Difference between glycolysis and pentose phosphate pathway

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• Citric acid cycle is a central metabolic pathway for metabolism of carbohydrates, fats and proteins. Citric acid cycle occurs in aerobic condition in mitochondria.
• At first carbohydrates, fats and proteins are catabolized by separate pathway to form acetyl-coA then Acetyl-coA enters into Citric acid cycle.
•  It is also known as Krebs cycle or Tri carboxylic acid (TCA) cycle.

Figure: Citric acid cycle

Two major reactions involved in citric acid cycle.

1. Formation of acetyl-coA
2. Reactions of citric acid cycle

1. Formation of acetyl-coA:

• Before entering into Krebs cycle, carbohydrates, fats and proteins are catabolized by separate pathway to form Acetyl-coA.
• For example: pyruvate formed by aerobic glycolysis is oxidized into acetyl-coA and CO2 by the enzyme Pyruvate dehydrogenase complex.

• It is an irreversible oxidative decarboxylation reaction in which a molecule of carbon in the form of CO2 is removed from pyruvate.
• In this reaction a molecule of pyruvate generate 1 NADH.
• This step is link between glycolysis and Krebs cycle.

2. Reactions of citric acid cycle

i. Formation of citrate (citric acid):

• It is a condensation reaction. Acetyl-coA condensed with oxaloacetate to form citrate and the reaction is catalyzed by the enzyme Citrate synthase.
• Oxaloacetate play catalytic role in citric acid cycle and at the end of process oxaloacetate is regenerated.

ii. Isomerization of citrate to Isocitrate:

• The enzyme aconitase catalyzes the isomerization of citrate to isocitrate with intermediate cis-aconitate.
• This is a reversible reaction.
• This reaction occurs in two steps- first dehydration and second hydration.

iii. Formation of α-ketoglutarate:

• This is an oxidative decarboxylation reaction.
• The enzyme isocitrate dehydrogenase catalyzes the oxidation of isocitrate to form α-ketoglutarate and CO2.
• In this step 1 molecule of NADH is generated.
• It is an irreversible reaction.

iv. Formation of succinyl-coA:

• This is also an oxidative decarboxylation reaction catalyzed by α-ketoglutarate dehydrogenase enzyme in which α-ketoglutarate is oxidized into succinylcoA and CO2.
• In this reaction, coA serves as carriers of succinyl group and NAD+ serves as electron acceptor.
• One molecule of NADH is generated in this step.

v. Formation of Succinate:

• Conversion of succinyl-coA to succinate is catalyzed by succinyl-coA synthetase or succinic thiokinase enzyme.
• This is a substrate level phosphorylation reaction in which CoA group ultimately donates its phosphate group to GDP forming energy rich GTP.
• A molecule of CO2 is released in this step.

vi. Formation of fumarate:

• Succinate dehydrogenase catalyzed the oxidation of succinate to from fumarate.
• It is reversible reaction.
• In this step a molecule of FADH2 is generated.

vii. Formation of malate:

• This reaction is catalyzed by fumarase (fumarate hydratase) in which fumarate is converted into malate.
• This is a hydration and reversible reaction.

viii. Formation of oxaloacetate: regeneration of oxaloacetate:

• Malate is oxidized into oxaloacetate generating a molecule of NADH.
• This reaction is catalyzed by malate dehydrogenase enzyme.
• Oxaloacetate is regenerated in this step and combines with acetylcoA and continues the cycle.

Significance of TCA cycle: Role of TCA cycle

i. Role in Central metabolic pathway:

• TCA cycle is a final common metabolic pathway of carbohydrates, fattyacids and aminoacids.
• At first all these biomolecules are catabolized by their separate metabolic pathways to generate acetyl-coA then acetyl-coA enters TCA cycle for further metabolism in aerobic condition.
• TCA is more efficient in energy conservation than other pathways of metabolism.

ii. TCA is an amphibolic pathway:

• It plays role in both catabolism and anabolism.

Catabolic role:

•  TCA is a catabolic pathway because it oxidizes acetyl-coA completely into CO2 and H2O and releases large amount of energy.

Anabolic role:

•  TCA is an anabolic pathway because it provides precursors for biosynthesis of other molecules in cells. Such as citrate, α-ketoglutarate, succinylcoA and oxaloacetate act as precursors for biosynthesis of various molecules.
• Glucose, purine and pyrimidine are synthesized from oxaloacetate.
• Fattyacids and steroids are synthesized from succinylcoA.
• Some aminoacids, purine and pyrimidine are synthesized from α-ketoglutarate.

iii. Citric acid cycle is an aerobic process:

•  NAD+ and FAD are electron acceptors in the TCA cycle. These are regenerated by Electron transport chain which requires oxygen as final electron acceptor. Hence overall TCA and ETC are aerobic process.

Citric acid cycle or Krebs cycle or Tricarboxylic acid (TCA) cycle

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• This pathway occurs in both aerobic and anaerobic condition
• Occur in prokaryotes only
• It occurs in cytoplasm
• Pyruvate and glyceraldehyde-3-phosphate produced from glucose by ED pathway

• At first glucose is phosphorylated to glucose -6-phosphate by the enzyme hexokinase.
• Glucose-6-phosphate is then oxidized to 6-phosphogluconolactone releasing a molecule of NADPH. This reaction is catalyzed by the enzyme glucose-6-phosphate dehydrogenase.
• Hydrolase enzyme converts 6-phopshogluconolactone to 6-phosphogluconate.
• 6-phosphogluconate undergoes dehydration reaction catalyzed by 6-phosphogluconate dehydratase to form 2-keto 3-deoxy 6-Phosphogluconate (KDPG).
• KDPG splits to form pyruvate and glceraldehyde-3-phosphate. It is catalyzed by KDPG aldolase enzyme
• Glyceraldehyde-3-phosphate is then metabolized by glycolysis to form pyruvate.

Significance of ED pathway

• This pathway used two specific enzymes ie. 6-phosphogluconate dehydratase and KDPG aldolase.
• This pathway generates 1 ATP, 1 NADH and 1 NADPH from one glucose molecule.

Entner-Doudoroff (ED) pathway

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• Glyoxylate cycle occurs in some microorganisms when acetate is sole source of carbon
• This cycle has two unique enzyme- isocitrate lyase and malate synthase which bypass some of the reaction of TCA cycle.

figure: Glyoxylate cycle

• Glyoxylate cycle is absent in higher organism.
• At first acetylcoA is produced from acetate or by oxidation of higher fattyacids.
• AcetylcoA then enter into TCA cycle and condensed with oxaloacetate to form citrate.
• Citrate then isomerized to isocitrate.
• Isocitrate lyase bypass the TCA cycle by splitting isocitrate into succinate and glycoxylate.
• Succinate metabolized by TCA whereas Glycoxylate condenses with another molecule of acetylcoA to form malate in the presence of malate synthase.
• Malate is converted into oxaloacetate by the enzyme malate dehydrogenase.

Significance of Glyoxylate cycle

• It is bypass reaction of TCA cycle
• It occurs in bacteria when they are cultured in acetate rich carbon source.
• When Higher fattyacids are oxidized into acetylcoA without forming puruvate acids, then acetylcoA enters into glyoxylate cycle.

 Glyoxylate cycle

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• Pentose phosphate pathway is an alternative pathway to glycolysis and TCA cycle for oxidation of glucose.
• It is a shunt of glycolysis
• It is also known as hexose monophosphate (HMP) shunt or phosphogluconate pathway.
• It occurs in cytoplasm of both prokaryotes and eukaryotes
• Pentose phosphate pathway starts with glucose and it is a multi-steps reaction.

• The sequence of reactions are divided into two types.

I) oxidative reaction phase
II) Non-oxidative reaction phase

Oxidative phase:

• First four reactions are irreversible and oxidative in which glucose molecule is oxidized twice generating two molecules of NADPH and glucose is converted into Ribose-5 phosphate.

1st reaction: conversion of glucose to glucose-6 phosphate.

• This reaction is catalyzed by the enzyme hexokinase and a molecule of ATP is utilized. This reaction is actually a primary step of glycolysis.

2nd reaction: conversion of glucose-6 phosphate to 6-phosphogluconolactone.

• This reaction is catalyzed by an enzyme glucose-6 phosphate dehydrogenase (G6PD) in the presence of Mg++ ion.
• In this reaction a molecule of NADPH is produced.

3rd reaction: conversion of 6-phosphogluconolactone to 6-phosphogluconate

• This reaction is a hydrolysis reaction catalyzed by hydrolase enzyme

4th reaction: conversion of 6-phosphogluconate to ribose-5 phosphate

• This reaction is catalyzed by the enzyme 6-phosphogluconate dehydrogenase to produce 3-keto-6-phosphogluconate which undergoes decarboxylation to produce ribulose-5 phosphate.
• In this reaction a molecule of NADPH is generated.

Non oxidative phase:

• Oxidative reactions is followed by a series reversible sugar phosphate inter-conversion reaction.
• Ribulose-5-phosphate is epimerized to produce xylulose 5-phosphate in the presence of enzyme phosphor pentose epimerase. Similarly ribulose-5-phosphate is also keto-isomerized into ribose 5-phosphate.
• Xylulose-5-phsphate transfer two carbon moiety to ribose 5-phospahate in the presence of enzyme transketolase to form sedoheptulose-7-phosphate and glyceraldehyde 3—phosphate.
• Sedoheptulose -7-phosphate transfer three carbon moiety to glyceraldehyde -3-phosphate to form fructose 6-phopsphate and erythrose 4-phosphate in the presence of enzyme transaldolase.
• Transketolase enzyme catalyse the transfer of two carbon moiety from Xylulose-5-phsphate to erythrose-4- phosphate to form fructose-6-phosphate and glyceraldehyde-3-phosphate.
• Fructose-6-phosphate and glyceraldehyde-3-phosphate is later enter into glycolysis and kreb’s cycle.
• The rate and direction of reversible reaction depends upon the needs of cell.
• If cell needs only NADPH then fructose-phosphate and glyceraldehyde-3-phosphate are converted back to glucose by reverse glycolysis, otherwise converted to pyruvate and enter TCA cycle generating ATPs.

Significance of Pentose phosphate pathway

• HMP is only the cytoplasmic pathway that generates NADPH
• NADPH is produced in this pathway acts as reducing agent during biosynthesis of various molecules eg. fattyacids.
• This pathway generates 3, 4, 5, 6 and 7 carbon compounds which are precursors for biosynthesis of other molecules. Eg nucleotides are synthesized from ribose-5-phsophate.
•  Pentose phosphate pathway is very essential for cell lacking mitochondria (eg.RBCs) for generation of NADPH.
• Triose, tetrose, pentose, hexose and heptose sugar are generated as intermediate products in pentose phosphate pathway.
• NADPH is also used to reduce (detoxify) Hydrogen peroxide in cell.
• Resistance to malaria in some Africans are associated with deficiency of glucose-6-phosphate dehydrogenase enzyme because malarial parasites depend upon HMP shunt to reduce glutathione in RBCs.

Pentose phosphate pathway (PPP) or hexose monophosphate (HMP) shunt

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