Under what conditions does Km represent the true binding affinity (i.e. Kd) of the substrate to the enzyme? 1. when kcat << k-1 2. when k-1 >> k1 3. when k1 >> kcat 4. when k1 = k-1
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Under what conditions does Km represent the true binding affinity (i.e. Kd) of the substrate to the enzyme?
1. when kcat << k-1
2. when k-1 >> k1
3. when k1 >> kcat
4. when k1 = k-1
An enzyme accelerates the rate of a chemical reaction several times as compared to the uncatalyzed reaction in water. It increases the rate of a chemical reaction by lowering the activation energy. However, an enzyme does not change the free energies of the initial and final states. The free energy of reaction, remains unchanged in the presence of an enzyme, so the relative amounts of reactants and products at equilibrium are unchanged. In other words, an enzyme does not change the position of equilibrium in a chemical reaction. It only accelerates the attainment of equilibria but do not shift their positions.
The formation of an enzyme-substrate complex is the first step in enzymatic catalysis. The enzyme first binds to the substrate, the compound to be catalyzed. Active site is the region of the enzyme where substrate (and the cofactor, if any) binds and catalysis occurs. The unique physical and chemical properties of the active site limit an enzyme's activity to specific substrates. The substrate binds to the active site of an enzyme by multiple weak non covalent interactions like hydrophobic interactions, H-bonds, ionic interactions or reversible covalent bonds. The free energy released in the formation of a large number of weak interactions between the enzyme and the substrate is termed binding energy. The binding between the enzyme and the substrate is highly specific. A given enzyme usually binds to only one kind of substrate. The specificity of binding depends on the precisely defined arrangement of atoms in an active site. The specificity of enzyme-substrate interactions arises mainly from hydrogen bonding and the shape of the active site, which rejects molecules that do not have a sufficiently complementary shape.
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