COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

GLYOXYLATE CYCLE

It had been observed by many plant physiologists that during the germination of fatty seeds, the fat content decreased with a simultaneous increase in glucose(i.e, carbohydrates).

Kornberg and Krebs (1957) framed a cycle which is known as Glyoxylic acid cycle or Glyoxylate Cycle through which the fats could be converted in to sucrose (i.e., carbohydrates) during the germination of fatty seeds in plants. The glyoxylate cycle (which is intimately associated with the Krebs’ cycle) is now known to occur in many other bacteria, yeasts, molds, and in higher plants and is completed in glyoxysomes, mitochondria and cytosol.

Various steps of this cycle occuring in higher plants especially during the germination of fatty seeds are as follows:

1) REACTION IN GLYOXYSOME

i) Acetyl-CoA produced after the β oxidation of fatty acids condenses with oxaloacetic acid to form citric acid.

CH3COCoA

Acetyl CoA CH2COOH

+ Citrate synthase C(OH)COOH + CoA

COCOOH CH2COOH

CH2COOH +H2O Citric acid

Oxaloacetic acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

ii) Citric acid is dehydrated to produce cis-aconitic acid in the presence of aconitase.

CH2COOH Aconitase CH2COOH C(OH)COOH CCOOH

CH2COOH - H2O CHCOOH Citric acid Cis -aconitic acid

iii) Cis aconitic acid reacts with one molecule of H2O to form iso-citric acid.

CH2COOH CH2COOH

CCOOH + H2O CHCOOH CHCOOH CHOHCOOH Cis -aconitic acid Iso-citric acid

iv) Iso- citric acid is broken down into glyoxylic acid and succinic acid by the enzyme Isocitratase.

CH2COOH Isocitratase CHO CH2COOH

CHCOOH COOH + CH2COOH

CHOHCOOH Glyoxylic acid Succinic acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

Iso-citric acid v) Glyoxylic acid combines with acetyl CoA in the presence of malate synthetase to produce malic acid.

CHO malate synthetase COOH

COOH + CH3COCoA CH2 + CoA

Glyoxylic acid Acetyl CoA +H2O CHOH COOH Malic acid

vi) Malic acid is oxidized in to oxaloacetic acid in the presence of Malic dehydrogenase and the coenzyme NAD.

COOH malic dehydrogenase COCOOH

+ + CH2 + NAD +NADH +H

CHOH CH2COOH

COOH Oxaloacetic acid Malic acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

2) REACTIONS IN MITOCHONDRION

vii) Succinic acid moves in to mitochondrion and is converted in to oxaloacetic acid as in the Krebs’ cycle.

CH2COOH By reaction of Krebs’ cycle COCOOH

CH2COOH CH2COOH Succinic acid Oxaloacetic acid

viii) Oxaloacetic acid produced in the above step is decarboxylated in the presence of ATP to form phosphoenol pyruvic acid.

COCOOH CH2

CH2COOH + ATP COPO3H2 + CO2 + ADP

Oxaloacetic acid COOH Phosphoenol pyruvic acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

3) REACTIONS IN CYTOSOL

ix) Phosphoenol pyruvic acid moves in to the cytosol and by the reverse reactions of glycolysis and with slight modification is converted in to glucose and fructose phosphates. x) Finally, glucose and fructose phosphates are converted in to sucrose.

SIGNIFICANCE OF GLYOXYLATE CYCLE

1) During germination of fatty seeds, the fats which are insoluble are hydrolysed in to fatty acids and glycerol. Fatty acids after β oxidation produce acetyl-CoA units which synthesize sucrose through glyoxylate cycle. Soluble sucrose is then supplied to different growing regions of the young germinating seedling till it develops its own photosynthetic system 2) Those microorganisms which can grow on ethyl alcohol or acetate as a sole source of energy and carbon, make use of this cycle in synthesizing longer carbon chains. 3) The glyoxylate cycle is an example of gluconeogenesis.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

GLUCONEOGENESIS

Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. Gluconeogenesis occurs through glyoxylate cycle, β oxidation, α oxidation.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.