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BIOSYNTHESIS OF NUCLEOTIDE COENZYMES AND THEIR ROLE IN METABOLISM
1. A DISCUSSION ON BIOSYNTHESIS OF
NUCLEOTIDE CO-ENZYMES AND THEIR ROLE
IN METABOLISM
MODERATOR-DR. M. B. BORA
PRESENTER-DR. NABA
References-
1. Harper,29th Ed
2. Biochemistry By D.Das,14th Ed.
3. Lehninger Principle Of
Biochemistry,5th Ed.
4. Lubert Stryer,6th Ed.
5. D.Voet & I.G.Voet,4th Ed.
6. Internet
2. NUCLEOTIDES ARE THE BUILDING BLOCKS OF AMINO ACID WHICH
CONSISTS OF-
-NITOGENOUS BASE NUCLEOSIDE
-PENTOSE SUGAR NUCLEOTIDE
-PHOSPHATE PHOSPHATE
Nitrogen
containing
base
Pentose
sugar
Phosphate
3. What are nucleotide coenzymes?
• A nucleotide coenzyme is a compound including in its structure
at least one simple nucleotide moiety.
• Nucleotide coenzymes function in association with specific
proteins or apoenzymes.
• Historically, the first nucleotide coenzyme discovered was
cozymase, or diphosphopyridine nucleotide.
4. GLIMPSE INTO THE HISTORY
The coenzyme NAD+ was first discovered
by British biochemists Arthur
Harden and William John Young in 1904.
They noticed that yeast extract lost it’s
ability to ferment glucose to ethyl alcohol
when the extract was dialysed. They called
the unidentified factor responsible for this
effect a coferment .
This heat-stable factor was identified as
a nucleotide sugar phosphate by Hans von
Euler-Chelpin(NP-1929).
In 1936, the German scientist, Dr. Otto
Heinrich Warburg(NP-1931) showed the
function of the nucleotide coenzyme in
hydride transfer and identified the
nicotinamide portion as the site of redox
reactions.
6. NAD⁺ BIOSYNTHESIS
Nicotinamide moiety is derived in human from-
Dietary Nicotinamide
Nicotinic acid or
Tryptophan.
• Nicotinate phosphoribosyl transferase catalyzes the
formation of nicotinate mononucleotide from nicotinate and
PRPP.
• Nicotinate mononucleotide may also be formed from ‘Quinolinate’,
a degradation product of tryptophan, catalysed by
quinolinate phosphoribosyl transferase (Liver and kidney).
• Nicotinate mononucleotide is joined to ATP-derived AMP- residue
by pyrophosphate linkage to form nicotinate adenine dinucleotide
(desamido NAD+) catalyzed by NAD+ pyrophosphorylase.
• NAD synthetase then convert intermediate to NAD+ by
transamidation reaction where glutamine is the NH₂ donor.
7. NADP BIOSYNTHESIS
• Nicotinamide moiety, derived from dietary
nicotinamide is joined with phosphoribosyl-residue
from PRPP to form nicotinamide mononucleotide
catalyzed by nicotinamide phosphoribosyl
transferase.
• It is then joined with AMP-residue, derived from
ATP by pyrophosphate linkage to form
NAD⁺(Nicotinamide adenine dinucletide).
• Finally, NADP is formed from ATP-dependent
phosphorylation of c₂′-OH of adenine residue by
NAD⁺kinase.
10. BINDING OF NAD⁺ AND NADP WITH PROTEIN
• Function as co-enzymes of
pyridine dependent
dehydrogenases.
• Binds to a specific domain
called ‘ROSSMANN’S FOLD’
(Michael Rossmann)by ionic
& hydrogen bond.
• This domain is formed of
supersecondary motif
consists of hexaparallel β-
pleated sheath and four
parallel α-helices in β-α-β-α-β
arrangements.
• Association is transient and
loose.
11. NAD⁺/NADPH IN OXIDATION AND REDUCTION REACTIONS
• It undergoes reversible reduction of
nicotinamide ring (benzenoid
form)(260nm) by accepting a hydride
ion(:H ̄ )and get itself reduced to
NADH/NADPH (quinonoid
form)(340nm)releasing second
proton to the aqueous medium.
• Since association is loose and
transient, nucleotide of both
coenzymes move readily from one
enzyme to another as water soluble
electron carrier.
12. NAD⁺ AND NADP⁺ PARTICIPATES IN DIFFERENT
METABOLIC REACTIONS
• NAD⁺ and NADP⁺ play distinctly different metabolic
reactions.
• Transfer of electrons from various substrates to NAD⁺
is facilitated in many tissue due to ↑[NAD⁺/NADH]
conc. ratio in their cells→ transfer of these reducing
equivalents to mitochondrial ETC→ production of
energy.
• On the contrary, transfer of electrons from NADPH to
lipogenic and steroidogenic tissue having
↑[NADPH/NADP⁺] conc. ratio →Reductive
biosynthesis.
• Most oxidative tissue involved in catabolic reactions
have ↑NAD⁺ and ↓NADH while tissues or organs
involved in anabolic reactions (reductive biosynthesis)
have ↓NADP⁺ and ↑NADPH.
13. ENZYMES USING NAD⁺/NADH
There are about 200
different types of enzymes
using NAD⁺/NADH as
their co-enzymes. Some
important enzymes are-
GLYCOLYSIS
Glyceraldehyde 3-P
Dehydrogenase
Lactate dehydrogenase
Pyruvate dehydrogenase
complex (PDH)
20. SOME NON REDOX ROLE OF NAD⁺
• NAD is the source of ADP-ribose for ADP-
ribosylation of proteins.
• Cyclic ADP-ribose and nicotinic acid
adenine dinucleotide, formed from NAD
act to increase intracellular calcium in
response to neurotransmitters and
hormones.
• BACTERIAL DNA LIGASE- uses NAD⁺ to
donate AMP to 5′ end of one nucleotide to
which another nucleotide add to it’s 3′ end
to form phosphodiester bond.
• SIRTUINS- NAD dependent deacetylase,
related to aging is a topic of extensive
research.
21. DRUGS AND DISEASES ASSOCIATED WITH NAD
• ISONIAZID- React with NADH to inhibit
Dihydro folate reductase(DHFR) and enoyl
acyl carrier protein reductase.
• MYCOPHENOLIC ACID AND TIAZOFURIN
inhibit IMP dehydrogenase.
• PALLEGRA- Due to deficiency of niacin or
tryptophan.
• Symptoms(3D)→Death (4D)
• Diarrhoea-
• Dementia- Tissue with high respiration rate such as
CNS is severely effected.
• Dermatitis-Due to ↑kynurenine/↓repair of UV induced
damage to epidermis.
• Potential role of NAD in therapy of patients suffering
from neurodegenerative diseases like Alzheimer’s
disease, parkinson’s disease is being studied.
22. FORMATION OF FMN AND FAD
• Formed from riboflavin in liver, intestinal mucosa and
other tissues.
• 5′-OH group of ribityl side chain of riboflavin is
phosphorylated by FLAVOKINASE and ATP forming
FMN.
• Then coupling of FMN and ATP-derived
AMP(adenylate) occures by pyrophosphate linkage
catalysed by FAD pyrophosphorylase to form FAD.
• FMN is not a true nucleotide since it’s ribityl residue is
not a true sugar.
24. FLAVOPROTEIN IN OXIDATION AND REDUCTION
• FLAVOPROTEIN-Enzymes
catalyzing oxidation and
reduction reactions using FMN
or FAD as coenzymes
(QUINONE).
• SEMIQUINONE-When one pair
of reducing equivalents are
transferred from substrate to N¹
of isoalloxazine ring forming
FMNH•/FADH•(450 nm).
• HYDROQUINONE-Two pair of
reducing equivalents are
transferred to N¹ and N¹⁰
producing
FMNH₂/FADH₂(360nm).
25. CHARACTERISTICS OF FLAVOPROTEIN
• Flavin nucleotide in most flavoproteins are bound rather tightly to the
protein. ex-succinate dehydrogenase- FAD is bound covalently like the
prosthetic groups.
• Cannot transfer electrons by diffusing from one enzyme to another, rather it
can temporarily hold electrons and transfer it to an electron acceptor.
• Tight association confers on the flavin ring a reduction
potential(E⁰),different from reduction potential of free flavin nucleotide.
• CRYPTOCHROMES- A family of flavoproteins, mediate the effect of blue
light on plant development.
• PHOTOLYASES-Found in bacteria & eukaryotes, uses energy of absorbed
light to repair chemical defect in DNA.
• METALLOFLAVOPROTEIN-Contain electron transferring metals like Fe
and Mo in addition to FAD, FMN.eg- Xanthine oxidase & NADH
dehydrogenase
• HEMOFLAVOPROTEIN-Contain both heme and flavin.eg-L-lactate dehydrogenase of yeast.
31. ROLE OF NAD⁺/FAD IN ETC
FLOW OF ELECTRONS THROUGH
VARIOUS COMPLEXES
32. FORMATION OF COENZYME-A FROM PANTOTHENATE
• Pantothenate at first is
phosphorylated by
pantothenate kinase to form 4-
phosphopantothenate.
• It is then coupled to cysteine
catalysed by
phosphopantothenoylcysteine
synthetase to form 4-
phosphopantothenoylcysteine.
• It undergoes decarboxylation to
form 4-phosphopantethiene.
• It is then coupled to AMP by
pyrophosphate linkage to form
dephospho-coA by
pyrophosphorylase.
• Undergoes phosphorylation to
form co-A by dephospho-coA
kinase.
34. ROLE OF COENZYME-A IN DIFFERENT METABOLISM
• Formation of acyl coA from free fatty acid which
undergoes β-oxidation of fatty acid.
• Formation of acetyl coA from pyruvate which is
aerobically oxidized in TCA cycle.
• Acetyl coA and malonyl-coA is used in synthesis and
elongation of fatty acid.
• Formation & utilisation of ketone bodies.
• Cholesterol synthesis.
• Formation of succinyl coA used for heme synthesis.
35. ADENOSINE 3′-PHOSPHATE 5′-
PHOSPHOSULFATE(PAPS):BIOSYNTHESIS AND USES
• Also known as ‘active
sulphate’.
• Sulphate(so₄² ̄) is
activated in two steps to
produce PAPS which
further undergoes
reduction to produce
sulphide(S² ̄) for
different sulfation
reactions catalyzed by
sulfotransferases.
36. FUNCTION OF PAPS
• PAPS of ‘active sulfate’ is the sulfide donor for different
reactions catalyzed by various sulfotransferases.
• Important compounds which are conjugated with
sulphate are-
Proteoglycans.
Steroid hormone.
Glycolipids(sulfatide).
Bilirubin.
Various drugs. eg-minoxidil sulphate
37. SULPHATED PROTEOGLYCANS
• HEPARIN-Fractionated form of
heparan sulphate derived from
mast cells.
• It binds with antithrombin to inhibit
thrombin.
• Therapeutic agent used to inhibit
coagulation.
• CHONDROITIN SULPHATE-
contribute to the tensile strength
of cartilage, tendon, ligaments
and wall of the aorta.
• KERATAN SULPHATE-present in
cornea, cartilage, bone and horny
structures formed of dead cells.
• DERMATAN SULPHATE-
contribute to the pliability of the
skin
• Also present in blood vessels and
heart valves.
38. SULFONATION OF STEROID HORMONE(DHEA-S)
• DHEA-S(Dehydroepiandrosterone sulphate) is the sulfate
ester of DHEA. This conversion is reversibly catalyzed
by sulfotransferase (SULT2A1) primarily in the adrenals,
the liver, and small intestine.
• In the blood, most DHEA is found as DHEA-S with levels that
are about 300 times higher than those of free DHEA as it is
more stable.
• Whereas DHEA levels naturally reach their peak in the early
morning hours, DHEA-S levels show no diurnal variation.
• From a practical point of view, measurement of DHEAS is
preferable to DHEA, as levels are more stable(IN
ADRENOGENITAL SYNDROME).
40. SUMMARY
• Nucleotide co-enzymes are compounds having at least one
simple nucleotide moiety in it’s structures.
• Nicotinamide containing coenzyme NAD⁺ and flavoprotein
take part in redox reactions of different metabolism while
NADP takes part in reductive biosynthesis.
• Non redox function is important for ADP-ribosylaton, drug
therapeutics etc.
• Flavoproteins like FMN or FAD is a part of COMPLEX I and
COMPLEX II in ETC.
• CoenzymeA takes part in different metabolisms like TCA
cycle,β-oxidation of fatty acid, synthesis and elongation of
fatty acid, cholesterol and heme synthesis etc.
• PAPS or active sulfate is the sulfide donor for different
sulfonation reactions catalyzed by different
sulfotransferases forming sulfated product of immense
biological significance.
