Biochemistry Enzymes Overview

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Enzyme Features and Classes

• Enzymes are biological catalysts that increase reaction rates without being consumed.
• They accelerate reactions by lowering the activation energy.
• Enzymes are primarily proteins, although some are catalytic RNA molecules called ribozymes.
• Enzymes operate optimally at pH 7.4 and 37°C (for most human enzymes).

Intended Learning Outcomes

• Students will understand enzymes as catalysts in biochemical reactions.
• Students will identify basic enzyme features.
• Students will describe the major enzyme classes.
• Students will grasp that enzymes reduce activation energy but do not alter equilibrium.

Enzymatic Equation

• Enzymatic reactions follow the general equation: E + S ⇌ ES ⇌ EP ⇌ E + P,
• Where E = enzyme, S = substrate, and P = product.
• Substrate is the molecule the enzyme acts on.
• Product is the molecule formed from the reaction.

Reaction Rate vs. Equilibrium

• Reaction rate measures substrate and product concentration change.
• Equilibrium is where these concentrations no longer change in a closed system.
• Equilibrium does not necessitate equal substrate and product concentrations.
• Enzymatic equations usually use the reversible sign to signify reversibility of the reaction.

Equilibrium in Simple Terms

• At equilibrium, the system balances; no net change in substrate or product quantities.
• Reactions still occur in both directions but their rates are equal.
• Equilibrium denotes a dynamic state where reaction rates are equal in both directions.

Enzymes Increase Reaction Rate

• Enzymes do not change the amount of product formed at equilibrium.
• Enzymes simply accelerate the time to reach equilibrium.

Enzymes Lower Activation Energy

• Enzymes lower activation energy in reactions using alternative routes.
• This alternate route is represented by a lower activation energy peak on a reaction progress diagram.

Changes in Transition State

• Transition state without enzyme is highly unstable requiring high activation energy.
• Transition state with enzyme is more stable, requiring less activation energy, through intermediate formation.

Enzyme Rates

• Enzymes dramatically increase reaction rates from billions to quadrillions of times faster in some cases.

What Enzymes Need To Work

• Some enzymes function without other compounds .
• Some enzymes require coenzymes generated from vitamins (e.g. NADH) or cofactors to function.
• Cofactors can alter enzyme conformation (shape) for better function (e.g. metal ions like Mg2+, Fe2+, Zn2+).

Definitions

• An enzyme acts as a catalyst, speeding up biochemical reactions.
• Coenzymes are vitamin-derived molecules helping enzymes.
• Cofactors are non-protein molecules (i.e. ions) aiding enzymes.
• Apoenzyme is the protein component of an enzyme requiring coenzymes to finalize functionality.
• Holoenzyme is the combination of apoenzyme and coenzyme for an active enzyme.

Enzyme Classifications

• There are six major enzyme classes based on the reaction they catalyze.
• Examples of classes include oxidoreductases (alcohol dehydrogenase), transferases (kinases), hydrolases (lipases), lyases (decarboxylases), isomerases (isomerase), and ligases (DNA ligase).

Summary

• Enzymes increase reaction rates by decreasing activation energy.
• Enzymes do not alter the equilibrium of the reaction.
• Some need cofactors/coenzymes.
• Enzymes are categorized into six classes.

Resources

• Lehninger Principles of Biochemistry (Seventh Edition, 2017) by Freeman and Company, specifically Chapter 6.