Dictyostelium discoideum or cellular slime mold is simple eukaryotic microorganism, which generally grows in forest soil and decaying leaves. This amoeba feeds on bacteria and grows as single cells. The development of Dictyostelium discoideum is simpler than that of mammalian cells. It uses many of the same signals that are found to function in higher eukaryotic organisms like plants and animals. Dictyostelium discoideum is an excellent system in which to study metabolic pathways which are simpler than that of the complex systems like mammalian system. Glucose is metabolized in glycolysis to yield pyruvate and lactate and further metabolized in the tricarboxylic acid cycle. Glucose can be polymerized into glycogen in addition to glycolysis process. In a metabolic pathway, the generation of glucose from certain non‐carbohydrate carbon substrates is called gluconeogenesis. In Dictyostelium discoideum, glucose is synthesized by the breakdown of pyruvate. Glycogen phosphorylase and amylase break down glycogen to form glucose. Glycogen synthase and glycogen phosphorylase are the key enzymes for the regulation. Both the enzyme equally regulated the process simultaneously, so that when one is activated, the other is deactivated. During gluconeogenesis, glucose is synthesized from pyruvate but sometimes during this process, three enzymes, glucose‐6‐phophatase, fructose‐1,6‐bisphosphatase, and phosphoenolpyruvate carboxykinase catalyze an irreversible reaction.
- eukaryotic system
- Dictyostelium discoideum
An amoeba is very interesting organism to study because it grows as single cells and develops as multi‐cellular organisms. They present a range of developmental processes which can be used to study of any molecular pathways like glycolysis or gluconeogenesis pathways . During evolution, the amoebozoa generated a large number of species which goes through the similar developmental stages, from unicellular to multi‐cellular stages [1–4]. The well‐characterized amoebozoan species is
2. The gluconeogenesis process in eukaryotic cell
Gluconeogenesis is a process by which carbohydrate is synthesized from non‐carbohydrate precursors like oxaloacetate and pyruvate (Figure 1). In the first step of the gluconeogenesis process, oxaloacetic acid is synthesized from pyruvic acid. On the other hand, in the citric acid cycle, oxaloacetic acid reacts with acetyl‐CoA. So, at low concentration of acetyl‐CoA and high concentration of ATP, gluconeogenesis proceeds. Gluconeogenesis starts in the mitochondria of the cells. In the first step, carboxylation of pyruvate occurs by pyruvate carboxylase enzyme and it forms oxaloacetate by using one ATP molecule. Oxaloacetate is reduced to malate by using NADH. After this step, the remaining steps of gluconeogenesis process occur in the cytosol. In the next step, malate is oxidized to oxaloacetate using NAD+. Oxaloacetate is first decarboxylated, and after that, it is phosphorylated by using the enzyme, PEP carboxykinase, and one GTP. In the next step, PEP converted into 2‐phosphoglycerate, 3‐phosphoglycerate and then 1,3‐bisphosphoglycerate by the enzyme enolase, phosphoglycerate mutase and phosphoglycerate kinase, respectively. In the next step of this reaction, 1,3‐bisphosphoglycerate converts into glyceraldehyde 3‐phosphate by the enzyme glyceraldehyde phosphate dehydrogenase. Now, the glyceraldehyde 3‐phosphate converts into fructose 1,6‐bisphosphate via two ways: one is direct conversion and another through the intermediate component called dihydroxyacetone phosphate. In the next step, fructose 1,6‐bisphosphate converts into fructose 6‐phosphate, using an enzyme, fructose 1,6‐bisphosphatase, one water molecule, and releasing one phosphate. This step is the rate‐limiting step in gluconeogenesis process. Glucose‐6‐phosphate is formed from fructose 6‐phosphate followed by glucose by the enzyme glucose‐6‐bisphosphatase. The reaction of the glucose formation occurs inside the endoplasmic reticulum, specifically in the lumen, where glucose‐6‐phosphate is hydrolyzed and produces glucose and releases an inorganic phosphate . 3.The developmental stages of
About 80 years ago, Ken Raper isolated
3.1. The microcyst
Encystment was a very common process to amoebae, but it was not known for
3.2. The macrocyst
In this stage of the sexual cycle, cells of two mating types fused . Under wet condition, the macrocyst form, which had three layered cellulose coat at maturity. After fusion, the cells formed giant cells which had at least two nuclei or many nuclei. This fused structure attracted other amoebae by chemotaxis to cAMP. The endocytes were formed by engulfing these cells, and after that the giant cells produced meiotic offspring . Macrocysts were formed from endocytes including hundred of cells.
3.3. Fruiting bodies
The fruiting body was formed through complex and polarized cell movements. In this stage, cells were not engulfed to form endocytes because one cell was recognized by the other cell. For the formation of fruiting body of
During mid‐developmental stage,
4. Size of the aggregates of
D. discoideum is depending upon the gluconeogenesis pathway indirectly
The group size of
The size of the terminal structures of
5. Gluconeogenesis process affected during the differentiation of myxamoebae of the cellular slime mold
Myxamoeba is a naked amoeboid uni‐nucleate protoplast that lacks both cilia and flagella. In the life cycle of
Wright et al. showed in their kinetic model, the glycogen content of the cell remained constant during the early developmental stage but it decreased when the culmination process occurred . White and Sussman suggested that the glycogen content of axenic cells was small . They also showed that the glycogen initially decreased because of the consumption of the bacterial glycogen for the development of axenically grown myxamoebae. Wright et al. assume in their model, during differentiation of
In the absence of glucose and the presence of very low concentrations of glycogen, myxamoebae grow and degrade all the glycogen during 4 h of development . Glycogen is synthesized during the late developmental stages (5–15 h) and finally broken down by the cell to synthesize saccharide. Hames and Ashworth showed that the amount of glycogen synthesized during the late developmental stage is larger than the glycogen content of the vegetative cells . This glycogen synthesis occurs during gluconeogenesis process when the cellular glucose remains at a constant low concentration. During differentiation, myxamoebal glycogen is not stored, but the gluconeogenesis process still can occur, if the cells initially have a large amount of glycogen.
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