A mechanism of catalysis is the way in which the chemical reaction is assisted in moving forward.

The mechanisms of enzyme catalysis vary, but they are all similar in that they increase the reaction rate by reducing the activation energy. After binding between the enzyme and substrate takes place, one or more mechanisms of catalysis lowers the energy of the reaction’s transition state by providing an alternative chemical pathway for the reaction.

There are several possible mechanisms of catalysis including: 

• Approximation: When the enzyme catalyzes the reaction by bringing reactants in close proximity.
• Covalent catalysis: When the enzyme catalyzes the reaction by forming a temporary covalent bond with the reactant.
• Acid-base catalysis: When the enzyme catalyzes the reaction by acting as an acid (proton donor) or base (proton acceptor).
• Metal ion catalysis: When the enzyme catalyzes the reaction by assisting in electrophilic or nucleophilic interactions or binds to substrate (increasing binding energy).

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. The valency of the metals is often important here with divalent (2+ charged ions) being popular in many biological catalysts.
• Coenzymes are organic helper molecules with a basic atomic structure made up of carbon and hydrogen.
• Dietary vitamins are the most common coenzymes. 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.

For example, 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.

Practice Questions

Khan Academy

• Muscles rely on lactate dehydrogenase 

 

MCAT Official Prep (AAMC)

Practice Exam 4 C/P Section Passage 9 Question 52

Key Points

• 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. Divalent (2+ charged) ions are the most common.

• 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.

Key Terms

Enzyme: A substance produced by a living organism which acts as a catalyst to bring about a specific biochemical reaction.

Substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.

Activation Energy: The minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction.

Catalyze: Cause or accelerate (a reaction) by acting as a catalyst.

Approximation: When the enzyme catalyzes the reaction by bringing reactants in close proximity.

Covalent catalysis: When the enzyme catalyzes the reaction by forming a temporary covalent bond with the reactant.

Acid-base catalysis: When the enzyme catalyzes the reaction by acting as an acid (proton donor) or base (proton acceptor).

Metal ion catalysis: When the enzyme catalyzes the reaction by assisting in electrophilic or nucleophilic interactions or binds to substrate (increasing binding energy).

Cofactors: Inorganic ions that assist an enzyme in its catalytic activity

Coenzymes: Small, organic molecules that assist an enzyme in its catalytic activity.

Water-soluble vitamins: Include the series of B-vitamins and Vitamin C and are a dietary requirement as precursors to coenzymes.