(a) Explain three principal ways how metabolic processes are regulated at the ce
ID: 215012 • Letter: #
Question
(a) Explain three principal ways how metabolic processes are regulated at the cellular level? (5 points)
(b) The conversion of glucose into two molecules of pyruvate will not proceed for long in cells if there is no regeneration of NAD+. Given what you know, concisely explain why it is important to regenerate NAD+ in cells and how it is regenerated. (5 points)
(c) Explain why it is advantageous for the key control point of a metabolic pathway to be at the beginning rather than at the end of pathway. Explain your answer based on your knowledge on metabolic pathways we have discussed so far in class. (5 points)
Explanation / Answer
The chemical reactions of metabolism are combined to form metabolic pathways where one compound is converted to another compound with the help of enzymes. Enzymes play important role in the fine regulation of metabolic pathways to maintain a constant condition in the cell in reaction to the change in the state of cell environment.
a)
Metabolic pathways are regulated in three ways. They are
1. Gene regulation: As enzymes are encoded by genes, one way of controlling the chemical reactions of the metabolic pathways is by gene regulation. The bacterial genes that control the enzymes for breaking down of sugar are turned off when the bacteria are not exposed to sugar. Genes are switched on when the sugar is available.
2. Cellular regulation: Metabolism is regulated at the cellular level. Cells receive the signals from the environment and allow the chemical reactions in the cell to get adapted to these signals. The signaling pathways often activate the protein kinases that attach the phosphate group to the target proteins. When the person is frightened, the phosphorylation of intracellular enzymes is stimulated in carbohydrate metabolism. If the person is not frightened, the enzymes are dephosphorylated restoring the carbohydrate metabolism.
3. Biochemical regulation: This involves the chemical reactions where the action of the enzymes is assisted by the presence of binding of a molecule to it. This molecule will help in the direct regulation of the enzyme action by binding to its regulatory site.
b)
In glycolysis, the step that is catalyzed by NAD+ has the reactant glyceraldehyde 3-P which gets converted into 1, 3-biphosphoglycerate. Formation of two energy phosphate molecule is catalyzed by NAD+ which is later converted into NADH. If oxygen supply is not there, the cell will have depleted NAD+ and glycolysis will cease till NAD+ is regenerated. NAD is regenerated in the cell due to the process called fermentation. In the fermentation process, pyruvate is converted into another compound in the presence of NADH.
c)
The biochemical pathways are regulated by the enzymes that catalyze the individual reactions. If the enzyme for a step is considered as active, then this specific step is known to take place faster unless the concerned enzyme is inactive. So, if the cell needs to regulate the metabolic pathway, it has to control any one of the enzymes in the pathway.
Often, the primary molecule that is regulated in a biochemical pathway is the enzyme that catalyzes the first step or first committed step. However, the most important key control point in the glycolysis is phosphofructokinase which catalyzes the conversion of fructose-6-P to fructose1, 6-bisphosphate. This reaction is observed as the committed step as it is considered as the main target for the regulation of glycolysis pathway.
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