1. Reactions
Reactions - Enzymes
Enzymes

2. Reactions Learning Objectives Discuss the importance of enzymes
Describe the structure of an enzyme
Explain the steps of enzyme catalysis
After this lesson you will be able to discuss the importance of enzymes, describe the structure of an enzyme, and explain the steps of enzyme catalysis.

3. Enzymes Reactions can occur at varying rates
Catalyst – a substance that affects the rate of a chemical reaction
Enzyme – a specialized protein that catalyzes biological reactions
Usually end with the suffix “-ase” and its name comes from the substrate
Ex) Maltase breaks down maltose
Have optimal temperatures and pH to maintain normal functioning
Some chemical reactions naturally occur quickly while others can take years.
A catalyst is a substance that speeds up a chemical reaction. Although a catalyst may form bonds during the reaction, it returns to its previous state after the reaction. In other words, a catalyst speeds up chemical reactions while remaining unchanged by the reaction.
An enzyme is a specific type of catalyst. Enzymes are specialized proteins that act as biological catalysts. They make it possible for the chemical reactions inside a cell to continue at a pace fast enough for life to continue.
Enzymes share several characteristics. The name of an enzyme usually ends in the suffix “–ase” and is often derived from the name of its substrate.
For example, maltase is an enzyme that acts to split the disaccharide, maltose, into two glucose molecules.
Enzymes work best at certain temperatures and pH. Optimum temperatures for enzymes in living cells are usually close to normal cell temperature, but the optimum pH may vary. For example, enzymes that function within the stomach to help break down foods are most effective at acidic pH values.

4. Enzymes Enzymes lower the activation energy of the reaction
Activation energy – the minimum amount of energy needed to cause a chemical reaction to occur
Without enzymes, reactions would proceed too slowly
Enzymes catalyze reactions by lowering the activation energy. Activation energy is the minimum amount of energy needed to cause a chemical reaction to occur. For example, for water to form from hydrogen and oxygen gas, energy must be supplied to the system to break the bonds between the atoms of the two gases. A spark is used to start the reaction. This energy, or spark, is the activation energy.
Enzymes lower the amount of activation energy needed to start the chemical reaction. Without enzymes, reactions would proceed too slowly for cells to live.

5. Enzymes Substrate – the reactant in an enzyme-catalyzed reaction
Active site – the area of the enzyme where the substrate binds
Two ideas on how enzymes attach to substrates:
Lock-and-key model
Induced-fit model
Induced fit – the slight change of shape of an enzyme after the substrate binds to it
The reactants on which an enzyme works are called substrates.
The location on the enzyme where the substrates bind is called the active site.
There are two theories of how enzymes bind to substrates.
The lock-and-key model suggests the substrate molecules fit into the enzyme just like a key fits into a lock. During this close association, the enzyme catalyzes a chemical reaction in the substrates.
The induced-fit model suggests that the enzyme is more flexible. In this model, chemical interactions between the molecules of the substrate and the amino acids of the enzyme cause the enzyme to change shape slightly.
This change in shape is called an induced fit. It enables the enzyme to hold the substrates tightly during the chemical reaction. The induced fit model is the most widely accepted theory of how enzymes bind to substrates.

6. Enzymes Induced-fit enzyme reaction Substrates enter
Induced fit around the substrate
Lowers activation energy
Products are released/enzyme returns to normal shape
Enzyme can be reused
The steps of an induced fit enzyme-catalyzed reaction are explained here.
Substrates enter the active site of the enzyme, forming an enzyme-substrate complex.
The enzyme changes shape slightly, creating an induced fit around the substrates. This lowers the activation energy, enabling the chemical reaction to proceed.
The products are released from the enzyme and the enzyme returns to its normal shape.
Because the enzyme has not endured any permanent changes during the process, it is immediately ready to catalyze another reaction.

7. Reactions Chemical Reactions Dehydration Synthesis

8. Reactions Learning Objectives
Summarize the dynamics of a chemical reaction
Explain how a dehydration synthesis occurs
List examples of dehydration synthesis in macromolecules
After this lesson you will be able to summarize the dynamics of a chemical reaction, explain how a dehydration synthesis occurs, and list examples of dehydration synthesis in macromolecules.

9. Chemical Reactions
Chemical reaction – a process in which bonds of compounds are broken and reformed into different compounds
Reactant (left side of equation) – the starting material in a chemical reaction
Product (right side of equation) – the compounds that result from a chemical reaction
Coefficient – the number in front of each molecule in a chemical equation that tells how many of each molecule is present
A chemical reaction occurs when compounds interact, bonds are broken, and new bonds form different compounds. For example, when methane (CH4) and oxygen (O2) interact, the bonds within the existing molecules are broken and new bonds create carbon dioxide (CO2) and water (H2O).
The starting materials in a chemical reaction, also called the reactants, are listed on the left side of the equation.
The end materials, also called the products, are listed on the right side of the equation.
The numbers in front of each molecule are called coefficients. Coefficients tell you how many of each molecule are present in the equation. For example, one molecule of methane reacts with two molecules of oxygen to form one molecule of carbon dioxide and two molecules of water.

10. Chemical Reactions
Molecules are rearranged during reactions, but composition stays the same
There are the same number of atoms in the products as there are in the reactants
Although molecules are rearranging during chemical reactions, the composition of matter remains the same. In other words, new atoms are not being created or destroyed in the process.
In the reactants, there are one carbon atom, four hydrogen atoms, and four oxygen atoms. During the reaction, the bonding between the atoms is rearranged. The products still contain the same number of atoms as the reactants: one carbon atom, four hydrogen atoms, and four oxygen atoms.

11. Dehydration Synthesis
Dehydration synthesis – a chemical reaction in which two molecules are bonded together and a molecule of water is removed
Also called condensation reaction
Hydrogen from one reacts with hydroxyl (-OH) of another
Water molecule is removed (hence “dehydration”)
The chemical reaction that bonds two monomers is called a dehydration synthesis (also called a condensation reaction).
During dehydration synthesis, the hydrogen (H) from one monomer reacts with the hydroxyl group (-OH) of another monomer.
A water molecule (H2O) is removed and a new bond between the monomers is formed. The name dehydration synthesis is appropriate for this reaction because “dehydration” refers to the loss of water and “synthesis” refers to the combination of two things to create something new.   

12. Dehydration Synthesis
Creates polymers present in macromolecules
Monosaccharides  Polysaccharides
Amino Acids  Proteins
Fatty Acids  Lipids
Hydrolysis – a chemical reaction in which the bonds between two molecules are broken by adding a molecule of water
Dehydration synthesis creates the polymers present in macromolecules.
Every time monosaccharides bond to form a disaccharide or a monosaccharide bonds to a polysaccharide, dehydration synthesis occurs. The same is true for the addition of amino acids to a polypeptide chain. Dehydration synthesis also creates triglycerides each time a fatty acid attaches to a glycerol molecule.
When the bonds between monomers are broken in hydrolysis, a water molecule must be added.