BSC 314 Lecture Notes - Lecture 23: Cytosol, Nicotinamide Adenine Dinucleotide, Dihydroxyacetone Phosphate
27 Jun 2018
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Glycolysis
Simply put, glycolysis (glyco = sugar; lysis = splitting) splits a 6 carbon sugar, glucose, ‐
into two molecules of 3 carbon pyruvate in a series of steps, each catalyzed by a ‐
particular enzyme. The energy of the sugar molecule is released in stages and
transferred to adenosine triphosphate (ATP) and the reduced coenzyme nicotinamide
adenine dinucleotide (NADH), each of which are then available for other energy ‐
requiring metabolic processes. (Note: Pyruvic acid formed in the reactions dissociates
to pyruvate plus H +; the acid and the pyruvate exist together in equilibrium, and so the
names “pyruvate” and “pyruvic acid” are used interchangeably by textbook authors.)
First steps (preparatory, energy-using stages)
The energy needed to start the reaction comes from an ATP molecule that is added,
together with a phosphate group, to one of the sugar carbons, thereby energizing the
sugar to glucose 6 phosphate.‐ ‐
The molecule is restructured, a second ATP enters, binding another phosphate group to
a different carbon atom, and the 6 carbon energized sugar molecule splits into two 3‐ ‐
carbon molecules, each with a phosphate group consisting of dihydroxyacetone
phosphate (DHAP) and glyceraldehyde 3 phosphate (PGAL).‐
Final steps (energy-releasing phase)
DHAP converts to PGAL. There are now twomolecules of PGAL going forward. Two
almost simultaneous reactions occur next: an oxidation and a phosphorylation. Energy
is harvested in the oxidation of PGAL (H atoms and their electrons are removed), and
the coenzyme NAD + is reduced (H atoms and electrons added) to form NADH, a high‐
energy compound.
The phosphorylation, using some of the oxidation energy, attaches an inorganic
phosphate group (P i) to the carbon ring (the bond energy is similar to that of the high
energy bonds of ATP). The substrate is now 1,3 bisphosphoglycerate, with two ‐
phosphate groups attached.
A phosphate group is released to phosphorylate a molecule of ADP, forming a molecule
of ATP. This kind of phosphorylation—formation of ATP by transferring a P i from a
metabolic intermediate compound—is called substrate level phosphorylation‐. Since
there are two PGAL molecules from each glucose molecule, two ATPs are formed at
this stage. Two ATPs were needed to activate the pathway, so at this point the energy
use and production are equal.
The substrate molecule is restructured: The remaining P i group is moved to another
binding site in the molecule, and a molecule of water is removed resulting in a high‐
energy, phosphorylated compound being formed (phosphoenolpyruvate).
A second substrate level phosphorylation takes place; the P‐ i is moved to ADP, and the
second molecule of ATP is formed.
The result of all of these reactions is: two molecules of 3 carbon pyruvate, four ATPs, ‐
and two NADHs for each molecule of glucose. The net gain of ATP, however, is only
two molecules of ATP since two molecules were used to initiate the process. The two
pyruvate molecules retain about 80 percent of the energy of the glucose molecule, and
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Document Summary
Simply put, glycolysis (glyco = sugar; lysis = splitting) splits a 6 carbon sugar, glucose, into two molecules of 3 carbon pyruvate in a series of steps, each catalyzed by a particular enzyme. The energy needed to start the reaction comes from an atp molecule that is added, together with a phosphate group, to one of the sugar carbons, thereby energizing the sugar to glucose 6 phosphate. There are now twomolecules of pgal going forward. Two almost simultaneous reactions occur next: an oxidation and a phosphorylation. Energy is harvested in the oxidation of pgal (h atoms and their electrons are removed), and the coenzyme nad + is reduced (h atoms and electrons added) to form nadh, a high energy compound. The phosphorylation, using some of the oxidation energy, attaches an inorganic phosphate group (p i) to the carbon ring (the bond energy is similar to that of the high energy bonds of atp).
