Question

4. Chemical reactions break existing bonds and make new bonds. Enzymes are biological catalysts (proteins) that speed up biological reactions. Just of note, there are only three known biological reactions that can occur at any measurable rate without enzymes – they control everything in biology! Consider the hydrolysis of ATP:

ATP + H2O <--> ADP + Pi

You know that there are MANY biological reactions that use the energy of ATP hydrolysis to power other reactions. Each of these reactions is catalyzed by an enzyme, part of which catalyzes the hydrolysis of ATP (as mentioned in lecture ATP is water is stable for millennia because the uncatalyzed rate is incredibly slow).

List three different things that an enzyme that catalyzes the hydrolysis of ATP might be doing to lower the activation energy and speed up this reaction. Note that your answer does not have to be specific to this reaction but could apply to any biological reaction.

Answer 1

For any chemical reaction, the reactant must acquire certain amount of energy to cross the transition state which is known as activation energy of the reaction. Catalysts generally lower the activation energy barrier which speeds up the reaction. Enzymes are biological catalysts. However one important thing must be clarified beforehand that enzymes do not alter the overall Gibss free change of the reaction.

The most effective way an enzyme catalyses the reaction is by providing an alternative route. Enzymes have active sites in them. Through these active sites they can specifically and covalantly bind to a substrate such as protein and they help the biological reaction to proceed. But this binding is specific and in the past it was termed as "lock and key" mechanism. However now a days a more common term "induced fit" is in use. Through binding enzyme can lower the activation energy by

1. Enzyme can fit with reactants (substrates) and make them close to one another for the reaction to happen faster. According to collision theory of chemical reaction, two molecules react when they collide into one another at certain orientations. Enzymes provides right orientation and close proximity of the reactants. This complexation leads to an alternative route for the reaction.

2. Many biological reactions as well as chemical reactions require right environment such as certain polar or non-polar or ionic environment for the them to happen easily. Enzymes can do that. They can provide required proton concentration to the substrates to make the reaction go faster. Here one should note that pH affects the activity of enzymes.

3. Enzymes can bind with the transition state formed by the substrates and help to stabilise it and in turn lowering the amount of energy needed to achieve it.