Why Is Gluconeogenesis Not Just Glycolysis in Reverse?
- Apr 20, 2017
- Biological Processes, MCAT Biology, MCAT Blog, PCAT Blog
Glycolysis is widely understood to be one of the most important metabolic pathways to study for the exam, but students often have difficulty studying gluconeogenesis in parallel to glycolysis. On a certain level, it can be tempting to think of gluconeogenesis as the reverse of glycolysis, because glycolysis breaks down glucose into two pyruvate molecules to obtain energy and feed into the citric acid cycle, whereas gluconeogenesis takes two pyruvate molecules and builds them into a glucose molecule. However, it’s not quite right to just think of gluconeogenesis as glycolysis in reverse, because there are some differences between the two that are important to understand.
First, glycolysis is an ancient metabolic pathway that is common to all forms of cellular life, and can be performed by all cells to obtain energy (a net of 2 ATP per glucose). In contrast, gluconeogenesis is much more specific; in humans, it is primarily carried out in liver cells, as well as to some extent in the adrenal cortex. It also has a much more specific goal: to produce glucose to be secreted into the bloodstream when blood glucose levels are low.
Second, the biochemical details of gluconeogenesis are different from those of glycolysis. The reason usually given for this is that gluconeogenesis needs to bypass the energetically favorable/irreversible steps of glycolysis. This statement is true, but it’s worth examining what’s going on here in somewhat greater detail. Let’s first quickly review the steps of glycolysis that gluconeogenesis bypasses:
- Step 1: glucose → glucose 6-phosphate (G6P), in a reaction catalyzed by hexokinase. In this step, an ATP is invested; more specifically, a phosphate group from ATP is transferred to glucose to form G6P.
- Step 3: fructose 6-phosphate (F6P) → fructose 1,6-bisphosphate (F1,6BP), which is catalyzed by phosphofructokinase-1. Mechanistically, this is similar to step 1; an ATP is invested, and the phosphate group is transferred to F6P to form F1,6BP. However, this is the committed step of glycolysis and is subject to especially strict regulation.
- Step 10: phosphoenolpyruvate (PEP) → pyruvate, which is catalyzed by pyruvate kinase. This step forms ATP, is subject to regulation, and is the end of glycolysis. It generates pyruvate, which the cell can further feed into the citric acid cycle.
Note that steps 1 and 3 of glycolysis (glucose → G6P and F6P → F1,6BP) involve phosphorylation, whereas step 10 (PEP → pyruvate) generates the end product. Let’s look at the various ways that gluconeogenesis differs from glycolysis, starting with the pyruvate end of the pathway.
The final step of glycolysis is the conversion of PEP to pyruvate. This step is highly energetically favorable, so the first challenge in gluconeogenesis is to bypass it. In the mitochondria, pyruvate carboxylase converts pyruvate to oxaloacetate by adding a COO- group. Oxaloacetate is briefly converted to malate for transport out of the mitochondria, where it is then converted immediately back to oxaloacetate. At this point, in the cytosol, PEP carboxykinase converts oxaloacetate to PEP. The reason for this intricate process is both because the direct conversion of PEP to pyruvate is irreversible and because the cell must avoid a futile cycle in which pyruvate from glycolysis is immediately converted back to PEP. This is why gluconeogenesis has a two-step pathway split up between the mitochondria and cytosol.
Moving upstream, in gluconeogenesis, the enzyme fructose 1,6-bisphosphatase catalyzes the hydrolysis of the phosphate group on C1 of F1,6BP, resulting in F6P. F6P is isomerized to G6P. Then, the conversion of G6P to glucose is the final step where gluconeogenesis bypasses glycolysis; in this step, glucose 6-phosphatase catalyzes a hydrolysis reaction in which G6P yields glucose and inorganic phosphate.
Let’s take a closer look at the bypassed reactions that involve phosphorylation. In steps 1 and 3 of glycolysis, an ATP is invested in putting a phosphate group onto the substrate. This in and of itself tells you that gluconeogenesis can’t just reverse glycolysis, because doing so would mean that gluconeogenesis would itself create ATP. That would be impossible! You may remember that the electron transport chain and ATP synthase are specialized structures that allow ATP to be formed. This is a complex, energetically costly process that can’t just randomly be replicated in a metabolic pathway. Also, it contradicts the basic logic of gluconeogenesis; the goal of gluconeogenesis is not to produce energy for the cell, but to produce glucose to be circulated in the bloodstream. To summarize:
- Steps 1 and 3 of glycolysis are bypassed by gluconeogenesis because the glycolytic steps involve transferring a phosphate group from ATP, and gluconeogenesis can’t regenerate ATP.
- Step 10 of glycolysis is bypassed by gluconeogenesis to work around an irreversible reaction and to avoid a futile cycle.
The figure below compares gluconeogenesis and glycolysis. While it is certainly useful to memorize the bypasses, understanding why certain steps are bypassed will help you consolidate your knowledge.
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