Question

Glucose labeled with \(^{14} \mathrm{C}\) at \(\mathrm{C}-6\) is added to a solution containing the enzymes and cofactors of the oxidative phase of the pentose phosphate pathway. What is the fate of the radioactive label?

Short answer

The \(^{14}C\) label stays on the ribulose-5-phosphate after the oxidative phase.

Step-by-step solution

Understand the Oxidative Phase of Pentose Phosphate Pathway

The oxidative phase of the pentose phosphate pathway involves the conversion of glucose-6-phosphate to ribulose-5-phosphate. It includes dehydrogenation, hydrolysis, and decarboxylation reactions. Key enzymes in this process are glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase.

Identify the Carbon Positions

Glucose has six carbon atoms labeled C-1 to C-6. In glucose labeled with \(^{14}C\) at C-6, it's important to track where C-6 flows through the pathway.

Decarboxylation Reaction

During the oxidative decarboxylation catalyzed by 6-phosphogluconate dehydrogenase, the C-1 carbon is lost as CO2. However, the C-6 atom is retained in the structure, since the loss occurs at the C-1 position.

Generate Ribulose-5-Phosphate

After the oxidation and decarboxylation steps, ribulose-5-phosphate is generated. The C-6 carbon from the original glucose molecule remains as one of the carbons in ribulose-5-phosphate, now as part of the five-carbon sugar.

Conclusion on Radioactive Fate

Since the label is on C-6, which is not involved in the decarboxylation affecting the C-1 position, the \(^{14}C\) label remains part of the ribulose-5-phosphate molecule. It continues through further non-oxidative rearrangements if those steps are considered in subsequent reactions.

Key concepts

Oxidative Phase of the Pentose Phosphate Pathway

The oxidative phase is the initial segment of the pentose phosphate pathway. Here, glucose-6-phosphate undergoes a series of reactions to become ribulose-5-phosphate. This process is crucial for producing NADPH and ribose-5-phosphate.
NADPH is a cofactor used in anabolic reactions and for maintaining the redox balance in cells. The pathway consists of three main reactions:

• Dehydrogenation: Glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase to form 6-phosphoglucono-δ-lactone.
• Hydrolysis: The lactone is then hydrolyzed by water to yield 6-phosphogluconate.
• Decarboxylation: Finally, 6-phosphogluconate undergoes decarboxylation, catalyzed by 6-phosphogluconate dehydrogenase, to produce ribulose-5-phosphate and carbon dioxide.

The key outcome of this phase is the generation of NADPH and ribulose-5-phosphate necessary for nucleic acid synthesis. Understanding these steps helps explain how carbon labels are processed through the pathway.

Role of Radioactive Labeling in Biochemistry

Radioactive labeling is a technique where biological molecules like glucose are tagged with radioactive isotopes. In our context, glucose is labeled with the radioactive isotope \(^{14}\mathrm{C}\) at specific carbon positions, like C-6. This allows researchers to track its path through metabolic pathways.
Radioactive carbon (^{14}\mathrm{C}) acts as a marker to observe how glucose is metabolized:

• Tracer for Metabolic Pathways: By following the \(^{14}\mathrm{C}\)-labeled glucose, scientists can determine which metabolites retain the label.
• Identifies Reaction Sites: The retention or loss of the label indicates whether specific carbon atoms are released or incorporated into new compounds during reactions.
• Kinetics and Dynamics: It assists in measuring reaction rates and understanding dynamic processes within cells.

This method provides insights into the functional and structural aspects of metabolic pathways and helps unravel complex biochemical processes.

Enzyme Reactions in the Pathway

Enzymes are proteins that act as catalysts in biochemical reactions, including those in the oxidative phase of the pentose phosphate pathway. They ensure that reactions occur swiftly and efficiently without the need for external energy sources.
Incorporating labeled glucose allows us to observe the pathway's enzymatic steps. Important enzymes in the oxidative phase include:

• Glucose-6-Phosphate Dehydrogenase (G6PD): Initiates the pathway by catalyzing the oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone.
• 6-Phosphogluconate Dehydrogenase: Catalyzes the oxidative decarboxylation of 6-phosphogluconate into ribulose-5-phosphate.

These enzymes work step-by-step to convert glucose into ribulose-5-phosphate. With labeled carbon atoms, we can track which carbon positions in the glucose are retained or lost as carbon dioxide.
This clarity adds a deeper understanding to the metabolic journey of the sugar through the oxidative phase and further explains how labeled carbons contribute to insights in metabolic research.