■ Explain the function of a coenzyme.
■ Why is it important for a cell that allosteric inhibition be reversible?
The three central metabolic pathways—glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle—modify organic molecules in a step-wise fashion to form:
■ Intermediates with high-energy bonds that can be used to synthesize ATP by substrate-level phosphorylation
■ Intermediates that can be oxidized to generate reducing power
■ Intermediates and end products that function as precursor metabolites
Note that the precursor metabolites can be siphoned off from these pathways for use in biosynthesis. The rate at which they are removed will dramatically affect the overall energy gain of catabolism. This is generally overlooked in descriptions of the ATP-generating functions of these pathways for the sake of simplicity. Recognize, however, that because these pathways serve more than one function, the energy yields are only theoretical.
The pathways of central metabolism are compared in table 6.7 (page 146). The entire pathways with chemical formulas and enzyme names are illustrated in Appendix IV.
Glycolysis is the primary pathway used by nearly all organisms to convert glucose to pyruvate (figure 6.15). In the 10-step pathway, one molecule of glucose is converted into two molecules of pyruvate. This generates a net gain of two molecules of ATP and two molecules of NADH. The overall process can be summarized as:
glucose (6 C) + 2 NAD+ + 2 ADP + 2 Pi d 2 pyruvate (3 C) + 2 NADH + 2 H+ + 2 ATP
In addition to generating ATP and reducing power (NADH), the pathway produces 6 different precursor molecules needed by E. coli (see table 6.3). Because none of the reactions in the pathway require molecular oxygen, it can occur under either aerobic or anacrobic conditions.
■ Step 1: A high-energy bond is expended to initiate the pathway when a phosphate group from ATP is transferred to glucose, forming glucose 6-phosphate. In bacteria, the high-energy phosphate bond is expended as glucose is being transported into the cell.
■ Step 2: A chemical rearrangement occurs to generate fructose 6-phosphate.
■ Step 3: A second high-energy phosphate bond from ATP is expended to form fructose 1, 6-bisphosphate, priming the molecule so it can be more readily split into two molecules. The step is non-reversible and serves as an important control point for the pathway. High levels of ATP inhibit the allosteric enzyme that catalyzes the step.
■ Step 4: The 6-carbon compound formed in the previous step is split to form two 3-carbon molecules— glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHP).
■ Step 5: The DHP is converted into glyceraldehyde 3-phosphate (G3P), resulting in a total of two G3P molecules at this point in glycolysis.
■ Step 6: This is a coupled reaction; energy released by the oxidation of G3P is used to add an inorganic phosphate group, creating 1,3-bisphosphoglycerate (BPG), which has a high-energy phosphate bond. During the oxidation, the electron carrier NAD+ is reduced to form NADH + H+. Note that this and subsequent steps of glycolysis each occur twice, once for each of the two G3P formed as a result of the previous step.
■ Step 7: Substrate-level phosphorylation occurs, as energy in the high-energy phosphate bond of BPG is used to generate ATP. Removal of the phosphate group from BPG forms 3-phosphoglycerate (3PG). At this
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