Glycolysis Investment Phase

Overview

The video covers the five reactions in glycolysis’s investment phase. One glucose molecule is converted into two three-carbon molecules of glyceraldehyde-3-phosphate (GAP), using two ATP molecules along the way. The first three steps prepare and phosphorylate the sugar; the fourth splits it into two three-carbon molecules; the fifth converts one of those molecules into GAP.

The Five Reactions

1. Glucose → glucose-6-phosphate

   • Enzyme: Hexokinase
   • Reaction: A kinase transfers a phosphate from ATP to glucose’s sixth carbon, producing glucose-6-phosphate and ADP.
   • Mechanism and significance: An aspartate residue in the enzyme helps remove a proton from the hydroxyl group, allowing it to attack ATP’s phosphate. Magnesium helps shield ATP’s negative charges.
   • The reaction is strongly favorable and effectively proceeds in one direction. Phosphorylation also traps glucose inside the cell, since glucose-6-phosphate cannot use the glucose transporters described in the video.
   • Hexokinase undergoes induced fit when glucose and ATP bind. The enzyme closes around them, helping prevent water from interfering with the intended phosphate transfer.

2. Glucose-6-phosphate → fructose-6-phosphate

   • Enzyme: Phosphoglucose isomerase
   • Reaction: A reversible isomerization changes the sugar from an aldose to a ketose, shifting the carbonyl position. The sugar also changes from a six-membered ring form to a five-membered ring form.
   • Mechanism: The ring opens, and enzyme acid–base groups help rearrange the open-chain sugar through an enediol intermediate. The molecule then closes into the new ring form. The lecture mentions lysine and glutamate residues in the process.
   • This step is reversible.

3. Fructose-6-phosphate → fructose-1,6-bisphosphate

   • Enzyme: Phosphofructokinase
   • Reaction: A second ATP donates a phosphate, this time to carbon 1, producing fructose-1,6-bisphosphate and ADP.
   • Mechanism: Like the first phosphorylation, this is a phosphate transfer and nucleophilic substitution.
   • This is the committed step of glycolysis and a key regulatory point: after it, the sugar is committed to continuing through the pathway. The video distinguishes bisphosphate—two phosphates attached at separate positions—from diphosphate, where the phosphates are connected to each other.
   • The sugar can interconvert between alpha and beta forms through ring opening and reclosing, a process called mutarotation.

4. Fructose-1,6-bisphosphate → two three-carbon products

   • Enzyme: Aldolase
   • Reaction: The six-carbon sugar is split into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP).
   • Mechanism: The lecture uses an enediol-based sketch to illustrate the rearrangement and carbon–carbon bond cleavage, while noting that this is a simplified account. In the enzyme, a lysine residue participates through a more complex intermediate. The reaction is reversible, and its direction depends on cellular conditions and the continued use of its products in glycolysis.

5. Dihydroxyacetone phosphate → glyceraldehyde-3-phosphate

   • Enzyme: Triose phosphate isomerase
   • Reaction: A reversible isomerization converts DHAP into GAP, the form that continues through glycolysis.
   • Mechanism: The enzyme uses acid–base chemistry, including histidine and glutamate residues, to form and resolve an enediol intermediate.
   • After this step, the original glucose has yielded two GAP molecules for the later payoff phase.

General Mechanistic Points

• The instructor often represents catalytic groups as generic acids or bases, rather than specifying every charge and molecular detail. The emphasis is on the glycolytic transformation.
• Enzyme residues can act as acids or bases by removing or donating protons.
• Several reactions use an enediol intermediate to rearrange the sugar’s carbonyl position.
• The first five steps consume two ATP total and prepare glucose for the energy-producing payoff phase.

Speakers and Sources

• One instructor/narrator from the Sparks Chemistry video.
• No guest speakers or other identifiable sources are featured.