Chapter 8 PP

Chapter 8: Enzymes – Basic Concepts and Kinetics

Introduction

This chapter explores the fundamental properties of enzymes, their catalytic mechanisms, specificity, and the kinetic models used to describe their activity. Enzymes are essential biological catalysts that accelerate chemical reactions in living organisms, often with remarkable specificity and efficiency.

Enzymes as Catalysts

Enzymes Are Remarkable Catalysts

• Definition: Enzymes are biological molecules, primarily proteins (some are RNA), that catalyze chemical reactions by lowering the activation energy required.

• Importance: They are crucial for speeding up vital biochemical reactions and are often targets for pharmaceutical drugs.

• Example: Omeprazole inhibits the K+/H+ ATPase enzyme in the stomach, reducing acid production and treating heartburn.

• Enzymes function by stabilizing the transition state, the high-energy intermediate in a reaction pathway.

Learning Objectives

• Describe the relationship between enzyme catalysis, reaction thermodynamics, and transition state formation.

• Explain the connection between the transition state and the enzyme's active site.

• Define and determine reaction velocity.

• Distinguish between reversible and irreversible enzyme inhibitors.

• Identify different types of reversible inhibitors.

• Understand the significance of studying enzymes at the single-molecule level.

Chapter Outline

• Enzymes as powerful and highly specific catalysts

• Gibbs free energy and enzyme thermodynamics

• Enzyme acceleration of reactions via transition state facilitation

• Michaelis–Menten kinetic model

• Enzyme inhibition by specific molecules

• Single-molecule enzyme studies

Enzyme Properties and Specificity

Enzymes Are Powerful and Highly Specific Catalysts

• Enzymes can accelerate reaction rates by factors of millions or more.

• Even simple reactions, such as the hydration of CO2 to form bicarbonate in red blood cells, require enzymatic catalysis (e.g., carbonic anhydrase).

• Example Reaction:

$\ce{CO2 + H2O ->[\text{carbonic anhydrase}] HCO3^- + H^+}$

Rate Enhancement by Selected Enzymes

Enzymes can increase reaction rates by many orders of magnitude compared to uncatalyzed reactions. The following table summarizes the rate enhancements for selected enzymes:

Enzyme	Nonenzymatic half-life	Uncatalyzed rate (kuncat, s−1)	Catalyzed rate (kcat, s−1)	Rate enhancement (kcat/kuncat)
OMP decarboxylase	78,000,000 years	2.8 × 10−16	39	1.4 × 1017
Staphylococcal nuclease	130,000 years	1.7 × 10−13	95	5.6 × 1014
RNase	69,000 years	1.0 × 10−11	60	6.0 × 1012
Carboxypeptidase A	7.3 years	1.9 × 10−8	578	3.1 × 1010
Triose phosphate isomerase	1.9 days	4.3 × 10−6	66,000	1.5 × 1010
Carbonic anhydrase	5 seconds	1.3 × 10−1	1 × 106	7.7 × 106

Source: Berg et al., Biochemistry, 9e, © 2019 W. H. Freeman and Company

Enzyme Specificity

• Enzymes are highly specific for their substrates due to precise molecular interactions.

• Example: Proteolytic enzymes catalyze the hydrolysis of peptide bonds, but may also hydrolyze ester bonds under laboratory conditions.

• Different proteolytic enzymes (e.g., trypsin vs. thrombin) exhibit varying degrees of specificity for substrate sequences.

Key Terms

• Substrate: The reactant molecule upon which an enzyme acts.

• Active site: The region of the enzyme where substrate binding and catalysis occur.

• Transition state: A high-energy, unstable intermediate state during a chemical reaction.

Example Application

• Enzyme inhibitors, such as omeprazole, are used therapeutically to modulate enzyme activity in disease states.

Additional info: The table above demonstrates the extraordinary catalytic power of enzymes, with rate enhancements ranging from millions to over a quadrillion times faster than uncatalyzed reactions.