Lesson 12 – Enzymes and Reactions

Learning Goals to understand the concept of Gibb’s free energy to compare endergonic and exergonic reactions to determine the role enzymes play in the reactions occurring in living things.

Reactions In a chemical reaction, bonds are being broken and reformed. We call the starting molecules the reactants and final result the products.

Reactions Everywhere! Chemical Reactions are occurring all the time in living systems: to break down food to convert energy to useable forms to build macromolecules to turn signals on and off to convert one substance to another and on and on….

Reactions Require Energy All chemical reactions require an initial investment of energy called the activation energy (Ea). This is the energy required to begin the reaction.

Exergonic Reactions Despite all reactions needing energy to begin, some reactions will release more energy (in the form of heat or work) than was put in. We can say that there is less free energy in the products than the reactants. These are called exergonic reactions. In biology, a good example of an exergonic reaction would be the hydrolysis of starch.

Life Needs Fast Reactions! Reactions left to themselves can be far too slow for living systems’ needs (that, or the energy required would be too high!) Enzymes are biological catalysts that make reactions occur more quickly.

Enzymes In both exergonic and endergonic reactions, enzymes lower the activation energy and speed up the rate of reaction.

How Enzymes Work Enzymes are proteins, with very specific sequences and shapes. Their shapes are very well adapted to fit certain molecules. We call the molecules that they work on substrates. We call the location they fit the active site.

Induced Fit Current theory suggests that there are slight changes in shape (conformational changes) that occur when a substrate binds into the active site. These changes cause a better fit, which makes breaking/making bonds more likely. Enzymes are quite specific, but substrates that have similar structures can compete for binding sites.

Example This enzyme is called sucrase, and it is responsible for helping the hydrolysis of sucrose into glucose and fructose by lowering the activation energy of the reaction. Molecules ending in ‘ase’ are enzymes, with the first part of the word usually a description of the reaction or molecule they are catalyzing.

Breaking Down Molecules Enzymes can break down many of the macromolecules we learned about this week, such as the starch in a potato. These would be exergonic reactions. Reactions that release energy from larger molecules are called catabolic reactions.

Building Bigger Molecules Enzymes help synthesize larger molecules, such as glycogen from glucose. These would be endergonic reactions. Reactions that store energy in larger molecules are referred to as anabolic reactions.

Review Identify as either anabolic or catabolic: a) Protein synthesis  b) Digestion  c) Cellular Respiration  d) Photosynthesis •Look up the definition for metabolism. Try to connect this to your prior understanding of what metabolism was (if any!). •Define Catalyst •Draw a labelled free energy diagram to illustrate the effect of an enzyme on the activation energy of a hypothetical reaction (exergonic) •Can an enzyme affect the free energy change of a reaction?