enzymatic/metabolic pathway?

Question

Could someone describe an enzymatic/metabolic pathway that occurs in the human body in its entirety and include an example that contains a negative feedback reaction.

Answer 1

One Thing is ACTH Release, the Final Product (Cortisol) Inhibits the Release of ACTH From the Anterior Pituitary (Via CRH From the Hypothalamus), See the Dexamethasone Suppression Test, Another Example of Something Very Similar In Mechanism is FSH an LH. Another Very Good Example is TSH. Another Good Example, Of a Enzymatic Pathway Regulation, is Hexokinase of Glycolysis.

Answer 2

A metabolic pathway is a series of chemical reactions occurring within a cell, catalyzed by enzymes, resulting in either the formation of a metabolic product to be used or stored by the cell, or the initiation of another metabolic pathway (then called a flux generating step). Many pathways are elaborate, and involve a step by step modification of the initial substance to shape it into the product with the exact chemical structure desired.

Eg.of negative feedback reaction

Hexokinase(in glycolysis)

Hexokinase is inhibited by glucose-6-phosphate (G6P), the product it forms through the ATP driven phosphorylation. This is necessary to prevent an accumulation of G6P in the cell when flux through the glycolytic pathway is low. Glucose will enter the cell but since the hexokinase is not active it can readily diffuse back to the blood through the glucose transporter in the plasma membrane. If hexokinase remained active during low glycolytic flux the G6P would accumulate and the extra solute would cause the cells to enlarge due to osmosis.

2.Many of the enzymes in the TCA cycle are regulated by negative feedback from ATP when the energy charge of the cell is high. Such enzymes include the pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. These enzymes, which regulate the first three steps of the TCA cycle, are inhibited by high concentrations of ATP. This regulation ensures that the TCA cycle will not oxidise excessive amounts of pyruvate and acetyl-CoA when ATP in the cell is plentiful. This type of negative regulation by ATP is by an allosteric mechanism.

Several enzymes are also negatively regulated when the level of reducing equivalents in a cell are high (high ratio of NADH/NAD+). This mechanism for regulation is due to substrate inhibition by NADH of the enzymes that use NAD+ as a substrate. This includes pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Citrate is used for feedback inhibition, as it inhibits the phosphofructokinase(enzyme in glycolysis) that makes Fructose 1,6-bisphosphate), a precursor of pyruvate. This prevents a constant high rate of flux when there is a build up of citrate and a decrease in substrate for the enzyme.