Acetyl-CoA acetyl transferase Aceto- l ep 1or cholesterol synthesis. 2 Acetyl- C
ID: 257348 • Letter: A
Question
Acetyl-CoA acetyl transferase Aceto- l ep 1or cholesterol synthesis. 2 Acetyl- CoA acetyl-CoAAcetyl-COA COA-SH CoA-SH HMG-CoA synthase B-Hydroxy-p-methylglutaryl- CoA (HMG-CoA) 2 NADPH+2 H HMG-CoA reductase ATPjid NADP COA-SH Mevalonate many steps Cholestery esters Cholestero intracellular) Oxysterols receptor-mediated endocytosis Cholesterol (extracellular) Figure 4 Generalized scheme for de nove cholesterol synthesis stions Do of the pathway? Contrast committed steps to ra all rate-limiting steps function as the committed step in a given pathway? . What is a committed step? Why do complex pathways have enzymes that are subject to regulation near the starr te-limiting steps. Are all committed steps rate-limiting steps?Explanation / Answer
A committed step in a metabolic pathway is the effectively irreversible enzymatic reaction after which the product can not be converted back to substrate. This is the pathway regulating step. HMG CoA reductase enzyme which irreversibly catalyses the synthesis of mevalonate is the first committed step in cholesterol synthesis.
The product of the first committed step can not be converted back to the substrate, and only can be proceeded to the final product of the pathway. Thus the regulation is required at the start of the pathway to maintain the substrate availability. It makes sure that proper compounds are produced in adequate amounts. It ensures that the intermediates are not accumulated and thus avoids a harmful environment to the cell.
Rate- limiting step is the slowest step in a biochemical reaction and this determines how fast the reaction should take place. This is also called rate-determining step. Sometimes the first committed step can be rate-limiting step as in cholesterol synthesis. HMG CoA is reduced to mevalonate by NADPH in the presence of HMG CoA reductase, this is the rate limiting step determining the speed of the pathway.
All committed steps are not necessarily the rate limiting steps. Committed steps are also regulated by activation and feedback inhibition so that the pathway can be carried out effectively avoiding the waste of energy.
In denovo cholesterol biosynthesis rate-limiting step is same as the first committed step.