Chapter 13, Problem 1P

Interpretation Introduction

Interpretation: The specific carbon atom eliminating as carbon dioxide from each of the glucose metabolism pathways can be determined by radioactive pathway for which the exact carbon atom of the six of glucose should be labelled as $\text{C}$ .

Concept introduction:

Pentose phosphate pathway: This is primarily catabolic and behaves as an alternative pathway of glucose oxidation and simultaneous generation of NADPH which is required in a many reduction bio-reactions. The pentose phosphate pathway breaks the six carbons skeleton of glucose to generate five carbon sugars, particularly ribose-5-phosphate, which is required for synthesis of numbers of 5-carbon amino acids like purine, pyrimidine etc. The pentose phosphate pathway rarely oxidise the glucose to completely to ${\text{CO}}_{\text{2}}$ and ${\text{H}}_2\text{O}$ .

Glycolysis pathway: Metabolic oxidation of glucose is essential for all living activities. Oxidative breakdown of glucose is accomplished in two stages. In the first stage, glucose molecule splits to produce two molecules of 3-C fragments e.g. pyruvic acid and/or lactic acid through a sequence of reactions called glycolysis.

Citric acid cycle: Pyruvate produced in glycolysis of glucose is oxidised to carbon dioxide and water through a sequence of complicated reactions cycle, popularly named as Krebs cycle. Citric acid and some other tricarboxylic acids occur in the intermediate steps of this reactions sequence; hence the name citric acid cycle and tricarboxylic acid cycle are also given. It is through this cycle of reactions that all food stuff and nutrients are completely oxidised in the cell. All food stuff including carbohydrate must be converted to pyruvate or some other intermediate products for their oxidation to produce carbon dioxide and water.

Pictorial representation:

  

Figure 1: Pentose phosphate pathway

  

Figure 2: Glycolysis pathway

  

Figure 3: Citric acid cycle

Explanation of Solution

From the pictorial representation it can be seen that in pentose phosphate pathway the ${\text{C}}_{\text{1}}$ carbon atom of the glucose molecule forms the carbon dioxide ( ${\text{CO}}_2$ ) in the reaction step shown below.

(a)	(b)
(c)
Figure: 4 The ${\text{CO}}_2$elimination takes place via different pathways of glucose metabolism: (a) Pentose phosphate pathway, (b) citric acid cycle and (c) glycolysis pathway Thus, the proportion of glucose metabolism can be determined by the radioactive determination and estimating the amount of radioactive carbon dioxide eliminating in each step of the different pathways. The isotope of carbon i.e. $\text{C}$must be incorporated at the ${\text{C}}_1$, ${\text{C}}_4$and ${\text{C}}_5$positions. So, the glucose molecule should be as shown in the figure.