Glycolysis 2: Videos & Practice Problems

Glycolysis involves a series of enzymatic reactions, starting with hexokinase, which phosphorylates glucose to form glucose 6-phosphate, committing to the pathway by expending ATP. The isomerization of glucose 6-phosphate to fructose 6-phosphate is reversible, facilitated by phosphohexose isomerase. Phosphofructokinase then phosphorylates fructose 6-phosphate, marking another commitment step. Aldolase splits fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), with G3P continuing in glycolysis. Understanding these steps is crucial for grasping metabolic pathways and energy transfer in cells.

Glycolysis 2

Glycolysis is a crucial metabolic pathway that converts glucose into pyruvate, generating energy in the form of ATP. The process consists of several enzymatic reactions, each characterized by specific enzymes, substrates, and changes in Gibbs free energy (ΔG). Understanding these components is essential for grasping the overall pathway.

The first step of glycolysis is catalyzed by the enzyme hexokinase, which phosphorylates glucose to form glucose 6-phosphate. This reaction has a significantly negative ΔG, indicating it is not readily reversible and represents a commitment step in the pathway. The reaction consumes one molecule of ATP, producing ADP and a proton as byproducts. This phosphorylation is critical as it traps glucose within the cell, preparing it for further breakdown.

In the second step, the enzyme phosphohexose isomerase converts glucose 6-phosphate into fructose 6-phosphate. This reaction has a ΔG close to zero, making it readily reversible. The isomerization involves a rearrangement of the molecular structure, transitioning from a six-membered ring to a five-membered ring, which is a relatively simple transformation.

The third step is catalyzed by phosphofructokinase, a key regulatory enzyme in glycolysis. This reaction also has a negative ΔG, marking it as another commitment step. Here, fructose 6-phosphate is phosphorylated to form fructose 1,6-bisphosphate, consuming another ATP molecule. This step is crucial as it commits the substrate to the glycolytic pathway, leading to energy production.

The fourth step involves the enzyme aldolase, which cleaves fructose 1,6-bisphosphate into two three-carbon molecules: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). Although the standard ΔG for this reaction appears positive, under cellular conditions, it is actually negative, indicating that the reaction is reversible. G3P can directly continue in glycolysis, while DHAP must be converted into G3P to proceed in the pathway. This conversion is facilitated by the enzyme triose phosphate isomerase, allowing both products to contribute to the subsequent steps of glycolysis.

Overall, glycolysis involves a series of reactions that not only transform glucose into pyruvate but also regulate energy investment and production through the careful management of ΔG and enzyme activity. Understanding these steps is fundamental for studying cellular respiration and energy metabolism.

Here’s what students ask on this topic:

What is the role of hexokinase in glycolysis?

Hexokinase is the enzyme that catalyzes the first step of glycolysis. It phosphorylates glucose to form glucose 6-phosphate, using one molecule of ATP in the process. This reaction has a highly negative ΔG, making it essentially irreversible and committing the glucose molecule to the glycolytic pathway. The phosphorylation of glucose also prevents it from diffusing out of the cell, thus trapping it inside for further metabolism.

Why is the reaction catalyzed by phosphohexose isomerase considered readily reversible?

The reaction catalyzed by phosphohexose isomerase, which converts glucose 6-phosphate to fructose 6-phosphate, has a ΔG close to zero. This indicates that the reaction is readily reversible under cellular conditions. The enzyme facilitates the isomerization by rearranging the molecular structure without significant energy changes, allowing the reaction to proceed in both directions depending on the cellular needs.

What is the significance of phosphofructokinase in glycolysis?

Phosphofructokinase (PFK) is a key regulatory enzyme in glycolysis, catalyzing the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate. This step is another commitment point in glycolysis, marked by a highly negative ΔG, making it essentially irreversible. PFK is allosterically regulated by various metabolites, making it a crucial control point for the glycolytic flux and overall cellular energy metabolism.

How does aldolase function in glycolysis?

Aldolase catalyzes the cleavage of fructose 1,6-bisphosphate into two three-carbon molecules: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). Although the standard ΔG for this reaction is positive, under cellular conditions, the reaction is driven forward by the rapid consumption of G3P in subsequent steps. DHAP is then converted to G3P by triose phosphate isomerase, ensuring that both molecules can continue through the glycolytic pathway.

What happens to dihydroxyacetone phosphate (DHAP) in glycolysis?

Dihydroxyacetone phosphate (DHAP) is converted to glyceraldehyde 3-phosphate (G3P) by the enzyme triose phosphate isomerase. This conversion is necessary because only G3P can proceed to the subsequent steps of glycolysis. The isomerization of DHAP to G3P ensures that both molecules produced by the aldolase reaction can be utilized in the energy-yielding phase of glycolysis, thereby maximizing the efficiency of glucose metabolism.