Enzymes: History and Definition

Enzymes: History and Definition

• Enzymes, in Greek, mean "in living" (en= in, zyme= living).
• Enzymes are biocatalysts or organic catalysts with high molecular weight (except RNA enzymes, ribozymes).
• The term "enzyme" was coined by Kuhne in 1878.
• The first enzyme extracted was from yeast by Buchner in 1897.
• The first purified enzyme was urease, isolated by James B. Summer in 1926.

Definition

• Enzymes are proteins (biological catalysts).
• They increase the rate of reactions by lowering activation energy.
• Enzymes catalyze nearly all biochemical reactions in cells.
• Enzymes have unique 3-dimensional shapes that fit the shapes of reactants.

Activation Energy

• Activation energy is the initial energy needed to start a chemical reaction.
• It's often supplied as heat from the surroundings.
• Think of activation energy as a barrier to making a product.
• The most stable product has the lowest energy.
• Most reactions require a "push" to get started, which is activation energy.

Characteristics of Enzymes

• Enzymes generally act quickly.
• The reaction speed depends on the substrate amount.
• Enzymes are not damaged during the reaction, so they can be reused.
• Enzymes can react in both directions.
• Enzymes are specific, limited to one particular reaction for a specific substrate.
• Enzyme molecules are typically larger than their substrates.
• All enzymes are proteins, but not all proteins are enzymes.
• Enzymes are complex globular proteins with three dimensions.

Nomenclature and Classification

Trivial Names

• Enzyme names typically end in "-ase" (e.g., dehydrogenase).
• Sometimes, the enzyme's name identifies the reacting substance (e.g., sucrase catalyzes sucrose reactions).
• Other times, the name describes the enzyme's function (e.g., oxidases catalyze oxidation).
• Some enzyme names are common, particularly for digestion enzymes (e.g., pepsin, trypsin).

Systemic Nomenclature

• According to the International Union of Biochemistry (IUB), an enzyme has two parts:
  • The first part is the name of the substrate.
  • The second is the reaction type, ending with the suffix "-ase" (e.g., lactate dehydrogenase, pyruvate carboxylase).

IUB Classification

• Enzymes are classified by the reactions they catalyze.
• Different classes have unique examples shown in the table.

EC Numbers

• EC numbers are four-digit codes for enzymes.
• The first digit represents the enzyme class.
• Subsequent digits specify the functional group, coenzyme, and substrate.
  • (e.g., alcohol: NAD oxidoreductase EC number 1.1.1.1).

Enzyme Terminologies

• Enzymes: Proteins produced by living cells that act as biological catalysts, accelerating biochemical reactions by lowering activation energy.
• Substrate: The substance upon which an enzyme acts in a biochemical reaction.
• Coenzymes: Small organic molecules that transport chemical groups (e.g., NAD, FAD).
• Cofactor: Non-protein, heat-stable, small-MW substances with the same function as coenzymes (e.g., Mg²⁺, Fe²⁺, Cu²⁺).

Ribozymes

• Ribozymes (RNA enzymes) catalyze chemical reactions by hydrolyzing their own phosphodiester bonds or bonds in other RNAs.

Enzyme Active Site

• The active site is a region within the enzyme that fits the shape of the substrate.

Zymogen

• Zymogens (proenzymes) are inactive forms of enzymes.
• The active site is masked by a polypeptide chain.
  • (e.g., pepsinogen→pepsin, trypsinogen→trypsin).

Isoenzymes

• Isoenzymes catalyze the same reaction in different tissues.
• (e.g., lactate dehydrogenase has 5 isoenzymes).

Mechanism of Enzyme Action

• The substrate (S) fits into the enzyme's (E) active site.
• An enzyme-substrate complex (ES) is formed.
• The reaction occurs in the ES complex, converting S to product (P).
• The product (P) is released from the enzyme.
• The overall reaction converts substrate to product.

Example of an Enzyme-Catalyzed Reaction

• Description of sucrose binding catalyzing sucrose hydrolysis.

Models of Enzyme Action

Lock-and-Key Model

• The active site is rigid.
• Only substrates with the exact shape fit.
• Only one substrate fits.

Induced Fit Model

• The active site is flexible.
• The enzyme's and substrate shapes adjust to improve the fit.
• There's a greater range of substrate specificity.

Factors Affecting Enzyme Activity

Temperature

• Enzymes are most active at optimal temperatures (usually 37°C in humans).
• Enzyme activity decreases at low temperatures; at high temperatures.
• Enzyme activity is lost due to denaturation.

pH

• Enzymes are most active at optimum pH.
• Enzyme activity is lost due to disruptions in tertiary structure in low or high pH.

Enzyme Concentration

• Reaction rate increases with enzyme concentration (at constant substrate concentration).
• Higher enzyme concentration = more substrate binds to the enzyme.

Substrate Concentration

• Reaction rate increases with substrate concentration (at constant enzyme concentration).
• Maximum activity with saturated enzymes.

Enzyme Inhibition

• Inhibitor is any substance that diminishes enzymatic reaction velocity.
• Types of inhibition:
  • Competitive: Inhibitor binds directly to the active site.
  • Noncompetitive: Inhibitor binds to an allosteric site, changing the active site's shape.

Explore the fascinating world of enzymes, their historical context, and their fundamental role as biological catalysts. This quiz will cover the origins of the term 'enzyme,' notable discoveries, and the concept of activation energy. Test your knowledge on how enzymes influence biochemical reactions and their unique structures.