metabolism of chos

8/3/2019 Metabolism of CHOs

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Metabolism of Carbohydrates

Difficulty Level : One

QNo: 1

QStem:The heptose ketose sugar formed in HMP shunt is :

Ch1: Glucoheptose

Ch2: Sedoheptose

Ch3: Sedoheptulose

Ch4: Mannoheptose

Reference: Harper's Biochemsitry 24th Edition. PP : 207

Explanation: The answer is 'C' :

A molecule of Sedoheptulose-7-P and one of Glyceraldehyde-3-P are formed

by the Transketolase reaction in HMP shunt.

Transketolase (with the help of coenzyme thiamine pyrophosphate, TPP)

transfers a two-carbon unit comprising carbon 1 and 2 of a ketose to thealdehyde carbon of an aldose sugar. Therefore, it converts an Aldose

(Ribose-5-P) to a ketose with two carbon more (Sedoheptulose-7-P).


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QNo: 2

QStem:Glucose yields how many ATPs by Glycolysis (1 NADH = 3 ATP)?

Ch1: 5

Ch2: 11

Ch3: 8

Ch4: 10

Reference: Chawla and Tarek, PP: 230


Oxidation of Glucose by Glycolysis yields :

y One NADH at Gl-3-P DH step.y Two ATPs are generated by substrate level phosphorylation at

phosphoglycerokinase & Pyruvate kinase steps.

y Since one glucose gives rise to two Gl-3-P, net yield is 2 NADH and4 ATP.

y One NADH can roughly be considered to yield 3 ATPs, therefore 2NADH = 6 ATP.Total = 10 ATP.

Two ATPs are utilized for phosphorylation of glu and fru-6-P.

Therefore net yield is 10 - 2 = 8 ATP

QNo: 3

QStem:Glycolysis of Glucose, under anaerobic conditions, yields how many ATPs?

Ch1: 2

Ch2: 6

Ch3: 8

Ch4: 10

Reference: Chawla and Tarek, PP: 230

Explanation: The answer is 'A' :

Under Anaerobic conditions, NADH cannot yield ATP as Electron transportchain is blocked. Therefore, Pyruvate is converted to Lactate by using one ( x

2 ) NADH.

Net yield = 2 ATP only.


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QNo: 4

QStem:Which one of the following is an intermediate in HMP shunt

Ch1: D-Ribulose

Ch2: D-Arabinose

Ch3: D-Xylose

Ch4: D-Lyxose

Reference: Harpers Biochemistry 26th Edition, pp: 164.


Ribulose-5-P is the first pentose sugar of HMP shunt generated by oxidative

decarboxylation of 6-Phosphogluconate with the generation of the second

molecule of NADPH.

The only potent distractant in the options is Xylose, but it is Xylulose-5-P

that is formed from Ribulose and not Xylose-5-P.

QNo: 86 QTopic: Metabolism of CHODifficulty Level: 2

QStem:Galactose is phosphorylated by galactokinase to form

Ch1: Galactose-6-phosphate

Ch2: Galactose-1,6-diphosphate

Ch3: Galactose-1-phosphate

Ch4: All of the above

Reference: Lehninger's Biochemistry. 5th Edition. PP : 536


D-Galactose, a product of hydrolysis of the disaccharide

lactose (milk sugar), passes in the blood from the intestine to the liver,

where it is first

phosphorylated at C-1, at the expense of ATP, by the

enzyme galactokinase:

Galactose + ATP --> galactose 1-phosphate + ADP


QStem:The reaction involved in conversion of succinyl CoA tosuccinate in TCA cycle

requires

Ch1: CDP

Ch2: ADP

Ch3: GDP


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Ch4: NADP+

Reference: Biochemistry by Lubert Stryer. 3rd Edition. PP : 376


The enzyme Succinyl CoA synthetase mediates this substrate level

phosphorylation reaction.

The cleavage of the thioester bond of Succinyl CoA is coupled to the

phosphorylation of GDP.


QStem:Hexokinase has a high affinity for glucose than

Ch1: Fructokinase

Ch2: Galactokinase

Ch3: GlucokinaseCh4: All of the above

Reference: Biochemistry by Lubert Stryer, 3rd Edition. PP : 361

Explanation: The answer is 'D' :

The students would be tempted to answer 'C' Glucokinase. But this is a

distractor.

All the enzymes in the options have lesser affinity (if not zero) than that of

hexokinase.

QNo: 65QTopic: Metabolism of CHODifficulty Level: 2

QStem:Cyclic AMP is formed from ATP by the enzyme adenylate

cyclase which is

activated by the hormone

Ch1: Insulin

Ch2: Epinephrine

Ch3: Testosterone

Ch4: Progesterone

Reference: Chatterjea and Shinde, 5th Edition, 2005. PP : 289

Explanation: The answer is 'B' :

Epinephrine and Glucagon stimulate adenylate cyclase on the plasma

membrane and enhance the synthesis of cAMP.

cAMP is the second messenger for these hormones and stimulates a


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cascade of kinase enzymes that result in phosphorylation of a number of

proteins.

The phosphorylation / dephosphorylation is a major mechanism of


QStem:The synthesis of adenylate cyclase is increased by :

Ch1: Thyroid hormones

Ch2: Epinepherine

Ch3: Prolactin

Ch4: Follicle Stimulating Hormone

Reference: .

Explanation: The Answer is 'A' :

Thyroid hormones stimulate the synthesis of a number of proteins includingadenylate cyclase.

Epinephrine, although acts through stimulation of adenylate cyclase, does

not increase its synthesis.

QNo: 15QTopic: Metabolism of CHO Difficulty Level: 2

QStem:Which one of the following is an intermediate in HMP shunt

Ch1: D-Ribulose

Ch2: D-Arabinose

Ch3: D-Xylose

Ch4: D-Lyxose

Reference: .


Ribulose-5-P is the first pentose sugar of HMP shunt generated by oxidative

decarboxylation of 6-Phosphogluconate with the generation of the second

molecule of NADPH.

The only potent distractant in the options is Xylose, but it is Xylulose-5-P

that is formed from Ribulose and not Xylose-5-P.


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QNo: 9 QTopic: Metabolism of CHO Difficulty Level: 3

QStem:Which of the following can be said to act as a carrier ofCitric acid cylce

Ch1: ACETYL CoA

Ch2: a-KETOGLUTARATE

Ch3: MALATE

Ch4: OXALOACETATE

Reference: .


Oxaloacetic acid (OAA) can be called as the carrier of TCA cycle because

Acetyl CoA enters the cycle after combining with OAA to from Citric acid.

At the end of each cycle, OAA is regenerated to accept a new Acetyl CoA

unit.

The pace of TCA cycle is also dependent upon the availability and


QStem:Which of the following glycolytic enzymes is inhibited by

fluoride:Ch1: Glyceraldehyde 3-P dehydrogenase

Ch2: Phospholglycerate kinase

Ch3: Enolase

Ch4: Fructose1-6 diphosphate

Reference: Chatterjea and Shinde, Sixth Edition PP : 272


Fluoride inhibits ENOLASE enzyme that carries out the formation of

Phosphoenol pyruvate from 2-Phosphoglycerate during glycolysis.

Fluoride is added to the vials for collection of blood samples for estimation

of blood glucose. Fluoride inhibits Enolase and hence stops the utilisation of

glucose by RBCs in the blood samples. In the absence of fluoride glucose

estimation could give false low values.


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QStem:Pompe's disease is characterized by the deficiencey of :

Ch1: Glucose-6-phosphatase

Ch2: Debranching enzyme

Ch3: Acid maltase

Ch4: Liver phosphorylase



Pompe's disease is caused by the inherited deficiency of Acid maltase,

lysosomal enzyme that hydrolyses oligosaccharides.

Inheritence : Autosomal recessive.

'Normal' type of glycogen is deposited in excess in heart, liver, smooth and

striated muscles.

Cardiomegali and muscle hypotonia are the features and cardiac failure is

the cause of death.


QStem:All of the following antibodies are Beta-cytolytic (Pancreatic

beta cells),

except:

Ch1: Insulin Ab

Ch2: Islet cell Ab

Ch3: Glutamic acid decarboxylase Ab

Ch4: Insulin receptor Ab

Reference: .


Type II diabetes mellitus is now recognised as an autosomal disease. A

number of antibodies have been implicated in the progressive destruction of

beta cells leading to DM.

Insulin antibodies which attach the insulin granules and deplete the beta

cells, Islet cell antibodies attach the membrane antigens of islet cells.


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Similarly Glutamic acid decarboxylase antobodies that attach the lysosomal

enzyme are also cytolytic and are being used as markers for developing DM.

Insulin receptor antibodies bind with the Insulin receptors in peripheral

tissues and make them unavailable to Insulin leading to insulin resistance.

These antibodies donot bind to the beta cells.


QStem:Which of the following enzyme carries out an anapleurotic

reaction:

Ch1: Pyruvate kinase

Ch2: Pyruvate carboxylase

Ch3: Pyruvate carboxykinase

Ch4: Pyruvate dehydrogenaseReference: Lehninger's Biochemistry, 5th Edition. PP : 617


Anapleurotic reactions or 'Filling-up' reactions are those that provide

intermediates of the TCA cycle and hence increase the pace of the cycle.

The major enzymes that carryout anapleurotic reactions are :

a) Malic enzyme - produces Malate from Pyruuvate

b) Pyruvate carboxylase - produces Oxaloacetate from Pyruvate

c) Transaminases that pour amino acid carbon skeleton into the cycle.


QStem:Lumber spinal fluid glucose concentration ischaracteristically low in patients

with:

Ch1: Meningioma

Ch2: Cerebral haemorrhage

Ch3: Multiple sclerosis

Ch4: Coccal Meningitis



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Bacterial infection in the CSF lowers the glucose concentration as the

bacterial utilise the glucose.

All the other options in the question are non-infectious disease conditions.


QStem:The GIT absorption of glucose is interferred by the

deficiency of:

Ch1: Vitamin A

Ch2: Thiamine

Ch3: Biotin

Ch4: InsulinReference: Chatterjea and Shinde, 5th Edition, 2005. PP : 264


The absorption of glucose from GIT is diminished in case of deficiency of a

number of B-complex vitamins e.g. Thiamine, Pyridoxine and pentothenic

acid.


QStem:Glucose absorption from intestine may be decreased in caseof severe -

Ch1: Oedema

Ch2: Nephritis

Ch3: Rickets

Ch4: Osteomyelitis



The transporters responsible for absorption of glucose need sufficient

concentration of Sodium for the abswortion of glucose since Sodium is Co-

transported along with Glucose.

Oedema, most often than not, is associated with low sodium concentrations

therefore low sodium availability can affect the glucose absorption.


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QStem:Glycogen synthetase activity is depressed by :

Ch1: Glucose

Ch2: InsulinCh3: Cyclic AMP

Ch4: Citrate



Epinephrine and glucagon hormones act through the formation of cAMP.The

mechanism involves a cascade of phosphorylation reactions mediated by a

number of kinases.

Glycogen Synthase is phosphorylated by the cAMP stimulated Protein

Kinase. The phosphorylated Synthase in inactive.


QStem:The branching enzyme acts on the glycogen when theglycogen chain has

been lengthened to a minimum of _____ glucose units:

Ch1: 5

Ch2: 8

Ch3: 11

Ch4: 16

Reference: Lehininger's Biochemistry, 5th Edition. PP : 568


Glycogen synthase cannot make the (a1-6) bonds

found at the branch points of glycogen; these are formed

by the glycogen-branching enzyme, also called amylo

(1-4) to (1-6) transglycosylase or glycosyl-(4-6)-transferase.

The glycogen-branching enzyme catalyzes transfer of a terminal fragment of

6 or 7 glucose residues from the nonreducing end of a glycogen

branch having at least 11 residues to the C-6 hydroxyl

group of a glucose residue at a more interior position of


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the same or another glycogen chain, thus creating a new

branch.


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QNo: 67QTopic: Metabolism of CHO Difficulty Level: 4

QStem:Which of the following is a substrate for Lactose synthase

enzyme

Ch1: UDP-GLucose

Ch2: UDP-Galactose

Ch3: Galactose

Ch4: Galactose-1-Phosphate



Lactose synthase consists of tow subunits viz . a Catalytic subunit

(galactosyl transferase) and a-lactalbumin.

The catalytic subunit transfers Galactosyl residue from a UDP-Galactose toGlucose and forms a B-1-4 glycosidic linkage.

Lactalbumin is the modifier subunit and serves to modify the specificity of

the catalytic subunit so that Galactose is transferred to Glucose rather than

N-acetylglucosamine (for the synthesis of glycoproteins).


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QStem:The coenzyme essential for activity of muscle Glycogenphosphorylase is

Ch1: Cyclic AMP

Ch2: NAD+

Ch3: Pyridoxal phosphate

Ch4: Coenzyme A

Reference: Lehininger's Biochemistry, 5th Edition. PP : 562


Pyridoxal phosphate is an essential cofactor in the

glycogen phosphorylase reaction; its phosphate group

acts as a general acid catalyst, promoting attack by Pi

on the glycosidic bond.

This is an unusual role for this

cofactor; its more typical role is as a cofactor in transamination and

decarboxylation reactions.


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QStem:Inactive muscle phosphorylase b is activated by activePhosphorylase kinase

which is stimulated by :

Ch1: Calcium ions

Ch2: Cyclic AMP

Ch3: Cyclic GMP

Ch4: A & B



The muscle glycogenolysis responds not only to the hormonal regulation but

is also synchronised with muscle contraction.

The hormonal arm works through cAMP whereas the synchronisation with

muscle contraction is provided by Calcium ions.

The delta subunit of Phosphorylase kinase is Calmodulin, a calcium binding

protein that regulates many enzymes in eukaryotes.


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QStem:Fructose - 1,6 -diphosphate is converted to Fructose-6-

phosphate by the

enzyme Fructose-1.6-bisphosphatase which is stimulated by

Ch1: Glucagon

Ch2: Insulin

Ch3: AMP

Ch4: None of the above



Interconversion of Fr-6-P and Fr-1,6-BP is a key point of control. AMP

stimulates Phosphofructokinase (PFK), whereas it inhibits the enzyme Fr-

1,6-

bisphosphatase (FBPase). Citrate has the opposite effect.

During starvation (high Glucagon), the level of Fr-2,6-BP drops due to

phosphoryltic inhibition of Phsophofructokinase2.

Low FBP2 decreases the activity of PFK and increases that of FBPase.

Consequently Fr-1,6-BP is converted to Fr-6-P to generate glucose by

gluconeogenesis.


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QStem:Phosphofructokinase 2 (PFK-2) is a 'tandem enzyme', whichholds another

enzyme activity on it, which is :

Ch1: Phosphofructokinase 1

Ch2: Fructose-bisphosphatase 2

Ch3: Aldolase

Ch4: Triose-phosphate isomerase



PFK2 mediates the synthesis of Fru-2,6-bisphosphate (FBP2), which ismajor

regulator of glycolysis and gluconeogenesis.

The enzyme that hydrolysis FBP2 is Fr-2,6-bisphosphatase. The twoenzymes

activities are present in a single 53 kD polypeptide chain, which thus is

called a tandem enzyme.

The activites of FBP2 and FBP2ase are reciproclly regulated by

phosphorylation of a single serine residue. When glucose is scarce,

Glucagon triggers this phosphorylation activating the FBPase and inhibits

PFK2.


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QStem:In liver, Glyceraldehyde-3-phosphate may be converted to

Ch1: Glycerol-3-Phosphate

Ch2: Dihydroxyacetone phosphate

Ch3: 1,3-Bisphosphoglycerate


Reference: Lehninger's Biochemistry, 3rd Edition. PP : 804


Glyceraldehyde-3-P has multiple fates in Liver - it can be converted to

Glycerol-3-P for the sythesis of triglycerides, or to DHAP for glycolysis or

to

1,3-bisphosphoglycerate by Rappaport Lubering shunt.


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QStem:In the presence of Arsenate

Ch1: Enzyme Glyceraldehyde3-P dehydrogenase is inhibited

Ch2: Enzyme Phosphoglycerokinase is inhibited

Ch3: Substrate level phosphorylation is uncoupled from glycolysis

Ch4: 1,3-di-arsenoglycerate is produced


Explanation: The best answer is 'A' :

Although the staement is not exactly true. The enzyme Glyceraldehyde-3-P

DH carries out the conversion of Glyal-3-P to 1,3-Diphosphoglycerate.

Arsenate competes with inroganic phosphate in the reaction and produces

1-Arseno-3-phsopho-glyceratewhich hydrolyses spontaneously to yield 3-phosphoglycerate and heat.

Therefore, the enzyme is not inhibited but is not able to form its normal

product.


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QStem:Coenzyme for the reoxidation of reduced lipoate in pyruvatedehydrogenase

reaction is

Ch1: FAD+

Ch2: NADP+

Ch3: NAD+

Ch4: FMN



Enzyme Dihydrolipoyl dehydrogenase, involved in the reoxidation of lipoic

acid, is a flavoprotein and contains FAD as the prosthetic group.

The hydrogen from reduced DH-lipoic acid are recieved by FAD to form

FADH2 which transfers them onward to NAD.

Therefore, although the final recipient in the reaction is NAD, the enzyme

first accepts the hydrogens on FAD molecule.


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QStem:The approximate number of copies of dihydrolipoyltransacetylase in E.Coli

Pyruvate dehydrogenase complex is :

Ch1: 8

Ch2: 24

Ch3: 34

Ch4: 1

Reference: Lehninger's Biochemistry, 3rd Edition. PP : 604


The PDH complex contains three enzymespyruvate

dehydrogenase (E1), dihydrolipoyl transacetylase

(E2), and dihydrolipoyl dehydrogenase (E3)each

present in multiple copies.

The number of copies of each

enzyme and therefore the size of the complex varies

among species.

In the bovine enzyme, 60 identical copies of E2 form a pentagonal

dodecahedron (the core) with a diameter of

about 25 nm. (The core of the Escherichia coli enzyme contains 24 copies ofE2.)


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QStem:Pyruvate is accumulated in case of the dietary deficiency ofvitamin -

Ch1: Pyridoxine

Ch2: Folic acid

Ch3: Cobalamine

Ch4: Thiamine

Reference: Biochemistry by Lubert Stryer. 3rd Edition. PP :

383.


The major enzymes that has Thiamine pyrophosphate as coenzyme are :

Pyruvate dehydrogenase, a-Keto-dehydrogenase and Transketolase.

Dietary deficiency or chronic alcoholism (excessive consumptoin of TPP)can

affect the activity of these enzymes. Lower activity of Pyruvate

dehydrogenase would lead to accumulation of pyruvate.


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QStem:Citrate is converted to isocitrate by aconitase which contains:

Ch1: Ca++

Ch2: Fe++

Ch3: Zn++

Ch4: Mg++



Citrate is converted to Isocitrate by a dehydration followed by hydration

steps. The enzyme Aconitase has been named after the intermediate

'Aconitate' in the reaction.

The enzyme contains four Iron atoms complexed to four inorganic sulfidesand four cysteine sulfur atoms. This Fe-S cluster binds Citrate and

participates in dehydrating tha rehydrating the bound substrate.

The enzyme Aconitase is thus an Iron-Sulfur protein.


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QStem:Glucose-6-phosphatase is absent from which of the following

tissues

Ch1: Brain

Ch2: Intestine

Ch3: Kidney

Ch4: Heart



The enzyme Glu-6-phosphatase is located on the luminal side of the smooth

endoplasmic reticulum in the Liver cells and is essential for

gluconeogenesis.

The enzyme is also present in kidney and intestine but is absent in the brain

tissue and muscles.

Brain and Muscle need a large amount of Glucose. The absence of Glu-6-

phosphatase helps them to retain glucose since Glu-6-P cannot escape the


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QStem:6-Phosphogluconate is converted to Ribulose-5-P in thepresence of coenzyme

Ch1: FAD+

Ch2: NAD+

Ch3: NADP+

Ch4: ATP



6-phosphogluconate

undergoes oxidation and decarboxylation by

6-phosphogluconate dehydrogenase enzyme to form the ketopentose

ribulose 5-phosphate.

The oxidation takes place in the presence of NAPD+ and requires Mg++ions.

This reaction provides the second molecule of NADPH generated in HMP

shunt.


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QStem:UDP-Glucose is oxidized to UDP-glucuronic acid by adehydrogenase in the

presence of

Ch1: FAD+

Ch2: NAD+

Ch3: NADP+

Ch4: ADP

Reference: .


The uronic acid pathway is primarily involved in the synthesis of Glucuronic

acid required for a) synthesis of glycosaminoglycans b) Glucuronides.

UDP-G Dehydrogenase oxidises the carbon C6 of UDPG to form UDP

glucuronic acid. The acceptor of hydrogen in the reaction is NAD+.


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QNo: 84 QTopic: Metabolism

of CHO Difficulty Level: 3

QStem:Fructokinase is present in which of the tissues

Ch1: Liver

Ch2: Adipose tissue

Ch3: Heart

Ch4: Brain



The pathway of entry of Fructose in the glycolytic pathway is different in the

peripheral tissues and Liver.

In the peripheral tissues, Fructose is phosphorylated by Hexokinase t Fru-6-

P.

In liver, fructokinase phosphorylates it to Fru-1-P. Fr-1-P then is acted upon

by Fr-1-P Aldolase to split it into glyceraldehyde and dihydroxyacetone


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QStem:Fructose-1-phosphate is splitted into glyceraldehyde anddihydroxyacetone

phosphate by the enzyme which is also known as

Ch1: Fructose-Biphosphate Aldolase

Ch2: Aldolase A

Ch3: Enolase

Ch4: Fructolase

Reference: IUB Website.


The enzyme commission of International Union of Biologists and

Biochemists now lists the Aldolase-1 and Aldolase-2 together as single

enzyme but the enzyme cleaving Fr-1-P is also listed as Fructose-


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QStem:In galactosemic individuals deficient in uridyltransferase,

UDP-Galactose may

be formed from

Ch1: Glucose

Ch2: UDP-Glucose

Ch3: Galactose-1-Phosphate




Galactosaemia is an inherited disorder (incidence 1 :18000) characterised

by impaired utilization of Galactose, hence its accumulation in blood.

The major enzyme defect is Gal-1-P Uridyl transferase but the deficiency of

Galactokinase and Epimerase are also known causes of Galactosaemia.

Even if the patients are kept on a Galactose free diet, the Galactose can be

formed from UDP-Glucose by epimerization to meet the body requirements

of galactose.

metabolism of chos

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