TABLE OF CONTENTS

Title Page    –         –         –         –         –         –         –         –         –         i

Certification –         –         –         –         –         –         –         –         –         ii

Dedication   –         –         –         –         –         –         –         –         –         iii

Acknowledgement –         –         –         –         –         –         –         –         iv

Table of Contents  –         –         –         –         –         –         –         –         vi

CHAPTER ONE: INTRODUCTION                 

1.1 Background of the Study – –         –         –         –         –         1

CHAPTER TWO: GLYOXYLATE CYCLE      –         –         –         5

2.1     Steps Involved in Glyoxylate Cycle     –         –         –         –         6

2.2     Similarities between Glyoxylate and TCA Cycle     –         –         10

2.3     Enzymes involved in Glyoxylate Pathway     –         –         –         10

CHAPTER THREE: THE BIOCHEMICAL IMPORTANCE

OF GLYOXYLATE CYCLE IN PLANT METABOLISM   

3.1     Conversion of Fat into Carbohydrate   –         –         –         –         15

3.2     Role of Glyoxylate Cycle in Gluconeogenesis         –         –         16

3.3     Products of Glyoxylate Cycle    –         –         –         –         –         17

3.3.1  Succinate Production       –         –         –         –         –         –         17

3.3.2  Production of Fumarate and Malate     –         –         –         –         20

3.3.3  Production of Glycolate   –         –         –         –         –         –         24

CHAPTER FOUR: SUMMARY AND CONCLUSION

4.1     Summary     –         –         –         –         –         –         –         –         29

4.2     Conclusion  –         –         –         –         –         –         –         –         29

References

 

CHAPTER ONE: INTRODUCTION

1.1     Background Information

It had been observed by many plant physiologists that during the germination of fatty seeds, the fat content decreased with a simultaneous increase in sucrose (i.e., carbohydrates). This apparent conversion of fats into sucrose remained a mystery till 1957 when Kornberg and Krebs discovered that a strain of bacterium Pseudomonas could readily convert C- labeled acetic acid into labeled malic acid and citric acid (these are inter mediates of Krebs’ Cycle) which involved the following reactions: (1) Acetyl-CoA combined with Oxaloacetic acid to form Citric Acid (Borzyskowskli et al., 2018).

Acetyl-CoA + Oxaloacetic acid → Citric Acid + CoA (Acetic acid first reacted with Coenzyme-A to form Acetyl CoA). (2) Acetyl CoA reacted with glyoxylic acid in the presence of the enzyme malate synthetase to produce Malic acid. Acetyl CoA + Glyoxylic Acid → Malic Acid + CoA The glyoxylic acid was obtained through the breakdown of Iso-Citric Acid (an inter mediate of Krebs’ Cycle) by the enzyme Isocitratase Isocitric acid → glyoxylic acid + succinic acid. On the bases of the above reactions Kornberg and Krebs (1957) framed a cycle which is called as Glyoxylic acid cycle or Glyoxylate Cycle through which the fats could be converted into sucrose (i.e., carbohydrate) during the germination of fatty seeds in plants. The glyoxylate cycle (which is intimately associated with Krebs’ Cycle) is now known to occur in many other bacteria, yeasts, molds, and higher plants and is completed in glyoxysomes, mitochondria and cytosol (Shimizu et al., 2019).

The glyoxylate cycle, also known as glyoxylate shunt (GS) was identified by Konberg and Krebs in 1957, explaining how organisms could grow on acetate as the sole carbon source for substances degraded exclusively to acetylmoiches e.g acetate, fatty acids and ketogenic amino acids, this pathway provides a simple and efficient strategy for anapterosis and gluconeogenesis and this, cell growth. The glyoxylate cycle is generally regarded as an ancilliary pathway of the TCA cycle, which widely acknowledged as the central metabolic hub of the cell. This pathway comprises two dedicated enzymes: Isocitrate lyase (ICL) and malate synthesis (MS). ICL catalyzes the aldol cleavage of Isocitrate to succinate and glycolate, while Ms catalyzes the synthesis of malate from glyoxylate, while Ms catalyses the synthesis of malate from glyoxylate and acetyl-CoA. The overall effect of this pathway is the formation of one malate from two molecules of acetyl-CoA. It by passed the oxidative decarboxylation steps of the TCA cycle and is also implicated in pathogenesis, antibiotic resistance and oxidative stress tolerance. In view of its significance in metabolism and pathogenicity, the enzymology and metabolic regulation, particularly for Escherichia coli, and the pathogenic bacterium mycobacterium tuberculosis (Dolan and Welch, 2018), undoubtedly, the gained knowledge lays a foundation for the bio-production of related chemicals in metabolic for the bio-production of related chemicals in metabolic engineering (Kabasch et al., 2019).

The glyoxylate cycle is involved in the synthesis of various chemicals. In recent years, studies on biosynthesis of organic acids, amino acids, and fatty acids related products through GS engineering have been reported some of the outstanding results are present to balance product output, reducing power regeneration and cell growth, the glyoxylate cycle needs to be reinforced, weakened, fine-tuned, or dynamically controlled in different production cases. When heterologously expressed in some strains lacking this pathway, the GS amplified carbon source spectrum and enabled more metabolic flexibility, thus facilitating bio-production (Kabasch et al., 2013; Schada Von, Borzyskowskli et al., 2018; Shimizu et al., 2019).