Enzymes and Bioenergetics

Have a basic understanding of metabolism & energetics of reactions.

• Reaction Rates

Be able to define substrate, product, active site, transition state, activation energy, endergonic reaction, exergonic reaction & free energy.

• Substrate- The substance that binds to the enzyme. You can think of it as the "starting material" for the reaction.
• Product- The substance produced by the reaction.
• Active site- The part of the enzyme that binds to the substrate.
• Transition state- The highest point on a reaction rate curve (see here). This is the stage at which bonds are broken and reformed in order to transition from substrate to product.
• Activation energy- The energy required to start a reaction. It is the difference between the energy of the substrates and the energy of the transition state.
• Endergonic reaction- A reaction that results in a net consumption of energy.
• Exergonic reaction- A reaction that results in a net release of energy.
• Free energy- Energy available for doing work (i.e. making stuff happen).

Appreciate that enzymes are biological catalysts.

I discussed what a catalyst is in an earlier post. Simply put, it is a substance that increases the rate of reaction by decreasing the amount of energy needed to reach the transition state. An enzyme is a catalyst that is also a biological molecule. Enzymes are often proteins, but they can be other biological molecules (such as RNA) as well. (Enzymes made out of RNA are called "ribozymes.")

Have an understanding of the mechanism of enzyme action in cells.

Enzymes help to speed up the many chemical reactions that occur in our cells so that they can progress at a reasonable pace under biological conditions. Without the appropriate enzymes, many reactions would take place so slowly as to be useless. Hence, you can control whether or not many chemical reactions run simply by controlling the level of enzymes.

Note that enzymes cannot make impossible reactions happen: they just speed up the reactions that are already possible.

Be able to describe the impact on enzyme activity of temperature, pH, substrate concentration. 

• Temperature- Generally, increased temperature increases the reaction rate. However, if temperatures get too high, enzymes can become denatured, which makes them unable to catalyse the reaction. Hence, enzyme activity increases with temperature up until around 40°C or so, after which enzyme activity rapidly decreases due to denaturing of enzymes.
• pH- All enzymes have an optimal pH at which they operate. For most enzymes, this is around 6-8, but can be much higher or lower (enzymes that operate in the acidic environment of the stomach tend to have a low optimal pH). This is because amino acid side chains can be protonated or deprotonated by an acidic or basic environment, thus altering the charge on the amino acids and the ability of the enzyme to interact with the substrate.
• Substrate concentration- A higher substrate concentration results in a faster reaction, as there are more interactions between the enzyme and substrate molecules. However, this only happens up to a point- once all enzymes are bound to substrate molecules, further addition of substrate will make no difference.