Under aerobic conditions, bacteria degrade glucose via glycolysis. The most comm
ID: 100219 • Letter: U
Question
Under aerobic conditions, bacteria degrade glucose via glycolysis. The most common form of glycolysis that you have studied in class is also called the Embden-Meyerhof-Parnas (EMP) pathway. Figure 1 shows few steps of the EMP pathway.
Figure 1: Simplified diagram of the glycolytic (EMP) pathway. In this figure, arrows indicate
the carbon #1 and 6 in glucose. DHAP and G-3-P stand for dihydroxyacetone phosphate and
glyceraldehyde-3-phosphate respectively. The phosphate groups are labeled Ph. Note that carbon
numbers in G3P and the subsequent molecules in the pathway are not indicated since they will
differ depending on the origin of G 3-P (directly from Y or from DHAP).
In bacteria, other pathways contribute to the oxidation of glucose molecules. These pathways include the hexose monophosphate pathway (HMP, Figure 2A) and in some gram-negative bacteria the Entner-Doudoroff (ED) pathway (Figure 3). Gram positive bacteria do not metabolize glucose through the ED pathway. The second step of the HMP cycle is a decarboxylation reaction leading to the conversion of 6-phosphogluconate into ribulose-5- phosphate (Figure 2B).
Some bacteria are lacking the enzyme that convert product Y (see figure 1) into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G-3-P) and don’t have a functional ED pathway. Could these bacteria rely on the HMP pathway for the production of
ATP? Briefly explain.
Figure 2: Oxidative HMP cycle. Panel A: Glucose is degraded into glyceraldehyde-3-phosphate
and CO2 in a reaction cycle. Panel B: reaction leading to the release of CO2.
3) To determine which glucose oxidizing pathway species of bacteria use, scientists have design respirometric experiments using glucose labeled with radioactive carbon (14C) in various position. Then, scientists collect the CO2 released soon after the bacteria were fed with the radiolabeled glucose. It is assumed that the released CO2 is the result of the decarboxylation of pyruvate (EMP and ED pathways) or 6-phosphogluconate (HMP pathway). Remember that, under aerobic conditions, the carboxyl group of pyruvate is released as CO2 when pyruvate reacts with coenzyme A to produce acetyl-coenzyme A.
a) Three separate analyses of glucose degradation by the gram-negative bacteria E. coli through radiorespirometry were performed using each time a different types of radiolabeled glucose molecules: glucose with carbon #1 labeled, or glucose with carbon #2 labeled, or glucose with carbons #3 and 4 labeled. The graph below (Figure 4) shows the results of the 3 experiments. Based on the pathways given in figures 1 to 3, rank (when possible) metabolic pathway (EMP, HMP, or ED) by order of predominance in E. coli? Briefly explain. (8 pts)
Figure 4: Graph showing the amount of 14CO2 release when glucose labeled on carbon #1(14C(#1)), #2 (14C(#2)) and #3 AND 4 (14C(#3&4) were used to feed E. coli. The Y-axis represents the amount of 14CO2 released.
b) Based on your knowledge of the fate of acetyl-coenzyme A under aerobic conditions, which metabolic pathway is likely at the origin of the 14CO2 released at the later stage of the experiment when glucose is labeled on carbon #2 or on carbon #1? Based on your answer in part b), would you modify your answer from part a)? Explain your reasoning. (6 pts)
c) The following two graphs show the same types of respirometric experiments with two strains of bacteria (A and B). Based on this graph, identify which strain is gram-negative. Briefly
explain. (6 pts)
Figure 5: Graph showing the amount of 14CO2 release at the early stage when glucose labeled on
carbon #1 (14C(1)), #3 (14C(3)) and #4 (14C(4) were used to feed two different bacterial strains.
The Y-axis represents the amount of 14CO2 released.
Explanation / Answer
Ans.) Yes, the bacteria can rely on the HMP pathway. The mentioned Y is fructose 1-6, bisphosphate. At first, glucose-6-phosphate is converted into fructose-6-phosphate. Eventfully this fructose-6-phosphate on the expanse of ATP molecule converted into fructose 1-6, bisphosphate.
3. a) In hexose monophosphate shunt (HMP pathway), the glucose is labeled at its 1st carbon that is released as carbon dioxide. The remaining five carbon sugars again go into into the glycolysis. In case of ED (Entner-Doudoroff) pathway, the carbon dioxide is released from the C1 and C4 of glucose, which eventually releases acetyl CoA (from 2nd and 5th carbon). On the other hand in the Embden-Meyerhof-Parnas (EMP) pathway, the carbon is released from the C3 and C4 of carbon. Therefore, as per the graph, the prevalence of pathways in E.coli is as followed -
EMB pathway > HMP pathway > ED pathway
b). The acetyl CoA synthesized in ED (Entner-Doudoroff) pathway, again enters into the citric acid cycle under aerobic conditions and releases two molecules of carbon dioxide. Consequently, the order of predominant pathways alters as -
EMB pathway > ED pathway > HMP pathway
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.