What is the Difference Between Glyoxylate and TCA Cycle

The main difference between glyoxylate and TCA cycle is that the glyoxylate cycle produces glucose from acetyl CoA, whereas the TCA cycle produces carbohydrates, fats, and proteins from acetyl CoA. 

Glyoxylate and TCA cycle are two metabolic pathways that occur in living organisms. Both pathways use acetyl CoA produced by β-oxidation. 

Key Areas Covered

1. What is a Glyoxylate Cycle
– Definition, Facts, Importance
2. What is a TCA Cycle
– Definition, Facts, Importance
3. Similarities Between Glyoxylate and TCA Cycle
– Outline of Common Features
4. Difference Between Glyoxylate and TCA Cycle
– Comparison of Key Differences

Key Terms 

Citric Acid Cycle, Krebs Cycle, Glyoxylate Cycle, TCA Cycle

What is Glyoxylate Cycle

The glyoxylate cycle is a variation of the TCA cycle that operates in plants, protists, fungi, and bacteria. In microorganisms, the glyoxylate cycle is important for synthesizing glucose from a two-carbon compound, acetyl CoA. Acetyl CoA is added to the citrate in the glyoxylate cycle to produce isocitrate. Then, isocitrate lyase converts isocitrate into glyoxylate and succinate. Malate synthase converts glyoxylate and succinate into malate by adding another acetyl CoA. Further, malate converts into oxaloacetate, which produces carbohydrates in gluconeogenesis. However, it does not operate in animals due to the lack of two enzymes necessary for the pathway: isocitrate lyase and malate synthase. Instead, animals can turn fat into glucose in the TCA cycle.

Figure 1: Glyoxylate Cycle

What is a TCA Cycle

TCA (tricarboxylic acid) cycle is a process that produces carbohydrates, proteins, and fat from the oxidation of acetyl CoA. It is also known as the citric acid cycle or Krebs cycle. During the TCA cycle, the acetyl part of the acetyl-CoA combines with an oxaloacetate molecule to form a citrate molecule, a six-carbon molecule. Then, the oxidation of citrate occurs in a series of steps in the TCA cycle into oxaloacetate:

1.  converts citric acid into α-ketoglutarate.
2. Α-ketoglutarate dehydrogenase and succinyl CoA synthetase convert α-ketoglutarate succinate.
3. Succinate oxidizes into malate.
4. Malate dehydrogenase converts malate into oxaloacetate. 
   
   

   Figure 2: TCA Cycle

Moreover, the importance of the TCA cycle is that it produces the five universal metabolic precursors, acetyl-CoA, pyruvate, oxaloacetate, succinate, and alpha-ketoglutarate (αKG) for the biosynthesis of molecules. Mainly, acetyl CoA comes from glycolysis, which produces pyruvate from glucose. Pyruvate dehydrogenase is the enzyme that converts pyruvate into acetyl CoA.

Similarities Between Glyoxylate and TCA Cycle

• Glyoxylate and TCA cycle are two metabolic pathways in plants, bacteria, and fungi.
• They use acetyl CoA produced by β-oxidation.
• Both are anabolic pathways.

Difference Between Glyoxylate and TCA Cycle

Definition

The glyoxylate cycle refers to a variation of the tricarboxylic acid cycle and is an anabolic pathway occurring in plants, bacteria, protists, and fungi. In contrast, the TCA cycle is a metabolic pathway connecting carbohydrate, fat, and protein metabolism.

Occurrence

The glyoxylate cycle only occurs in plants, protists, fungi, and bacteria, while the TCA cycle occurs in all living organisms.  

Process

The glyoxylate cycle produces glucose from acetyl CoA, while the TCA cycle produces carbohydrates, fats, and proteins through the oxidation of acetyl CoA.  

Importance

The glyoxylate pathway allows growth in the absence of glucose, while the TCA cycle provides the five universal metabolic precursors, acetyl-CoA, pyruvate, OAA, succinate, and alpha-ketoglutarate (αKG), for the biosynthesis of molecules. 

Conclusion

In brief, the glyoxylate and TCA cycle are two metabolic pathways that use acetyl CoA. The glyoxylate cycle only occurs in plants, fungi, protists, and bacteria. Also, it produces glucose from acetyl CoA. Additionally, it allows growth in the absence of glucose. In comparison, the TCA cycle occurs in all living organisms. It produces carbohydrates, proteins, and fats from acetyl CoA. Importantly, it provides acetyl-CoA, pyruvate, OAA, succinate, and alpha-ketoglutarate (αKG) for the biosynthesis of molecules. Therefore, the main difference between glyoxylate and TCA cycles is their process. 

References:

1. (2023b, August 31). 6.6: Glyoxylate Pathway. Biology LibreTexts. 
2. John E. Cronan, Jr., Laporte, D., John E. Cronan, Jr. D. of M. and B., & David LaporteDepartment of Biochemistry, M. B. & B. (2005, May 12). Tricarboxylic acid cycle and glyoxylate bypass. EcoSal Plus. 

Image Courtesy:

1. “Glyoxylatepath” By Adenosine at English Wikipedia – Own Work (CC-BY SA 2.5) via Commons Wikimedia
2. “Citric acid cycle with aconitate 2” By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins – Own work (CC BY-SA 3.0) via Commons Wikimedia