Explain using electron-pushing arguments why glycolysis must go through G6P to F6P step (C1 aldehyde to C1 ketone). Focus on the subsequent aldolase-catalysed reaction and explain why glucose 1,6 bisphosphate would not be able to go through the aldolase reaction.
To understand why glycolysis goes through the conversion of glucose-6-phosphate (G6P) to fructose-6-phosphate (F6P) before the subsequent aldolase-catalyzed reaction, it is important to first establish the electron-pushing arguments and reaction mechanism of each step.
In the initial step of glycolysis, glucose is phosphorylated to form glucose-6-phosphate (G6P) through the action of the enzyme hexokinase. The conversion from G6P to F6P involves the conversion of an aldehyde group (C1 of G6P) to a ketone group (C1 of F6P).
The conversion of G6P to F6P can be explained using electron-pushing arguments as follows:
1. Deprotonation: A base abstracts a proton (H+) from the carbon next to the carbonyl (C2) of G6P, forming an enediolate intermediate. This enediolate intermediate is stabilized by resonance throughout the glucose ring.
2. Cleavage of the C1-O bond: The electron pair from the carbon-carbon double bond of the enediolate attacks the electrophilic carbon (C1) of G6P, leading to the cleavage of the C1-O bond.
3. Protonation: A proton (H+) is transferred from a nearby acidic group (e.g., the His residue in the active site of the enzyme) to the newly created oxygen, resulting in the formation of F6P.
Now, let's focus on the subsequent aldolase-catalyzed reaction and why glucose-1,6-bisphosphate (G1,6BP) would not be able to go through this reaction. The aldolase catalyzes the cleavage of the carbon-carbon bond between C3 and C4 of F6P, leading to the formation of dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
The reason why G1,6BP cannot undergo the aldolase-catalyzed reaction is primarily due to the enzyme's active site and the required orientation of the substrate. The active site of aldolase is specifically designed to accommodate F6P, placing C3 and C4 in the necessary position for cleavage.
If G1,6BP was to enter the aldolase active site, the phosphate group at position 1 would interfere sterically with the active site, preventing the correct orientation of the substrate for cleavage. Additionally, the presence of a phosphate group at C1 would disrupt the electronic and steric environment necessary for the aldol cleavage reaction to occur.
Therefore, for glycolysis to proceed efficiently, G1,6BP must be converted to F6P through the action of the enzyme phosphofructokinase-1 (PFK-1) before entering the aldolase-catalyzed reaction. This ensures that the necessary rearrangements and cleavage can occur at the appropriate sites within the active site of aldolase.