Question

1. Why does a noncompetitive inhibitor decrease the Vmax of an enzyme?

2. The insertion of cholesterol into the lipid bilayer does what to the bilayer?

Answer 1

1.inhibitor can bind to an enzyme with or without a substrate at different places at the same time. It changes the conformation of an enzyme as well as its active site, which makes the substrate unable to bind to the enzyme effectively so that the efficiency decreases. A noncompetitive inhibitor binds to the enzyme away from the active site, altering the shape of the enzyme so that even if the substrate can bind, the active site functions less effectively. Most of the time, the inhibitor is reversible. However, this inhibition decreases the turnover number, meaning the rate of reaction decreases. As the inhibitor binds to the enzyme and the enzyme-substrate complex, it reduces the concentration of enzyme available for proper catalysis. Fewer functional enzymes leads to fewer available active sites and thus a smaller Vmax. Unlike competitive inhibition, raising [S] (substrate concentration) is pointless with noncompetitive inhibition.

2.Cholesterol is a major constituent of the eukaryotic cell membrane. ... For example, when present at high concentrations, cholesterol enhances the mechanical strength of the membrane, reduces its permeability, and suppresses the main-phase transition of the lipid bilayer.

The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and other membranes surrounding sub-cellular structures. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role, even though they are only a few nanometers in width,because they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps.