Glycolysis: Videos & Practice Problems

Glycolysis is a crucial metabolic pathway consisting of 10 biochemical reactions divided into two main phases: the energy-consuming phase (Phase A) and the energy-producing phase (Phase B). In Phase A, the first five reactions convert one molecule of glucose into two molecules of glyceraldehyde 3-phosphate (G3P).

During this phase, three reactions are irreversible and each consumes one molecule of adenosine triphosphate (ATP). The process begins with glucose, which requires the consumption of one ATP to add a phosphate group, resulting in glucose 6-phosphate. This compound can then be converted into fructose 6-phosphate through a reversible reaction.

Next, another ATP is consumed to add a second phosphate group, transforming fructose 6-phosphate into fructose 1,6-bisphosphate, where "bis" indicates the presence of two phosphate groups. The fourth reaction allows for a split, leading to the formation of either dihydroxyacetone phosphate or two molecules of glyceraldehyde 3-phosphate. The fifth reaction is reversible, allowing for interconversion between these two products.

This overview of Phase A highlights the essential steps and transformations that occur as glucose is prepared for further energy extraction in Phase B of glycolysis, which will be discussed later.

In the initial phase of glycolysis, two key reactions occur: phosphorylation and isomerization. The first reaction involves the enzyme hexokinase, which catalyzes the phosphorylation of glucose. This process utilizes ATP as both an energy source and a phosphate donor. During this reaction, hexokinase transfers a phosphate group from ATP to glucose, resulting in the conversion of ATP to ADP and transforming glucose into glucose 6-phosphate (C₆H₁₃O₉P).

The reaction can be summarized by the following equation:

Glucose + ATP → Glucose 6-phosphate + ADP

In the second reaction, the enzyme phosphoglucoisomerase facilitates the isomerization of glucose 6-phosphate to fructose 6-phosphate. Both compounds are isomers, meaning they share the same molecular formula (C₆H₁₃O₉P) but differ in their structural arrangement. This transformation does not alter the number of carbon, hydrogen, oxygen, or phosphate atoms; it simply rearranges the connections between them.

Overall, these two reactions are crucial steps in glycolysis, setting the stage for further metabolic processes by modifying glucose into a form that can be more readily utilized in energy production.

In the third reaction of glycolysis, we focus on the phosphorylation of fructose 6-phosphate, which is a crucial step in the metabolic pathway. This reaction, like the first, involves the consumption of an ATP molecule, highlighting the energy investment phase of glycolysis. The enzyme responsible for this reaction is phosphofructokinase, a type of kinase that facilitates the transfer of a phosphate group from ATP to the substrate.

During this process, the enzyme catalyzes the conversion of fructose 6-phosphate by replacing a hydrogen atom with a phosphate group. The reaction can be summarized as follows:

Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate + ADP

In this equation, ATP donates a phosphate group, resulting in the formation of fructose 1,6-bisphosphate, which is essential for the continuation of glycolysis. This step is significant as it not only modifies the substrate but also prepares it for subsequent reactions in the glycolytic pathway, ultimately leading to the production of energy in the form of ATP.

In the fourth reaction of glycolysis, the enzyme aldolase plays a crucial role by cleaving the carbon-carbon bond in the fructose 1,6-bisphosphate molecule. This enzymatic action results in the formation of two triose phosphates, specifically dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). The reaction can be summarized as follows:

Fructose 1,6-bisphosphate → Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphate

During this process, aldolase effectively cuts the fructose molecule in the middle, leading to the production of these two important intermediates that continue through the glycolytic pathway. Understanding this reaction is essential, as both DHAP and G3P are pivotal for energy production in cellular metabolism.

In glycolysis, the final reaction of Phase A involves an isomerization process where dihydroxyacetone phosphate (DHAP) is converted into glyceraldehyde 3-phosphate (G3P). This reaction is catalyzed by the enzyme triose phosphate isomerase, which facilitates the conversion between these two molecules. The isomerization is reversible, meaning that G3P can also be converted back into DHAP.

In terms of molecular structure, DHAP contains a ketone functional group, indicating that the central carbon is a ketone carbon. Conversely, G3P features an aldehyde functional group, characterized by a carbonyl group (C=O) bonded to a hydrogen atom. This structural distinction is crucial for understanding the biochemical pathways involved in glycolysis.

Overall, this reaction marks a significant step in glycolysis, highlighting the importance of isomerases in facilitating the interconversion of different sugar phosphates, which are essential for energy production in cellular metabolism.

Glycolysis is a crucial metabolic pathway that occurs in the cytoplasm of cells, where glucose is broken down to produce energy. In the first phase of glycolysis, often referred to as Phase A, energy is consumed rather than produced. This phase involves several key reactions, primarily phosphorylation reactions, which are catalyzed by enzymes known as kinases. These enzymes facilitate the transfer of phosphate groups from ATP to specific molecules, thereby activating them for subsequent reactions.

One important aspect of Phase A is that it does not produce ATP; instead, it consumes energy. For instance, the bond cleavage in reaction 4 does not lead to ATP production, as this phase focuses on energy investment. Instead, hydrolysis reactions are responsible for energy release, which occurs in later stages of glycolysis.

During this phase, the cleavage of glucose results in the formation of two triose phosphates: Dihydroxyacetone Phosphate (DHAP) and Glyceraldehyde 3 Phosphate (G3P). These two molecules can interconvert through isomerization, a reaction catalyzed by the enzyme Triose Phosphate Isomerase. This enzyme is specifically an isomerase, which facilitates the rearrangement of molecular structures without adding or removing atoms.

In summary, while kinases play a vital role in the phosphorylation reactions of glycolysis, the incorrect statement regarding Phase A is that bond cleavage in reaction 4 produces ATP, which is not the case. Instead, this phase is characterized by energy consumption and the formation of triose phosphates.

Phase B of glycolysis encompasses the final five reactions that convert two molecules of glyceraldehyde 3-phosphate (G3P) into two molecules of pyruvate, while simultaneously extracting energy. This phase is crucial as it produces a total of 2 NADH and 4 ATP molecules. Notably, the last reaction of this phase, known as reaction 10, is irreversible, marking a significant point in the glycolytic pathway.

The process begins with the incorporation of inorganic phosphate to form 1,3-bisphosphoglycerate (1,3-BPG), during which NADH is generated, representing a high-energy molecule. In the subsequent reversible reaction (reaction 7), 1,3-BPG is converted into 3-phosphoglycerate (3-PG) through the loss of a phosphate group, which is then used to phosphorylate ADP, resulting in the production of ATP.

In reaction 8, the phosphate group on 3-PG is repositioned to create 2-phosphoglycerate (2-PG). Following this, reaction 9 involves the removal of water, leading to the formation of phosphoenolpyruvate (PEP), a high-energy intermediate. Finally, in the last step, the removal of the final inorganic phosphate from PEP results in the production of ATP and the formation of pyruvate.

Since the process starts with two G3P molecules, each step occurs twice, yielding a total of 2 NADH and 4 ATP molecules by the end of phase B. This overview highlights the essential transformations and energy yield during this critical phase of glycolysis, setting the stage for a deeper exploration of each individual reaction.

In glycolysis, reaction 6 plays a crucial role as it involves the oxidation of two molecules of glyceraldehyde 3-phosphate (G3P) to produce 1,3-bisphosphoglycerate (1,3-BPG). This reaction is catalyzed by the enzyme glyceraldehyde 3-phosphate dehydrogenase, which belongs to the class of enzymes known as dehydrogenases, responsible for facilitating oxidation reactions.

During this process, NAD⁺ is reduced to NADH, highlighting the importance of this coenzyme in cellular respiration. The reaction can be summarized as follows: G3P, in the presence of inorganic phosphate (Pi) and the enzyme glyceraldehyde 3-phosphate dehydrogenase, is converted into 1,3-BPG. Notably, a hydrogen atom from G3P is transformed into a phosphate group (PO₄^2-), which is incorporated into the product.

1,3-BPG is a key intermediate that will eventually be converted into pyruvate, the end product of glycolysis. It is essential to remember that this reaction occurs twice in glycolysis due to the presence of two G3P molecules, emphasizing the significance of oxidation in energy production. Understanding the role of dehydrogenases and the transformation of substrates into high-energy intermediates is vital for grasping the overall metabolic pathway of glycolysis.

In reaction 7 of glycolysis, a crucial phosphate transfer occurs, specifically involving 1,3-bisphosphoglycerate (1,3-BPG) and resulting in the formation of 3-phosphoglycerate (3-PG). This reaction is catalyzed by the enzyme phosphoglycerate kinase, which plays a vital role in transferring phosphate groups. During this process, 1,3-BPG, which contains two phosphate groups, donates one inorganic phosphate to adenosine diphosphate (ADP), converting it into adenosine triphosphate (ATP). This transformation is significant as it generates energy for the cell.

The reaction can be summarized by the following equation:

\[\text{1,3-BPG} + \text{ADP} \xrightarrow{\text{phosphoglycerate kinase}} \text{3-PG} + \text{ATP}\]

As a result, the loss of one phosphate group from 1,3-BPG not only leads to the production of 3-PG but also contributes to the energy currency of the cell by synthesizing ATP. This step highlights the importance of kinase enzymes in metabolic pathways, particularly in energy transfer and storage.

Isomerization reactions play a crucial role in metabolic pathways, and one such example involves the conversion of 3-phosphoglycerate (3-PG) to 2-phosphoglycerate (2-PG). This reaction occurs twice due to the presence of two molecules of 3-PG. The enzyme responsible for this transformation is phosphoglycerate mutase, which is a specific type of isomerase. Isomerases are enzymes that facilitate the rearrangement of atoms within a molecule, resulting in isomers—compounds that share the same molecular formula but differ in the connectivity of their atoms.

In this particular reaction, phosphoglycerate mutase catalyzes the movement of an inorganic phosphate group from the third carbon (C3) of 3-PG to the second carbon (C2). This process can be represented as follows:

3-PG (C3) + Pi → 2-PG (C2)

Here, the phosphate group, initially attached to the oxygen on C3, is relocated to C2, resulting in the formation of 2-PG. Importantly, this reaction is reversible, meaning that 2-PG can be converted back to 3-PG if needed. The ability of mutases to facilitate such transformations highlights their significance in metabolic regulation and energy production.

In glycolysis, several reactions occur, each with distinct roles in energy production and consumption. One key reaction involves the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, which is catalyzed by a kinase. During this process, an inorganic phosphate is transferred from 1,3-bisphosphoglycerate to ADP, resulting in the formation of ATP. This reaction is significant as it represents one of the few steps in glycolysis where ATP is produced.

In contrast, the conversion of 3-phosphoglycerate to 2-phosphoglycerate is an isomerization reaction facilitated by mutases. This step involves the rearrangement of the phosphate group but does not yield ATP. Similarly, the transformation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate is an oxidation reaction that produces NADH through the reduction of NAD⁺, but again, no ATP is generated in this step.

Additionally, the initial conversion of glucose to glucose-6-phosphate is an energy-consuming reaction, requiring the investment of ATP to add a phosphate group to glucose. This highlights the importance of understanding the energy dynamics within glycolysis, where certain reactions consume ATP while others produce it, ultimately contributing to the cell's energy balance.

In glycolysis, reaction 9 involves a reversible dehydration reaction where two molecules of 2-phosphoglycerate (2-PG) are converted into phosphoenolpyruvate (PEP). This transformation is catalyzed by the enzyme enolase. During this reaction, a water molecule (H₂O) is removed, which is essential for the formation of a double bond between the carbon atoms of the 2-phosphoglycerate molecules. Since carbon is tetravalent, it requires four bonds to maintain stability. The loss of a hydrogen atom and a hydroxyl group (OH) facilitates this double bond formation, resulting in the production of phosphoenolpyruvate. This step is crucial in the glycolytic pathway, highlighting the importance of dehydration reactions in metabolic processes.

In the final step of glycolysis, known as reaction 10, a phosphate transfer occurs, resulting in the conversion of phosphoenolpyruvate (PEP) to pyruvate. This reaction is catalyzed by the enzyme pyruvate kinase, which facilitates the transfer of an inorganic phosphate group from PEP to adenosine diphosphate (ADP), forming adenosine triphosphate (ATP). This process is crucial as it is irreversible and represents a key regulatory point in glycolysis.

During this reaction, PEP loses its inorganic phosphate group, leading to the formation of pyruvate. The pyruvate molecule features a carbonyl group where the inorganic phosphate was previously attached, and the CH₂ group is transformed into a methyl group. This transformation marks the completion of glycolysis, which consists of a series of ten enzymatic reactions that ultimately yield pyruvate as the final product.

Overall, this step not only highlights the importance of energy production through ATP synthesis but also emphasizes the role of kinases in facilitating phosphate transfers within metabolic pathways.

In glycolysis, the conversion of phosphoenolpyruvate (PEP) to pyruvate is catalyzed by the enzyme pyruvate kinase. This reaction, known as reaction 10 of glycolysis, is a phosphate transfer reaction, which is characteristic of kinases. During this process, pyruvate kinase facilitates the transfer of a phosphate group from PEP to adenosine diphosphate (ADP), resulting in the formation of adenosine triphosphate (ATP) and pyruvate.

It is important to note that other enzymes mentioned, such as pyruvate dehydrogenase and phosphoenolpyruvate carboxykinase, do not catalyze this specific reaction. Pyruvate dehydrogenase is involved in oxidation reactions, while phosphoenolpyruvate carboxykinase is responsible for the carboxylation of PEP, which is not relevant to the conversion of PEP to pyruvate. Therefore, the correct enzyme that catalyzes this crucial step in glycolysis is pyruvate kinase.