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Chapter 18: Problem 2

Determine the anticipated location in pyruvate of labeled carbons if glucose molecules labeled (in separate experiments) with \(^{14} \mathrm{C}\) at each position of the carbon skeleton proceed through the glycolytic pathway

Short Answer

Expert verified
In pyruvate, carbons labeled 1 and 2 in glucose will become carbons 3 and 2 respectively. Similarly, carbons labeled 3, 4, and 5 in glucose end as carbon 1 in pyruvate. Carbon 6 in glucose is lost as carbon dioxide during decarboxylation step.

Step by step solution

01

Understand the structure of glucose and locate labeled carbon

Recall that glucose is a six-carbon molecule. In separate experiments, carbon atoms 1 to 6 are individually labeled with \(^{14} \mathrm{C}\), an isotope of carbon. The position numbering is based on the structural formula of glucose which begins from the aldehyde group (in glucose, this is the top carbon, also known as the anomeric carbon, in the ring structure).
02

Analyze the Glycolytic Pathway

First, consider the glycolytic pathway, from glucose to pyruvate. Glucose is first phosphorylated to form glucose-6-phosphate, then converted to fructose-6-phosphate, followed by fructose-1,6-bisphosphate. Fructose-1,6-bisphosphate is then cleaved into two triose phosphates: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GA3P). Finally, these are converted to pyruvate.
03

Trace the Labeled Carbons through Glycolysis

The key step in tracking the position of carbon atoms is when fructose-1,6-bisphosphate splits into DHAP and GA3P. The top part of the glucose molecule (carbon 1 and 2) forms DHAP which is rapidly isomerized into GA3P. Hence, both molecules continue in the same manner through glycolysis and form pyruvate.
04

Determine Position of Carbon in Pyruvate

In the last step of glycolysis, GA3P is converted into pyruvate. The carbon which was labeled as carbon 1 and 2 in glucose becomes carbon 3 and 2 respectively, in pyruvate. Now they're placed on the bottom half of the pyruvate molecule. Carbon 3,4, and 5 will be carbon 1 in pyruvate and is on the top half of the pyruvate molecule. Carbon 6 in glucose will not appear in pyruvate as it is released as carbon dioxide during the decarboxylation step of pyruvate formation in glycolysis.

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Most popular questions from this chapter

In an erythrocyte undergoing glycolysis, what would be the effect of a sudden increase in the concentration of a. ATP? b. AMP? c. fructose- 1,6 -bisphosphate? d. fructose- 2,6 -bisphosphate? e. citrate? f. glucose- 6 -phosphate?

Fructose bisphosphate aldolase in animal muscle is a class I aldolase, which forms a Schiff base intermediate between substrate (for example, fructose- 1,6 -bisphosphate or dihydroxyacetone phosphate and a lysine at the active site (see Figure 18.12 ). The chemical evidence for this intermediate comes from studies with aldolase and the reducing agent sodium borohydride, \(\mathrm{NaBH}_{4}\). Incubation of the enzyme with dihydroxyacetone phosphate and \(\mathrm{NaBH}_{4}\) inactivates the enzyme. Interestingly, no inactivation is observed if \(\mathrm{NaBH}_{4}\) is added to the enzyme in the absence of substrate. Write a mechanism that explains these observations and provides evidence for the formation of a Schiff base intermediate in the aldolase reaction.

Based on your reading of this chapter, what would you expect to be the most immediate effect on glycolysis if the steady-state concentration of glucose- 6 -P were \(8.3 \mathrm{m}\) M instead of \(0.083 \mathrm{mM}\) ?

Discuss the cycling of NADH and NAD \(^{+}\) in glycolysis and the related fermentation reactions.

(Integrates with Chapter 3 .) Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate \(+\mathrm{H}_{2} \mathrm{O}\). The standard free energy change, \(\Delta G^{\circ},\) for this reaction is \(+1.8 \mathrm{kJ} / \mathrm{mol}\). If the concentration of 2 -phosphoglycerate is \(0.045 \mathrm{m} M\) and the concentration of phosphoenolpyruvate is \(0.034 \mathrm{m} M\), what is \(\Delta G\), the free energy change for the enolase reaction, under these conditions?
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