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Solution to Short answer type questions- Carbohydrate Metabolism
Q.1-Give the details of the reaction catalyzed by each of the following enzymes-
The conversion of Fructose to Fructose-1-phosphate is catalyzed by Fructokinase. It is the first step of fructose metabolism.
A specific kinase, Fructokinase, in liver, kidney, and intestine,catalyzes the phosphorylation of fructose to fructose 1-phosphate. This enzyme does not act on glucose, and, unlike Glucokinase, its activity is not affected by fasting or by insulin, which may explain why fructose is cleared from the blood of diabetic patients at a normal rate.
b) Glyceraldehyde-3- phosphate dehydrogenase
The enzyme glyceraldehyde 3-phosphate dehydrogenase, is NAD+ dependent and catalyzes the oxidation of Glyceraldehyde-3-phosphate to 1, 3 bisphosphoglycerate.
1,3-Bisphosphoglycerate is an acyl phosphate. Such compounds have a high phosphoryl-transfer potential; one of its phosphoryl groups is transferred to ADP in the next step in glycolysis. The reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase is really the sum of two processes: the oxidation of the aldehyde to a carboxylic acid by NAD+ and the joining of the carboxylic acid and orthophosphate to form the acyl-phosphate product.
Glyceraldehyde-3-phosphate+ NAD++ Pi—>1, 3 bisphosphoglycerate+ NADH+ H+
The enzyme is inhibited by the —SH poison iodoacetate,which is thus able to inhibit glycolysis. Arsenate is also inhibitor of this enzyme. The toxicity of arsenic is the result of competition of arsenate with inorganic phosphate (Pi) in the above reactions to give 1-arseno-3-phosphoglycerate,which hydrolyzes spontaneously to 3-phosphoglycerate without forming ATP.
Citrate is isomerized to Isocitrate by theenzyme Aconitase (aconitate hydratase); the reaction occurs in two steps: dehydration to cis-aconitate and rehydration to isocitrate.
Aconitase is an iron-sulfur protein, or nonheme ironprotein. It contains four iron atoms that are not incorporated as partof a heme group. The enzyme is inhibited by Fluoroacetate. The poison Fluoroacetateis toxic, because fluoroacetyl-CoA condenses with oxaloacetate to formfluorocitrate, which inhibits Aconitase, causing citrate to accumulate. This isan example of suicidal inhibition.
d) Malic enzyme
Malate to pyruvate conversion is catalyzedby the “malic enzyme” (NADP malate dehydrogenase). This reaction provides an alternative source of NADPH, which is used for thereductive biosynthesis.
e) Xylitol dehydrogenase
The enzyme catalyzes the conversion of L-Xylulose to Xylitol which is subsequently converted to D- Xylulose. The enzyme is NADP dependent. The deficiencycauses Essential Pentosuria. L- Xylulose is excessively excreted inurine.
L –Xylulose+ NADPH+H+————->Xylitol————–>D-Xylulose
In erythrocytes, the reaction catalyzed by phosphoglyceratekinase may be bypassed to some extent by the reaction of bisphosphoglyceratemutase, which catalyzes the conversion of 1,3-bisphosphoglycerate to2,3-bisphosphoglycerate, followed by hydrolysis to 3-phosphoglycerate and Pi,catalyzed by 2,3-bisphosphoglycerate phosphatase. This alternativepathway involves no net yield of ATP from glycolysis. However, it does serve toprovide 2,3-bisphosphoglycerate, which binds to hemoglobin, decreasing itsaffinity for oxygen, and so making oxygen more readily available to tissues.
Glucose enters glycolysis by phosphorylation to glucose 6-phosphate, catalyzed by hexokinase, usingATP as the phosphate donor. Under physiologic conditions, the phosphorylation of glucose to glucose 6-phosphate can be regarded as irreversible. Hexokinaseis inhibited allosterically by its product, glucose 6-phosphate. The deficiencyof hexokinase causes hemolytic anemia.
Glucose + ATP———–>Glucose-6-phosphate
h) UDP- Glucuronyl transferase
Bilirubin +2UDP-Glucuronic Acid————–>Bilirubin Diglucuronide +2UDP
The conjugation of Bilirubin is catalyzed by a specific glucuronyl transferase.The enzyme is mainly located in the endoplasmic reticulum, uses UDP-Glucuronic acid as the Glucuronyl donor, and is referred to as Bilirubin-UGT. Bilirubin Monoglucuronide is an intermediate and is subsequently converted to thediglucuronide. Most of the bilirubin excreted in the bile of mammals is in the form of bilirubin diglucuronide.
i) UDP- G dehydrogenase- It catalyzes the conversion of UDP glucose to UDP Glucuronic acid.
In liver, the uronic acid pathwaycatalyzes the conversion of glucose to Glucuronic acid, ascorbic acid (exceptin human beings and other species for which ascorbate is a vitamin), andpentoses. Glucose 6-phosphate is isomerized to glucose 1-phosphate, which thenreacts with uridine triphosphate (UTP) to form uridine diphosphate glucose(UDPGlc) in a reaction catalyzed by UDPGlc pyro phosphorylase, as occurs in glycogen synthesis.
UDPGlc is oxidized at carbon 6 byNAD-dependent. UDPGlc dehydrogenase in a two-step reaction to yield UDP-glucuronate
UDP-Glucose+2 NAD+ ——–>UDP Glucuronic acid+ 2NADH+2H+
UDP-glucuronate is the source ofglucuronate for reactions involving its incorporation into Proteoglycans or forreactions of substrates such as steroid hormones, Bilirubin, and a number o fdrugs that are excreted in urine or bile as glucuronide conjugates.
j) Galactose-1-phosphate Uridyl transferase
Galactokinase catalyses the phosphorylation of galactose, using ATP asphosphate donor
Galactose + ATP———–>Galactose-1-phosphate
Galactose 1-phosphate reacts with uridine diphosphate glucose (UDPGlc) to form uridine diphosphate galactose (UDPGal) andglucose 1-phosphate, in a reaction catalyzed by galactose 1-phosphate uridyl transferase.
Galactose-1-phosphate+ UDP-Glucose——————>UDP –Galactose +Glucose-1-P
The conversion of UDP Gal to UDPGlc iscatalyzed by UDPGal 4-epimerase. Inability to metabolize galactose occurs in the galactosemias, which may be caused by inherited defects of Galactokinase, uridyl transferase, or 4-epimerase, though deficiency of uridyl transferase is the best known.
Q.2- Mention the defect in each of the following diseases-
Disease Enzyme Deficient
a) Essential Pentosuria Xylitol dehydrogenase
b) Von-Gierke’s disease Glucose-6-phosphatase
c) Hereditary Fructose Intolerance Aldolase B
d) Benign Galactosemia Galactokinase
e) Classical Galactosemia Galactose-1-PUridyl transferase
f) Amylopectinosis Branching Enzyme
g) Favism Glucose-6-phosphatedehydrogenase
h) Mc Ardle’s syndrome Muscle Phosphorylase
Q.3- Name the enzyme for each of the following reactions-
1) UDP Glucose—————————>UDP- Glucuronic acid.
2) Phospho enolPyruvate—————> Pyruvate
(Phosphoenolpyruvate Carboxy kinase)
3) 3-phosphoglycerate<-—————–>1,3 Bisphosphoglycerate
4) Pyruvate <——————————->Lactate
(ALT- Alanine aminotransferase)
6) Pyruvate———————————> Oxalo acetate
7) Pyruvate——————————–>Acetyl co A
(Pyruvate dehydrogenase complex)
11) Fructose-1-phosphate—-.>Glyceraldehyde + Dihydroxy acetone phosphate
13) Glucose———————————–> Sorbitol
15) Glucose-1-P +UTP——————->UDP glucose+PPi
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