29. The free energy required to synthesize a mixed anhydride bond of 1,3-bisphosphoglycerate is generated by the oxidation of .
(A) an aldehyde to acid
(B) an alcohol to acid
(C) an alcohol to aldehyde
(D) NADH to NAD+
Free Energy Required to Form the Mixed Anhydride Bond in 1,3-Bisphosphoglycerate
Correct Answer
(A) An aldehyde to acid
Introduction
One of the most fascinating reactions in glycolysis is the formation of 1,3-bisphosphoglycerate (1,3-BPG), a molecule containing a high-energy mixed anhydride (acyl phosphate) bond. Unlike ATP-dependent phosphorylation reactions, the energy required to create this high-energy phosphate bond is not supplied directly by ATP. Instead, the energy is obtained from the oxidation of glyceraldehyde-3-phosphate (G3P), making this reaction a classic example of energy coupling in metabolism.
The enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes this step, simultaneously oxidizing an aldehyde group, reducing NAD⁺ to NADH, and incorporating inorganic phosphate (Pi) to produce 1,3-bisphosphoglycerate. This reaction is central to cellular energy metabolism because it conserves the energy released during oxidation within a high-energy phosphate bond, which is subsequently used to generate ATP by substrate-level phosphorylation.
Understanding the Concept Behind the Question
During glycolysis, glyceraldehyde-3-phosphate (G3P) contains an aldehyde (-CHO) functional group. The enzyme glyceraldehyde-3-phosphate dehydrogenase catalyzes the oxidation of this aldehyde into a carboxylic acid equivalent, while NAD⁺ accepts the released electrons and becomes NADH.
Instead of releasing the oxidation energy as heat, the cell conserves it by forming a high-energy acyl phosphate bond, producing 1,3-bisphosphoglycerate.
The overall reaction is:
Glyceraldehyde-3-phosphate + Pi + NAD⁺ → 1,3-Bisphosphoglycerate + NADH + H⁺
The oxidation occurring here is specifically:
Aldehyde → Carboxylic Acid
Therefore, the free energy required for forming the mixed anhydride bond originates from the oxidation of an aldehyde to an acid.
Hence, the correct answer is Option (A).
Why Option (A) Is Correct
Oxidation of an Aldehyde to an Acid
Glyceraldehyde-3-phosphate contains an aldehyde group (-CHO) at carbon 1. During the GAPDH reaction, this aldehyde undergoes oxidation, and the released energy is captured in the newly formed acyl phosphate (mixed anhydride) bond of 1,3-bisphosphoglycerate.
Simultaneously:
- NAD⁺ is reduced to NADH.
- Inorganic phosphate is incorporated.
- No ATP is consumed.
The oxidation of an aldehyde to a carboxylic acid is highly exergonic, providing sufficient free energy to synthesize the unstable high-energy phosphate bond.
Therefore,
Option (A) is correct.
Why Option (B) Is Incorrect
Oxidation of an Alcohol to an Acid
Although oxidation of a primary alcohol to a carboxylic acid is a common biochemical reaction, it does not occur during this step of glycolysis.
Glyceraldehyde-3-phosphate already contains an aldehyde group, not an alcohol group. Therefore, the substrate is oxidized directly from an aldehyde to an acid equivalent.
Hence,
Option (B) is incorrect.
Why Option (C) Is Incorrect
Oxidation of an Alcohol to an Aldehyde
This reaction represents only the first stage of alcohol oxidation.
Examples include:
Ethanol → Acetaldehyde
However, glyceraldehyde-3-phosphate already possesses an aldehyde functional group before entering the reaction catalyzed by GAPDH.
Therefore, this oxidation does not occur during the formation of 1,3-bisphosphoglycerate.
Hence,
Option (C) is incorrect.
Why Option (D) Is Incorrect
Oxidation of NADH to NAD⁺
During glycolysis, NAD⁺ is reduced to NADH, not the reverse.
The reaction is:
NAD⁺ + 2e⁻ + H⁺ → NADH
Thus, NADH is produced, not oxidized.
Oxidation of NADH occurs later during oxidative phosphorylation or other metabolic pathways, but not during the GAPDH reaction.
Therefore,
Option (D) is incorrect.
The GAPDH Reaction
The sixth step of glycolysis is catalyzed by glyceraldehyde-3-phosphate dehydrogenase.
The reaction is:
Glyceraldehyde-3-phosphate + Pi + NAD⁺
↓
1,3-Bisphosphoglycerate + NADH + H⁺
This reaction simultaneously involves:
- Oxidation
- Reduction
- Phosphorylation
- Energy conservation
It is one of the most elegant examples of coupled biochemical reactions.
Why Is 1,3-Bisphosphoglycerate a High-Energy Molecule?
The phosphate attached to carbon 1 forms an acyl phosphate (mixed anhydride) bond, one of the highest-energy phosphate bonds found in metabolism.
Hydrolysis of this bond releases a large amount of free energy, allowing the next glycolytic enzyme, phosphoglycerate kinase, to synthesize ATP directly through substrate-level phosphorylation.
Thus, the energy released during aldehyde oxidation is temporarily stored in this high-energy intermediate instead of being lost as heat.
Biological Importance of This Reaction
The GAPDH reaction represents a crucial energy-conserving step in glycolysis. Instead of dissipating the energy released during oxidation, cells capture it in two useful forms:
- NADH, which later contributes to ATP production during oxidative phosphorylation.
- 1,3-Bisphosphoglycerate, which directly generates ATP in the next glycolytic step.
Without this coupling mechanism, glycolysis would produce much less usable energy, making cellular metabolism far less efficient.
Comparison of the Given Reactions
| Reaction | Occurs During Formation of 1,3-BPG? | Correct? |
|---|---|---|
| Aldehyde → Acid | Yes | ✅ |
| Alcohol → Acid | No | ❌ |
| Alcohol → Aldehyde | No | ❌ |
| NADH → NAD⁺ | No | ❌ |
Common Mistakes in Competitive Examinations
A common mistake is choosing Option (D) because NADH is mentioned in the glycolytic reaction. Students often overlook that NAD⁺ is reduced, meaning the substrate—not NADH—is undergoing oxidation.
Another frequent error is selecting Option (B) because alcohol oxidation to acid is a familiar biochemical reaction. However, glyceraldehyde-3-phosphate already contains an aldehyde, not an alcohol.
Students should remember that the substrate entering the GAPDH reaction is glyceraldehyde, making the oxidation aldehyde → acid.
High-Yield Points
-
Enzyme:
Glyceraldehyde-3-phosphate dehydrogenase
-
Substrate:
Glyceraldehyde-3-phosphate
-
Product:
1,3-Bisphosphoglycerate
-
Oxidation:
Aldehyde → Carboxylic acid equivalent
- NAD⁺ is reduced to NADH.
-
High-energy bond:
Acyl phosphate (mixed anhydride)
- Next reaction generates ATP by substrate-level phosphorylation.
Frequently Asked Questions
Why is 1,3-bisphosphoglycerate called a mixed anhydride?
Because the phosphate group is linked to a carboxylic acid through an acyl phosphate bond, which is a type of mixed anhydride possessing very high free energy.
Why isn’t ATP required to form 1,3-BPG?
The energy released during oxidation of the aldehyde group is sufficient to drive formation of the high-energy phosphate bond without ATP consumption.
What happens to the NADH produced?
Under aerobic conditions, NADH transfers its electrons to the mitochondrial electron transport chain, contributing to ATP production through oxidative phosphorylation.
Key Takeaways
The formation of 1,3-bisphosphoglycerate represents one of the most important energy-conserving reactions in glycolysis. The high-energy mixed anhydride bond is generated by coupling the oxidation of the aldehyde group of glyceraldehyde-3-phosphate with the reduction of NAD⁺ to NADH. Rather than releasing the oxidation energy as heat, the cell stores it in both NADH and the acyl phosphate bond of 1,3-bisphosphoglycerate, allowing efficient ATP production in subsequent steps of glycolysis.
Final Answer
Correct Option: (A) An aldehyde to acid
Explanation
During glycolysis, the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) catalyzes the conversion of glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. In this reaction, the aldehyde (-CHO) group of glyceraldehyde-3-phosphate is oxidized to a carboxylic acid equivalent, while NAD⁺ is reduced to NADH. The free energy released during this oxidation is conserved by forming a high-energy mixed anhydride (acyl phosphate) bond in 1,3-bisphosphoglycerate. Therefore, the energy required to synthesize this high-energy bond is generated by the oxidation of an aldehyde to an acid, making Option (A) the correct answer.
