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What are we doing????
• Pentose Phosphate Pathway – Aka HMP shunt (hexose monophosphate)
• This process is used to produce NADPH +H– Needed for synthesis reactions of cholesterol,
steroids, and nitric oxide– Also needed to maintain glutathione in reduced
form (metabolism of drugs)• Also used to create Ribose 5 phosphate
– Used to make nucleotides and nucleic acids
Where is this
• HMP shunt occurs in the liver, adrenal cortex, testes, ovaries, RBC, adipose tissue, and lactating mammory glands.– The reactions specifically take place in the cytosol
• There are two phases of the HMP shunt– The oxidative (irreversible) phase– The nonoxidative (reversible) phase
• Glucose is the starting substance for pentose phosphate pathway
• First reaction is catalyzed by Hexokinase
( Glucose to Glucose -6-P)
• For one complete set of reaction of Pentose P. pathway 3 molecules Glucose- 6 - P are required
• At the completion of the reaction 2 ½ equivalent of Glucose-6-P are reformed
Oxidative (Irreversible) phase
• Two reactions → Conversion of 6 carbon sugar P to 5 carbon sugar P. – G6P → 6 phosphogluconate → ribulose 5 phosphate
• Production of NADPH + H+• Production of CO2
• The major enzymes are – two different Dehydrogenases– Glucose 6 Phosphate Dehydrogenase– 6 Phosphogluconate dehydrogenase
• Second phase is Non- oxidative phase – reversible
• Pentose sugar P. undergo isomerization and transfer reactions, ultimately leading to the reformation of 6 carbon sugar (as Glucose 6 PO4)
• These reactions are catalyze by specific isomerases/epimerases and transferases
TDP
TDP
Coenzymes required for HMP
• You need to conenzymes for these reactions
• NADP– Used to make NADPH H+ in the oxidative rxns
• Thiamine Diphosphate– Used for ketolase (part of the nonoixdative rxns)– What other 2 enzymes uses TDP?
Regulation
• Only one enzyme this time!• Glucose 6 P Dehydrogenase
– NADPH itself acts as a –ve regulator (inhibitor) for the enzyme
• Insulin acts as an inducer (stimulant) for the production of Glucose-6-P. Dehydrogenase – Thus insulin promotes Pentose P. Pathway– Do you think G6PD works phosphorylated or
dephosphorylated?
Glucose 6 P Dehydrogenase Deficiency
• X- linked inheritance– One of the major causes of hemolytic anemia (mainly
induced by oxidative drugs)• Pts w/ Glu-6-P dehydrogenase deficiency are a
sensitive to oxidative stress – may be due to drugs, chemicals, infections, certain food – (eg.fava bean – causes favism)
• However, these individuals are resistant to Malaria → caused by Plasmodium Falciparum
Symptoms of Glu-6-P dehydrogenase deficiency :• Hemolytic anemia
– i) Exposure to compounds having oxidizing effects such as primaquine, fava bean:
– ii) Infection – inflammatory response – free radical generation –oxidative effect – hemolysis
• HMP shunt is very crucial in RBC for the generation of NADPH + H+ , which in turn keeps Glutathione in reduced state.
• Reduced glutathione is actively involved as antioxidant (protects against free radicals), thereby protecting the integrity of cell & cell membrane
• Since the body has no protection against free radicals, they cause damage to RBC cell membranes
• Neonates with Glu-6-P dehydrogenase deficiency develop neonatal jaundice
• Different types of point mutations in Glu-6-P dehydrogenase gene is responsible for this genetic defect– Severity depends on residual activity of the enzyme
Other functions of NADPH H+
• Required for phagocytosis in WBC - – For the generation of superoxide radicals in an event → Respiratory
Burst– Enzyme used→ NADPH oxidase– Genetic deficiency of this enzyme → Chronic granulomatous disease
• characterized by severe, persistent infections
• NO is synthesized by utilizing Arginine, O2 and NADPH by the action of enzyme NO synthase
• NADPH is very essential for detoxification of drugs, that utilize cytochrome P 450
2
That’s it for HMP Shunt, Whats Next?
• Bioenergetics and Oxidative Phosphorylation
Exergonic vs Endergonic
• 1. Exergonic reaction: Net loss of Free Energy. Indicated with the sign - ΔG
• • 2. Endergonic reaction: Net gain of Free
Energy. Indicated with the sign +ΔG
• ∆G sign (+ or -) predicts direction of the reaction
Example for coupling endergonic reaction with exergonic reaction: (∆G is additive)
• GLUCOSE GLUCOSE 6 P ATP ADPBreak upGlucose + Pi Glucose 6 P ∆G = + 3.3 kcal. ATP ADP + Pi ∆G = - 7.3 kcal
SumGLUCOSE + ATP GLUCOSE 6 P +ADP ∆G = - 4.0 kcal.
All this stuff can occur backwards so it if happen the net G would + 4.0kcal
• In a reaction ∆G depends on the conc. of the reactant (substrate) and the product
• A B• For forward reaction (A to B)• ∆G = ∆Go +RT ln
• ∆Go is standard free energy change under standard condition(when both reactant and product are present at equal conc(1 mol/L)
• R is the gas constant; T is absolute temperature• ln is natural logarithm • (A) & (B) are actual conc of reactant & product
• Under standard condition!!!!!! ∆G = ∆Go
(B)
(A)
• Under standard condition the free energy change (∆G0) of a metabolic reaction is related to its equilibrium constant (Keq)
• A B
• Consider Equilibrium constant (Keq) of this reaction
• Keq =
• If Keq is more than 1 ∆G0 is – • If Keq is less than 1 ∆G0 is + • If Keq is 1 ∆G0 is 0
• At equilibrium ∆G0 is zero
•
(B)eq
(A)eq
• ∆G of a sequence of reactions is additive
• A B C D
• A B G1
• B C G2
• C D G3
• ---------------• A D GS (sum of overall free energy)
ATP is the major high energy currency of the body. ∆G on hydrolysis of ATP is -7.3 Kcal/mol (approx)
• Sources of ATP:
• Substrate level Phosphorylation:
• Examples: Refer 2 reactions in glycolysis and a single reaction in TCA cycle
• Oxidative Phosphorylation:• Formation of high energy compounds through
the mediation of Electron transport Chain (ETC) in mitochondria
OXIDATIVE PHOSPHORYLATION
• Oxidation reaction is the one in which electron/s (reducing equivalent/s) is/are lost
• Reduction reaction is the counterpart of oxidation reaction. Here electron/s (reducing equivalent/s) is/are gained
• Oxidation and reduction reactions are always coupled to each other
Oxidation → adding to a charge (0 → +1) adding on a protonReduction → subtracting the charge (0 → -1) removing a proton
• Redox Potential: • The capacity of a redox pair to donate (lose)
electron or gain electron is expressed in terms of Redox Potential (E0)
• Redox potential is the electromotive force, expressed in terms of volts or millivolts
• In general a redox pair with low redox potential exhibits a tendency to donate the electron to the pair with high redox potential
• NAD / NADH +H+ ----------- - 0.32• Cyto.c(Fe+++)/ Cyto.c(Fe++)- +0.22
• During the oxidative reactions of Glycolysis, β -oxidation and TCA cycle energy rich electrons (reducing equivalents) are generated
• These electrons are conveyed to the Electron transport chain (ETC), present in inner mitochondrial membrane
• In ETC these electrons are transferred through a series of components and finally accepted by Molecular oxygen to form water
• Each of the components of ETC is involved in Oxidation and reduction reactions
• During the transfer of electron across ETC, substantial Free energy is released– This energy is utilized for the synthesis of ATP using ATP synthase
• Importance of Mitochondria:• Mitochondria constitute important sites for the
generation as well as capture of energy• Two important electron releasing pathways are located
in mitochondria (matrix), – β-oxidation & TCA cycle
• The components responsible for capturing energy are also located in inner membrane of mitochondria – ETC (Electron transport chain)– ATP Synthase
• Composition of ETC:• Altogether there are 4 complexes and 2
mobile carriers • The complexes are numbered I, II, III and IV• Each complex comprises an enzyme and
certain components, which take part in Electron transfer
• Two components of ETC are called Mobile carriers. – Coenzyme Q,(also called as Ubiquinone)
• It is the lone Lipid component of ETC
– Cytochrome c
• Complex 1 → NADH transfers electron (e-) to FMN– Makes FMNH2
• Complex 2 → succinate transfers e- to FADFMNH2 and FADH2 e- are moved to CoEnzymeQ• Complex 3 → CoQ moves those e- to Cyto bc1
– Cytochromes use iron to accept and donate electrons– Cyto BC1 moves the e- to cytochrome c
• Complex 4 → Cyt C moves e- to Cyto A+A3– Cyto A+A3 donates electron to oxygen to create a molecule of water
FMN-FeS Cyt.b-FeS-Cyt.c1 Cyt.a.a3 H20
½ O2+2H+ CoQ Cyt.c
FeSFAD
NAD
PYRUVATEISOCITRATEα-KETOGLUTARATEMALATEβ-OH ACYL CoAβ-OH BUTYRATE
I
II
III IV
SUCCINATEACYL C0A
GLYCEROL 3 P
• All the cytochromes of ETC are hemoproteins. They contain Iron, which takes part in reversible oxidation and reduction reactions (Fe++ Fe+++) during electron transport
• Besides, Cytochromes a & a3 contain Copper, that also take part in reversible oxidation and reduction reactions (Cu+ Cu++)
• Cytochrome C is known to have a role in apoptosis of the cell– Whenever it gets out of the mitochondria, apoptosis
begins.
• In ETC the components are arranged in the order of increasing Redox potential– That means the electrons are transferred from the
component of lower redox potential to the component of higher redox potential
– So which has a lowr redox potential? Complex1 or Coenzyme Q?
• All the components of ETC participate in oxidation reduction reactions (Electron transport)
• Molecular oxygen is the ultimate acceptor of electrons to form H2O (water)
• By calculation, two electrons are required to form one molecule of H2O
Site of ATP Synthesis• Synthesis of ATP is
accomplished by an enzyme complex, present in inner mitochondrial membrane.
• It is called as ATP synthase (also called as F0 F1 ATPase)
• It has two portions - F0 & F1
• F1 ( head piece) exhibits the ATP synthase activity
How does ATP synthase work• Transport of electrons through ETC, results in liberation of free energy ( -ΔG)
– The liberation of free energy is adequate at complexes I, III and IV• This free energy is utilized to pump out Protons (H+) through inner
mitochondrial membrane (from matrix to the space between two membranes)
• As a result of proton pumping both proton gradient and electrical gradient (Electrochemical gradient) are created across the inner mitochondrial membrane – This generates Proton Motive Force
• The proton motive force is used as a driving force for the synthesis of ATP through ATP Synthase , present in inner mitochondrial membrane
• During the synthesis of ATP by ATP synthase, protons move back to the mitochondrial matrix, thereby dissipating (abolishing) the gradient
ATP synthase
• F0 part → used to allow protons across membrane• F1 part → used to make ATP off the energy from F0
• When a pair of electron is transported through complex I, III & IV of ETC , altogether approx. 10 protons are pumped out
• On the other hand when a pair of electrons are transported through complex III & IV (bypassing complex I) approx. 6 protons are pumped out
P/O ratio• This refers to the number of ATP molecules formed per consumption
of 1 atom of oxygen
• When a substrate (eg.Pyruvate, Isocitrate, α-Ketoglutarate, Malate, β-OH Acyl CoA or β-OH Butyrate) is oxidized through NAD dependent enzyme, P/O ratio is 2.5
• But when a substrate such as Succinate (in TCA cycle) or Acyl CoA (in β-oxidation) is oxidized through FAD dependent enzyme, 1.5 moles of ATP are formed per atom of oxygen consumed (P:O ratio 1.5)– because here the electron transport bypasses complex I, thereby the
subsequent proton pumping & Gradient generated is sufficient to synthesize only 1.5 ATP molecules
• It should also be obvious that you need ADP to run this enzyme. Without ADP you cannot make ATP.
• Now we are going to get into some things that make non of this ETC stuff work.
• Compounds that Interfere the Process of Oxidative Phosphorylation:
• A) Inhibitors of ETC:• Affecting Complex I: Amobarbital, Rotenone &
Piericidin A
• Affecting Complex II: TTFA (Thenoyl trifluoro acetone) & Carboxin
• Affecting Complex III: Antimycin A
• Affecting Complex IV: Cyanide, Carbon monoxide (CO) Sodium azide & Hydrogen sulfide (H2S)
• Uncouplers:• These are the agents, which dissociate
phosphorylation from oxidation– As a result the free energy released is not captured to
form ATP – However, oxidation process continues
• The action of uncouplers is attributed to the abolition of electrochemical gradient across the inner mitochondrial membrane– Does not generate proton motive foce → no ATP
synthesis• 2,4 – Dinitrophenol & Dinitrocresol• Uncoupling proteins are found in brown adipose
tissue → mostly in animals
• Naturally occurring uncouplers, present in the body:
• Thermogenin → uncoupling protein 1 (UCP 1):– It is a protein present in Brown Adipose Tissues. – Brown adipose tissues are concerned with more heat
production. – It is present in animal system to adapt for cold
climate. – Prominent in hibernating animals & in new born
human infants
• Inhibitor of Oxidative Phosphorylation:Oligomycin: It binds to the F0 region of ATP synthase and closes the proton (H+) channel • Thus it prevents the reentry of protons back into
the mitochondrial matrix.
• Inhibitor of ATP transport from matrix of Mitochondria to cytosol
• Atractyloside: It is a plant toxin, inhibits ATP transporter – Allows ATP transport from mit. matrix across inner
mit.membrane using ATP ADP exchange
Transportation of Extramitochondrial reducing equivalents (from NADH + H+) into mitochondria
• Reducing equivalents (from NADH + H+) generated in cytoplasm during glycolysis is tranported into mitochondria;
• --1. Through Malate shuttle
• --2. Through Glycerol 3 phosphate shuttle
• Genetic Disorders caused by mutation of mitochondrial DNA:
• Mitochondrial DNA code for the synthesis of a few proteins/enzymes (13), involved in oxidative phosphorylation. – mutation of mitochondrial DNA leads to defect in oxidative
phosphorylation• The rate of mutation of mitochondrial DNA is about 10
times higher than that of nuclear DNA• Mutation of mitochondrial DNA mainly affects those
tissues, where the requirement of ATP is high– Examples: CNS, Cardiac & skeletal muscles, kidney, liver
• Mitochondrial DNA is maternally inherited– Mitochondria in sperm are lost after fertilization
• Common Clinical symptoms:– Myopathy, encehalopathy, Ataxia, Retinal
degeneration and Loss of function of external ocular muscles
• Examples of Disorders caused due to mutation in mit.DNA:– 1. Leber’s Hereditary Optic Neuropathy:
Symptoms are – Rapid optic nerve death and Blindness
– 2. Mitochondrial encephalomyopathy, lactic acidosis and stroke (MELAS):
OMG that’s it for ETC!!!!
• Ok that was brutal but it is over
• Luckily the final two topics are not nearly as heavy
• First we are going to do metabolism of monosaccharides and disaccharides.
Fructose• Remember that sucrose is broken down
into glucose and fructose.• In muscle the only enzyme is hexokinase
– Fructose → Fructose 6 Phosphate• Liver Kidney and Intestine has 2 enzymes
– Fructokinase → makes Frutcose 1 phosphate– Aldolase B → uses F1P and F 16 BP as
substrates to make DHAP and glyceraldehyde• So fructose can be used in glycolysis
Two diseases here• Essential Fructosuria → Frucotokinase Deficiency
– Benign, asymptomatic → not a big deal• Hereditary Fructose Intolerance
– Aldolase B deficiency (next slide)
Hereditary Fructose Intolerance
• Lack of Aldolase B → can be lethal• Accumulation of Fructose 1-p
– Hypoglycemia seen inspite of Glycogen reserves – F 1P and F1,6 BP allosterically inhibit the Glycogen phosphorylase
• Drop in Pi → Decrease in ATP and increase in AMP – degradation of AMP → Hyperuricemia
• Decrease in ATP → Inhibits gluconeogenesis and protein synthesis– Why else is gluconeogenesis inhibited?
• Prolonged intake of fructose → vomiting, poor feeding, jaundice, hepatomegaly, lactic acidosis, Hyperlipidemia, hemorrhage and eventually hepatic failure and death
• Treatment → Avoid fructose and sucrose in diet
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Sorbitol pathway (polyol pathway)
lens, retina, schwann cells liver, kidney, RBC, ovaries & seminal
vesiclesin seminal vesicles as fructose is the major source of energy for sperms
In Diabetes- High Glucose Sorbitol Accumulates osmotic effects cell swelling Causes cataract, peripheral neuropathy, nephropathy, retinopathy
55
Galactose Metabolism (From Lactose-Milk sugar)
Lactose, glycoproteins, Glycolipids and Glycosaminoglycans
Classic Galactosemia
Where have you seen these reactions?
56
Clinical Significance of Galactose Metabolism• Classic Galactosemia is a major symptom of two enzyme defects.
• Galactose-1-phosphate uridyl transferase or• Galactokinase
Manifest by a failure of neonates to thrive, Vomiting and diarrhea following ingestion of milk, hence individuals are termed lactose intolerant
Accumulation in tissues of • Galactose & Galactose 1-phosphate acute neonatal
complications • Galactitol Cataracts• Decrease in UDP-galactose abnormal glycoprotein and
glycolipid synthesis
57
Clinical Significances of Galactose Metabolism• Present in first weeks of life poor feeding, weight loss,
vomiting, diarrhea, lethargy and hypotonia• Liver dysfunction, bleeding tendencies, bilateral cataracts and
septicemia may be seen• Prolonged exposure severe mental retardation
• Controlled by exclusion of galactose from the diet – Does not prevent long term complications cognitive impairment,
premature ovarian failure, progressive neurological impairment **Galactosemia diagnostic triad Mental retardation, Cirrhosis Cataract **
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Mannose
• Digestion of many polysaccharides and glycoproteins yields mannose
• Phosphorylated by hexokinase to generate mannose-6-phosphate.
• Converted to fructose-6-phosphate, by the enzyme phosphomannose isomerase, and – then enters the glycolytic pathway or – Is converted to glucose-6-phosphate by the gluconeogenic
pathway of hepatocytes.
Lactose Synthesis
• Occurs in the golgi, Two glucoses are required – One glucose is converted to UDP-Galactose
• Glucose → glucose-6-phosphate → glucose-1P→ UDP-glucose → UDP-galactose
– Another glucose is used without modification.• Glucose passes across the golgi apparatus membrane into the
golgi apparatus lumen by a glucose transporter (GLUT1). • UDP-galactose is actively transported into the Golgi apparatus
lumen
• Lactate synthase → 2 part enzyme– A subunit → ß1,4 galactosyltransferase (active enzyme)– B subnit → α lactalbumin
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Lactose Synthesis• In most tissues this enzyme component found in the Golgi
apparatus. During glycoprotein biosynthesis adds galactose to oligosaccharides UDP-D-galactose + N-acetylglucosamine ß1,4 galactosyltransferase
D-galactosyl-N-acetyl-D-glucosamine + UDP• Unique among all glycosyltransferases in that its substrate
specificity can be modified by addition of α-lactalbumin. • Together, ß1,4 galactosyltransferase and α-lactalbumin form the
lactose synthase complex.
In Lactating mammary glandUDP-D-galactose + D-glucose D-lactose + UDPEnz -a LA
Ok last part
• Glycosaminoglycans!– Long unbranched polysaccharides containing a
repeating disaccharide unit.• Disaccharide units contain either of two modified
sugars– N-acetylgalactosamine (GalNAc) or – N-acetylglucosamine (GlcNAc) and – A uronic acid such as glucuronate or iduronate
• Located primarily on the surface of cells or in the extra cellular matrix (ECM).
GAGs of significance
• 1. Hyaluronic acid***– Non sulfated & Not covalently linked to proteins– Synovial fluid, vitreous humor, ECM of loose – Functions as Lubricant and shock absorber in connective tissue
• 2. Dermatan sulfate → Skin, blood vessels, heart valves• 3. Chondroitin sulfate → Cartilage, tendons, ligaments and aorta• 4. Heparin → only intracellular GAG → mast cells lining • arteries in liver, lungs and skin → Anticoagulant• 5. Heparan sulfate → Basement membrane and cell surfaces • 6. Keratan sulfate → No Uronic acid. Cornea, bone, cartilage
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Proteoglycans• GAGs linked to core proteins Proteoglycans • GAGs extend perpendicularly from the core in a brush-
like structure.• Linkage of GAGs to protein core involves a specific
trisaccharide – Two galactose residues and a xylulose residue (GAG-
GalGalXyl-O-CH2-protein). • The trisaccharide linker is coupled to the protein core
through an O-glycosidic bond to a Serine residue in the protein.
• Some forms of Keratan sulfates are linked to the protein core through an N-asparaginyl bond.
64
Proteoglycans
• The protein cores of Proteoglycans are rich in Serine and Threonine residues, which allows multiple GAG attachments
• Many such Proteoglycans monomers aggregate on a molecule of Hyaluronic acid to form Proteoglycans aggregates through ionic interactions
65
Functions of Proteoglycans• Structural support to tissues especially in cartilage and
connective tissue by forming large aggregates of macromolecules
• Ability to withstand torsion and shock– Negative charge counter ions are attracted draw
water molecules swelling and stiffening of tissues rigidity with flexibility and compressibility
• Binding of proteins and enzymes to vascular walls • EX- Syndecan-Membrane, Versican, aggrecan- Extracellular Neurocan,cerebrocan- Nervous system
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Synthesis of GAGsSynthesis of amino sugars
• N-acetyl glucosamine & N acetyl galactosamine• Fructose 6 phosphate can be the direct precursor
Glucose
Glucose 6 phosphate
Fructose 6 phosphate
Glucosamine 6 phosphate
Gln Glu
Glucosamine 1 phosphate
UTPPPi
UDP glucosamineGAG
67
Synthesis of acidic sugars• D-Glucuronic acid synthesised by uronic acid pathway L-Iduronic acid by Uronosyl 5epimerase from D-Glucuronic acid
Glucose 6 phosphate Glucose 1 phosphate UDP glucose
UTP
PPiNAD
NADH+H+
UDP Glucuronic acid
UDPGlc dehydrogenase
L-gulonateAscorbic acid or
Vitamin C L-gulonolactone oxidase
ABSENT IN HUMANS
GAG
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Core protein Synthesis• Synthesised on RER • As it moves along the ER, carbohydrate chain formation
initiated
Core proteinR-CH2-OH R-CH2-O-Xyl
UDP-Xyl
UDP
2 gal
R-CH2-O-Xyl-gal-galXylosyl transferase
Followed by addition of alternating acidic and amino sugars. Sulfation and acetylation occur after incorporation of monosaccharide residuesSource of sulfate is PAPS 3’-phosphoadenosyl-5’-phosphosulfate*****. Enzyme is sulfotransferase
69
• Degradation of GAGs In lysosomes As extra cellular phagocytosed fused with a lysosome
endoglycosidases desulfated & deacetylated further action of acid hydrolases
Lysosomal storage diseases, result from defects in enzymes responsible for Degradation of GAGs. Lead to an accumulation of GAGs within cells
MucopolysaccharidosesHereditary disorders caused by the accumulation of GAG in various tissues due to the defect in the lysosomal hydrolases of GAG catabolism• share many common clinical features → Severe mental retardation in
all• multisystem involvement, organomegaly,, and abnormal facies, • dysostosis multiplex → defect in ossification of cartilages
– Skeletal deformities• Hearing, vision, airway, cardiovascular function and joint mobility are
affected.
• Hunter's syndrome → iduronate sulfatase deficiency– No corneal clouding (hunters need to see)
• Hurler's syndrome → L iduronidase deficiency– CORNEAL CLOUDING
Both have defective heparan and dermatan sulfate degradation
71
Glycoproteins • Proteins with oligosacchrides• Oligosaccharides covalently attached to proteins glycoproteins
• Predominant sugars glucose, galactose, mannose, Fucose, GalNAc, GlcNAc and NANA
• Membrane bound GP participate in Cell surfaces recognition (hormones, viruses)Cell surface antigenicity (blood group antigens)
– Communication between cells, – For maintaining cell structure
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• Carbohydrates linked to protein through • O-glycosidic bond
– To the hydroxyl of serine, threonine or hydroxylysine
– Linkage to hydroxylysine through galactose or glucosylgalactose.
– In ser- and thr-type O-linked glycoproteins, the carbohydrate directly attached to the protein is GalNAc.
• N-glycosidic bond. – Through the amide group of asparagine.– In N-linked glycoproteins, carbohydrate
attached to protein is GlcNAc. – Common core of carbohydrate attached
to the polypeptide.– Consists of mannose residues– Most common type of glycoproteins
Biosynthesis of Glycoproteins:• Protein part is synthesized in ribosomes, attached to
endoplasmic reticulum (RER)• Attachment of Oligosaccarides:• i) O-Linked – In Golgi apparatus****• ii) N- Linked – Initially in the lumen of ER.****• - Dolichol phosphate is required• Incorporation of individual carbohydrate residues is
catalyzed by specific glycosyl transferases • UDP is the common nucleotide required for the
incorporation of most of these carbohydrate residues• Mannose & Fucose require GDP as carrier• NANA (Sialic acid) is incorporated through CMP (as CMP
NANA)
Degradation of glycoproteins • Exoglycosidases remove sugars sequentially from the non-reducing end • Endoglycosidases cleave carbohydrate linkages from within• Importance of mannose 6 P.
– The enzymes destined for lysosomal locations are tagged with mannose 6.P– Mannose residues are incorporation in ER– Phosphorylation of mannose in golgi
• Defects in the genes encoding specific glycosidases, → incomplete degradation and subsequent over-accumulation of partially degraded glycoproteins → lysosomal storage diseases
• Without proper targeting, Newly synthesized lysosomal enzymes are secreted into extracellular medium instead of being targeted to lysosomes– Lysosomal enzymes are present at elevated levels in serum and body fluids
and Leads to the formation of dense inclusion bodies formation in the fibroblasts.
I-cell diseaseCaused by a deficiency of the ability to phosphorylate mannose (Phospho transferases) for lysosomal acid hydrolases. Characterized by severe psychomotor retardation, skeletal abnormalities, coarse facial features, painful restricted joint movement, and early mortality.Lysosome contents build up because the enzymes used for lysosomes. These look like inclusion bodies → I cells I cell disease is a devastating disorder that arises from defect in the lysosomal enzyme targeting. Instead of ending up in lysosomes many lysosomal enzymes are ended up in extracellular location. Which one of the following is most likely the molecular basis of the disease?A. Deletion of the gene encoding N-ethylamaide sensitive factor proteinB. A mutation in the gene which encode a protein which is essential for the formation of secretory vesiclesC. Deletion in a gene that encodes a protein which transfers a phosphate group to mannose residuesD. A mutation in the promoter sequence that is required for initiation of transcription of lysosomal enzymesE. A missence mutation in the signal peptide that causes the signal peptide of the lysosomal enzyme to lose its function
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That’s all
