Question

Ribose 5 -phosphate labeled with \(^{14} \mathrm{C}\) Eat \(\mathrm{C}-1\) is added to a solution containing transketolase,transaldolase, phosphopentose epimerase, phosphopentose isomerase, and glyceraldehyde 3 -phosphate. What is the distribution of the radioactive label in the erythrose 4-phosphate and fructose 6 -phosphate that are formed in this reaction mixture?

Short answer

The label is on C-3, C-4, or C-5 of fructose 6-phosphate; erythrose 4-phosphate remains unlabeled.

Step-by-step solution

Understanding the Role of Ribose 5-phosphate

Ribose 5-phosphate, which is labeled with radioactive carbon at position 1 (C-1), is a substrate in the pentose phosphate pathway. This pathway involves several enzymes, including transketolase and transaldolase, which are responsible for transferring carbon groups between sugar molecules.

The Action of Transketolase

Transketolase catalyzes the transfer of a two-carbon unit from ribose 5-phosphate to glyceraldehyde 3-phosphate, resulting in the production of sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate. The radioactive label remains on the C-1 of the ribose unit being transferred.

The Action of Transaldolase

Transaldolase transfers a three-carbon unit from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate, forming erythrose 4-phosphate and fructose 6-phosphate. In this process, the original labeled carbon from ribose remains as the C-1, C-2, or C-3 from the sedoheptulose 7-phosphate depending on the carbon rearrangements.

Analyzing Erythrose 4-Phosphate and Fructose 6-Phosphate

Following these enzyme-catalyzed rearrangements, erythrose 4-phosphate does not have the labeled carbon because the label is transferred to fructose 6-phosphate. The radioactive carbon originally from ribose 5-phosphate settles in the fructose 6-phosphate molecule, specifically on carbons 3, 4, or 5.

Key concepts

Transketolase

Transketolase is a key enzyme in the pentose phosphate pathway. This enzyme facilitates the transfer of two-carbon units between sugar molecules.
It acts by moving these units from ketose to aldose sugars. For example, it transfers two-carbon fragments from ribose 5-phosphate.
Understanding its role helps explain how carbon atoms flow through metabolic pathways. Transketolase is particularly significant when dealing with reactions between ribose 5-phosphate and glyceraldehyde 3-phosphate.
It produces sedoheptulose 7-phosphate, maintaining the radioactive carbon label in the process. This means that during this conversion,
the radioactive carbon remains intact in the newly formed sedoheptulose 7-phosphate molecule. It's essential for students to remember that this enzyme doesn't shift the position of the radiolabel; it only facilitates a two-carbon transfer.
Therefore, tracing the fate of radiolabeled atoms becomes easier when transketolase's role is considered.

Transaldolase

Transaldolase is another crucial enzyme in the pentose phosphate pathway. Unlike transketolase, it deals with three-carbon unit transfers. The action of transaldolase comes into play after the work of transketolase, converting sedoheptulose 7-phosphate into two different products. Transaldolase transfers a three-carbon unit from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate. The products of this reaction are erythrose 4-phosphate and fructose 6-phosphate. During this process, the enzyme facilitates the movement of the radiolabel across different carbon atoms. The presence of radiolabeled carbon can be tracked from ribose 5-phosphate through to fructose 6-phosphate. This is because transaldolase's rearrangement keeps the radioactive carbon within the resulting sugar molecules.

Radioactive Carbon Labeling

Radioactive carbon labeling is a technique used to trace the path of carbon atoms in metabolic pathways. It allows scientists to monitor the behavior and transformation of molecules inside cells. In this exercise, ribose 5-phosphate is labeled at its C-1 position with the radioactive isotope carbon-14. As the reactions progress through the pentose phosphate pathway, this radioactive label acts as a beacon. It reveals how carbons from ribose 5-phosphate move and transform into different molecules like sedoheptulose 7-phosphate and fructose 6-phosphate. In our scenario, the radiolabel is not found in erythrose 4-phosphate, indicating its loss during molecular conversions. However, it appears in fructose 6-phosphate, highlighting how these rearrangements affect where labeled carbons settle.

Sugar Molecule Rearrangement

Sugar molecule rearrangement is at the core of the pentose phosphate pathway's functions. This rearrangement involves the shifting of carbon units across different sugar molecules, leading to the formation of new sugars. In the pathway, enzymes like transketolase and transaldolase play major roles in these rearrangements. They ensure the transfer of various sized carbon groups. These rearrangements ensure efficient binding and processing of sugars, adapting to the cell's metabolic needs. In this exercise, we can see these rearrangements transforming ribose 5-phosphate into different sugar molecules by shifting carbon groups. Ultimately, the distribution of radioactive carbon among different molecules is determined by these meticulous rearrangements in the pathway, demonstrating how enzymes and molecule transformations maintain balance in the cell's metabolic processes.