I really need help with question (a) and (b). Thank you! Carbohydrate metabolism
ID: 267093 • Letter: I
Question
I really need help with question (a) and (b). Thank you!
Carbohydrate metabolism in the thermophilic archaeal bacterium Thermoproteus tenax is rather peculiar compared to the types of organisms usually studied in introductory biochemistry. For example, the phosphofructokinase reaction in T. tenax is reversible, and is dependent upon pyrophosphate rather than ATP. In addition, T. tenax has two different glyceraldehyde-3-phosphate dehydrogenase (GAPDH) isoenzymes. One is well known and, although it requires NADP+ as a cofactor instead of NAD+, it resembles the GAPDH enzyme found in E. coli and is referred to as the “phosphorylating GAPDH”. In contrast, the second isoenzyme is irreversible and requires NAD+ as a cofactor and is referred to as the “nonphosphorylating GAPDH”. In this case, we will consider the properties of the latter enzyme. The balanced equation of the reaction catalyzed by the nonphosphorylating NAD+-dependent
glyceraldehyde-3- phosphate dehydrogenase is shown below.
T. tenax stores energy in the form of glycogen, which is degraded to glucose-1-phosphate. The glucose-1-phosphate is then converted to glucose-6-phosphate and then enters the glycolytic pathway. The two GAPDH enzymes are probably differently regulated in T. tenax. The authors of this study propose that “phosphorylating, NADP+-dependent” GAPDH is involved in efficient ATP production whereas the “non-phosphorylating, NAD+-dependent” GAPDH is somewhat involved in ATP production but is also involved in providing intermediates for cellular biosynthetic reactions.
The Hill coefficients for NAD+ binding to the T. tenax GADPH in the presence and absence of NADP+ were measured and are shown on the graph in Figure 20.2. What is the significance of the change in the value of the Hill coefficient? Is this consistent with (a) the shape of the curve and (b) the information given in the background on the enzyme’s structure?
0 GAPDH + NAD T. tenax CH 3-phosphoglycerate Glyceraldehyde-3-phosphateExplanation / Answer
The change in the value of hill coefficient is significant because it indicates positive cooperative binding of ligands to the enzyme in the presence of NADP+?.
a)Yes, the hill coefficient is consistent with the shape of the curve. As seen in the graph, in case of control, hill coefficient is equal to 1.This indicates independent binding of the ligand or non-cooperative binding. While in case of NADP+ as the effector, hill coefficient value increases to 2 , indicating positive cooperative binding.
b) Yes, the change in hill coefficient value is consistent with the information given in the background on the enzyme’s structure. According to the subunit arrangement of the enzyme, NAD+-dependent GAPDH of T. tenax is activated by a series of metabolites as it is involved in ATP production and also in providing intermediates for cellular biosynthetic reactions.