Case 20 NAD-dependent Glyceraldehyde-3-phosphate Dehydrogenase from Thermoproteu

Question

Case 20 NAD-dependent Glyceraldehyde-3-phosphate Dehydrogenase from Thermoproteus tenax Focus concept Glycolytic enzymes from Thermoproteus tenax are regulated in an unusual manner. Prerequisites .The glycolytic pathway Enzyme kinetics and inhibition. The cooperative nature of regulated enzymes. Background Carbohydrate metabolism in the thermophilic archacal 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 we studied in class 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. 0 GAPDH NAD T. tenax ?? ? H- -C-OH O +NADH + H or Glyceraldehyde-3-phosphate 3-phosphoglycerate Figure 20.1: Non-phosphorylating NAD-dependent GAPDH in T. tenax. 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.

Explanation / Answer

1. Reaction catalyzed by the enzyme pyruvate kinase, phosphofructokinase and pyruvate kinase that catalyzed irreversible reaction.

2. GADPH stand for glyceraldehyde 3-phospahte dehydrogenase that perform the formation of 1,3 phosphoglycerate from glyceraldehyde 3- phosphate. This reaction is reversible . Here, substrate get phosphorylated by enzyme.

3. The enzyme GADPH phosphorylates the substrate in E. Coli but in T.tenax it is dephosphorylating the substrate as well as phosphorylating the substrate.

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