Comprehensive Study Guide on the Phases and Enzymatic Reactions of Glycolysis

Overview of Glycolysis: Phase One and Phase Two

Glycolysis is a central metabolic pathway described in two distinct phases that convert a single molecule of glucose into two molecules of pyruvate. In the first phase of glycolysis, five specific chemical reactions occur to convert one molecule of glucose into two separate molecules of Glyceraldehyde-3-phosphate, identifying this as the preparatory or priming stage. The second phase involves the conversion of these Glyceraldehyde-3-phosphate molecules through another series of reactions to ultimately produce pyruvate. This second stage is characterized as the payoff phase, as it yields a total of four molecules of ATP, providing two molecules of ATP for each molecule of pyruvate produced.

Phase One: The Preparatory and Priming Reactions

The initial reaction in the first phase of glycolysis is known as the first priming reaction. In this step, glucose is converted into Glucose-6-phosphate, abbreviated as G6P. This phosphorylation is catalyzed by the enzymes Hexokinase or glucokinase. The reaction requires the consumption of energy in the form of ATP, which is converted to ADP. This process is dependent on the presence of magnesium ions, denoted as $Mg^{2+}$. Following the formation of G6P, the enzyme Phosphoglucoisomerase facilitates the isomerization of Glucose-6-phosphate into Fructose-6-phosphate, or F6P.

The third step in this phase is referred to as the second priming reaction. During this reaction, Fructose-6-phosphate is further phosphorylated to become Fructose-1,6-bisphosphate, abbreviated as F1GBP. This step is catalyzed by the enzyme Phospho-fructo Kinase and, like the first priming step, requires the input of ATP and the cofactor $Mg^{2+}$, resulting in the production of ADP. Following this, the enzyme aldolase facilitates the cleavage of the six-carbon Fructose-1,6-bisphosphate into two three-carbon molecules: Dihydroxyacetone phosphate, known as DHAP, and Glyceraldehyde-3-phosphate, or G3P. While not explicitly detailed as a separate step in all diagrams, DHAP is converted to a second molecule of G3P, ensuring that a single glucose molecule yields two molecules of G3P to enter the second phase.

Phase Two: Oxidation and ATP Generation

The second phase of glycolysis begins with the conversion of Glyceraldehyde-3-phosphate into 1,3-Bisphosphoglycerate, or 1,3BPG. This reaction is catalyzed by G3P dehydrogenase and involve the reduction of $NAD^+$ to $NADH$. Specifically, for every molecule of glucose, two molecules of $NAD^+$ are converted into two molecules of $NADH$ during this dehydrogenase reaction. This step is crucial for the subsequent energy-harvesting reactions that characterize the remainder of the pathway.

The energy stored in the high-energy intermediate 1,3BPG is harvested in the first ATP-forming reaction. The enzyme Phosphoglycerate kinase transfers a phosphate group from 1,3BPG to ADP, forming ATP and 3-Phosphoglycerate, or 3PG. This reaction requires magnesium ions ($Mg^{2+}$). Subsequently, the enzyme phosphoglycerate mutase facilitates the rearrangement of 3-Phosphoglycerate into 2-Phosphoglycerate, or 2PG, in a reaction that also utilizes $Mg^{2+}$ as a cofactor.

Terminal Steps and Pyruvate Formation

The penultimate step in glycolysis involves the dehydration of 2-Phosphoglycerate to form Phosphoenolpyruvate, commonly referred to as PEP. This reaction is catalyzed by the enzyme Enolase and involves the release of a water molecule, $H_2O$, while requiring $Mg^{2+}$ ions. The final step in the glycolytic pathway is the second ATP-forming reaction. In this irreversible step, the enzyme Pyruvate kinase transfers the remaining high-energy phosphate group from PEP to ADP to produce ATP and the final product, Pyruvate. This reaction is highly regulated and requires both potassium ions ($K^+$) and magnesium ions ($Mg^{2+}$) for optimal enzymatic activity. As two molecules of PEP were generated from the original glucose molecule, this final stage produces two molecules of ATP and two molecules of pyruvate, completing the payoff phase.