) == (R1)4b (INACTIVE)
WHEN ATP BINDS TO HEXAMERIZATION SITE CATALYTICALLY ACTIVE HEXAMERS (R1)6
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ThioredoxinPhysiologic reducing agent of RNRCys pair can swap H atoms with disulfide
formed regenerate original enzyme Thioredoxin gets oxidized to disulfide
Oxidized Thioredoxin gets reduced by NADPH ( final electron acceptor)mediated by thioredoxin reductase
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Thymine Formation Formed by methylating deoxyuridine
monophosphate (dUMP) UTP is needed for RNA production,
but dUTP not needed for DNA If dUTP produced excessively, would
cause substitution errors (dUTP for dTTP) dUTP hydrolyzed by dUTPase (dUTP diphosphohydrolase) to dUMP
methylated at C5 to form dTMP rephosphorylate to form dTTP
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CHIME EXERCISE: dUTPase 1DUD: Deoxyuridine-5'-Nucleotide Hydrolase
in a complex with a bound substrate analog, Deoxyuridine-5'-Diphosphate (dUDP).
Explore dUTPase as follows:
Find the substrate in its binding site Find C5 on the Uracil group. Is there enough
room to attach a methyl group to C5? Locate the ribose 2’ C. What protein group
sterically prevents an –OH group from being attached to the 2’ C atom?
Find the H-bond donors and acceptors (to the uracil base) from the protein. What would be the effect on the H-bonding if the base was changed to cytosine?
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Tetrahydrofolate (THF) Methylation of dUMP catalyzed by thymidylate synthase Cofactor: N5,N10-methylene THF
Oxidized to dihydrofolate Only known rxn where net oxidation state of THF
changes THF Regeneration:DHF + NADPH + H+ THF + NADP+ (enzyme: dihydrofolate
reductase)
THF + Serine N5,N10-methylene-THF + Glycine (enzyme: serine hydroxymethyl transferase)
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dUMP dTMP
NADPH + H+
NADP+
SERINE
GLYCINE
REGENERATION OF N5,N10 METHYLENETETRAHYDROFOLATE
DHFN5,N10 – METHYLENE-THF
THF
dihydrofolate reductaseserine hydroxymethyl transferase
thymidylate synthase
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dUMP dTMP
NADPH + H+
NADP+
SERINE
GLYCINE
INHIBITORS OF N5,N10 METHYLENETETRAHYDROFOLATE REGENERATION
DHFN5,N10 – METHYLENE-THF
THF
dihydrofolate reductaseserine hydroxymethyl transferase
thymidylate synthase
METHOTREXATE AMINOPTERIN TRIMETHOPRIM
FdUMP
X
X
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Anti-Folate Drugs Cancer cells consume dTMP quickly for
DNA replication Interfere with thymidylate synthase rxn to
decrease dTMP production (fluorodeoxyuridylate – irreversible inhibitor) – also
affects rapidly growing normal cells (hair follicles, bone marrow, immune system, intestinal mucosa)
Dihydrofolate reductase step can be stopped competitively (DHF analogs) Anti-Folates: Aminopterin, methotrexate,
trimethoprim
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ADENOSINE DEAMINASE DEFICIENCY IN PURINE DEGRADATION, ADENOSINE
INOSINE ENZYME IS ADA
ADA DEFICIENCY RESULTS IN SCID “SEVERE COMBINED IMMUNODEFICIENCY”
SELECTIVELY KILLS LYMPHOCYTES BOTH B- AND T-CELLS MEDIATE MUCH OF IMMUNE RESPONSE
ALL KNOWN ADA MUTANTS STRUCTURALLY PERTURB ACTIVE SITE
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Adenosine DeaminaseCHIME Exercise: 2ADA Enzyme catalyzing deamination of Adenosine to
Inosine / barrel domain structure
“TIM Barrel” – central barrel structure with 8 twisted parallel -strands connected by 8 -helical loops
Active site is at bottom of funnel-shaped pocket formed by loops
Found in all glycolytic enzymes Found in proteins that bind and transport
metabolites
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ADA DEFICIENCY*** IN-CLASS QUESTION: EXPLAIN THE
BIOCHEMISTRY THAT RESULTS WHEN A PERSON HAS ADA DEFICIENCY
(HINT: LYMPHOID TISSUE IS VERY ACTIVE IN DEOXYADENOSINE PHOSPHORYLATION)
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ADA DEFICIENCY ONE OF FIRST DISEASES TO BE TREATED
WITH GENE THERAPY
ADA GENE INSERTED INTO LYMPHOCYTES; THEN LYMPHOCYTES RETURNED TO PATIENT
PEG-ADA TREATMENTS ACTIVITY LASTS 1-2 WEEKS