Enzymes are the molecules formed mostly of proteins that help to catalyze the biochemical reactions in the body. They have an active site where the substrate can bind and the reaction is catalyzed.

Enzymes lower the activation energy of a biochemical reaction that is, the amount of energy that is needed for the reaction to begin. Enzymes work by binding to reactant molecules and holding them in such a way that the chemical bond-breaking and bond-forming processes take place more readily. Thus, the reaction speeds up.

Current research supports a model known as induced fit to explain the catalyzing action of the enzymes. As the enzyme and substrate come together, their interaction causes a mild shift in the enzyme’s structure that confirms an ideal binding arrangement between the enzyme and the substrate. An enzyme-substrate complex is formed. This dynamic binding maximizes the enzyme’s ability to catalyze its reaction.

Enzymes are divided into six classes based on the type of reaction they catalyze. The six classes are:

1. Oxidoreductases: These enzymes transfer electrons or hydrogen from one molecule to another thus, they perform oxidation and reduction reactions. Example: alcohol dehydrogenase that catalyzes the oxidation reaction of ethanol to form an aldehyde.

2. Transferase: These enzymes move a functional group from one molecule to the other. Example: peptidyl transferase that transfers lysine residue from tRNA during translation.

3. Hydrolases: These enzymes catalyze a reaction in which a molecule reacts with water to break and form two different molecules by hydrolysis. Example: serine hydrolases that cause hydrolysis of the peptide bond.

4. Lyases: These enzymes catalyze a reaction in which a molecule breaks to form two different molecules without reacting with water. Example: Arginosuccinate lyase that breaks argininosuccinate into arginine and fumarate.

5. Isomerases: These enzymes catalyze the reaction in which a molecule is converted into its isomer. Example: phosphoglucoisomerase that converts glucose-6-phosphate to fructose-6- phosphate during glycolysis.

6. Ligases: These enzymes catalyze the reaction in which two molecules join to form one molecule. Example: DNA ligase which joins two strands of the DNA.

The enzyme’s active site binds to the substrate. Since enzymes are proteins, this site is composed of a unique combination of amino acid residues (side chains or R groups). Each amino acid residue can be large or small; weakly acidic or basic; hydrophilic or hydrophobic; and positively-charged, negatively-charged, or neutral. The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site. A specific chemical substrate matches this site like a jigsaw puzzle piece and makes the enzyme specific to its substrate. When the enzyme catalyze the reaction by bringing reactants in close proximity, it is known as an approximation. It can also catalyze the reaction by forming a temporary covalent bond with the reactant. This is known as covalent catalysis.

Many enzymes only work if bound to non-protein helper molecules called cofactors and coenzymes. Cofactors are inorganic ions such as iron (Fe²⁺) and magnesium (Mg²⁺). For example, DNA polymerase requires a zinc ion (Zn²⁺) to build DNA molecules. Coenzymes are organic helper molecules with a basic atomic structure made up of carbon and hydrogen. The most common coenzymes are dietary vitamins. Vitamin C is a coenzyme for multiple enzymes that take part in building collagen, an important component of connective tissue. Other water-soluble vitamins such as B vitamins also act as coenzymes. Pyruvate dehydrogenase is a complex of several enzymes that requires one cofactor and five different organic coenzymes to catalyze its chemical reaction. The availability of various cofactors and coenzymes regulates enzyme function.

Local conditions such as a change in temperature and pH affect the activity of an enzyme. When the temperature is low, the activity of an enzyme is affected thus, the reaction becomes slow. When the temperature is increased, the activity of enzyme increases thus, the reaction becomes fast. But when the temperature extends beyond the optimal limit, the enzyme denatures and the reaction becomes slow. This denaturation causes a change in the active site that affects the binding of the substrate. When the pH is within the optimal limit, the activity of the enzyme is maximum and the reaction can be catalyzed. The reaction will be fast. When the pH is increased or decreased, the activity of the enzyme is affected thus, the reaction becomes slow.

Key Points

• The enzyme’s active site binds to the substrate.
• The induced-fit model states a substrate binds to an active site and both change shape slightly, creating an ideal fit for catalysis.
• When an enzyme binds its substrate it forms an enzyme-substrate complex.
• The activation energy reacquired to start a reaction is reduced by the enzymes.
• Enzymes promote chemical reactions by bringing substrates together in an optimal orientation, thus creating an ideal chemical environment for the reaction to occur.
• The six major classes of enzymes are Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases.
• Specific substrate/substrates bind to the active site of enzymes
• The positions, sequences, structures, and properties of the amino acid residues at the active site create a very specific chemical environment to which a specific chemical substrate matches.
• An enzyme catalyzes a reaction by either bringing the reactants or substrates together or by bonding with them to help in the reaction.
• Cofactors are the inorganic ions such as iron (Fe²⁺) that help the enzyme in catalysis.
• Coenzymes are organic molecules such as vitamin C that help the enzyme in catalysis.
• Water-soluble vitamins include vitamin B and vitamin C that are coenzymes.
• When the temperature is reduced, the activity of an enzyme is affected.
• When the temperature is increased, activity of an enzyme is increased but excess raise in temperature can cause denaturation of the enzyme.
• Increased or decreased pH from the optimal pH reduces enzyme activity.

Key Terms

• substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.
• induced fit: Proposes that the initial interaction between enzyme and substrate is relatively weak, but that these weak interactions rapidly induce conformational changes in the enzyme that strengthen binding.
• active site: The active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed.
• catalyze: accelerate or speed up
• denature: the destruction of the protein/ enzyme by changes like increased temperature
• optimal: the most favorable temperature or pH for an enzyme is considered as optimal temperature or pH