Biochemistry Enzymes Overview

Learning Objectives

• Describe enzymes and their common properties
• List enzyme classifications in correct order
• Describe factors changing enzyme activity
• Explain the relationship between Km and activity
• Explain enzyme inhibition and inhibition types

Metabolic Reactions

• Enzymes catalyze metabolic reactions—both anabolic and catabolic reactions
• Enzymes speed up reactions by lowering activation energy
• Enzymes facilitate biochemical modifications

Glycolysis

• Glycolysis is a metabolic pathway that converts glucose to pyruvate
• Enzymes are involved in each step of glycolysis—facilitating reactions

Activation Energy

• Activation energy is the energy needed for a reaction to occur
• Enzymes decrease activation energy
• Reaction speed increases
• Enzymes are not consumed in the reaction

Activation Energy and Catalysts

• Catalysts speed up reaction rates by decreasing the activation energy
• Enzymes are biocatalysts, acting as catalysts in biological systems

Enzymes

• Enzymes are biocatalysts, increasing reaction rates
• They bind to substrates, speeding up reactions
• They are not consumed in reactions

Properties of Enzymes

• Most enzymes are proteins—an exception is ribozymes (catalytic RNA).
• Enzymes catalyze reactions with high efficiency and specificity
• Enzymes change reaction rate, not equilibrium

Efficiency

• Enzyme-catalyzed reactions are highly efficient, increasing speeds by 10³ to 10⁸ times
• Carbonic anhydrase is a fast enzyme, hydrating 10⁶ CO₂ molecules per second

Specificity

• Reaction specificity: A given enzyme catalyzes a specific reaction type
• Substrate specificity: A given enzyme uses specific molecules or chemically related molecules

Enzyme Nomenclature: Classification

• Enzymes are classified by the type of reaction they catalyze
• An enzyme's classification is represented using an Enzyme Commission (EC) number

Enzyme Structure

• Apoenzyme: Protein part
• Cofactor: Non-protein part
• Holoenzyme: Catalytically active form
  • The active site facilitates substrate interaction

Enzyme Structure: Active Site

• Active sites have key amino acids
• Amino acids precisely positioned to interact with substrates

Enzyme Activity

• Activity (V) represents the speed of the reaction
• Measured in µmol substrate converted to product in a given time
• UNIT Activity: Enzyme activity that converts 1 µmol substrate into product in 1 minute, under optimal conditions

Factors Affecting Enzyme Activity

• Enzyme concentration: Increased enzyme increases reaction speed
• Substrate concentration: More substrate increases the reaction speed but only up to a certain point
• Temperature: Optimal temperature maximizes reaction speed
• PH: Optimal pH maximizes reaction speed
• Inhibitors: Substances that reduce enzyme activity

Enzyme Concentration

• Increased enzyme concentration increases reaction speed and product formation

Substrate Concentration

• As substrate concentration increases, the reaction speed increases but is limited by the enzyme availability
• Km represents the substrate concentration where the reaction rate is half its maximal value (½ Vmax)
• Affinity between the enzyme and substrate depends on Km

Temperature

• Optimal temperature maximizes reaction speed
• Excessive heat can inactivate enzymes

pH

• Optimal pH maximizes reaction speed
• Extreme pH values inactivate enzymes

Inhibitors

• Inhibitors decrease enzyme activity
• Reversible inhibitors can be removed from the enzyme
• Irreversible inhibitors permanently inactivate the enzyme
• Competitive inhibitors compete with the substrate for the active site
• Non-competitive inhibitors bind to the enzyme at a site separate from the active site

Reversible Inhibition

• Competitive inhibitors compete for active site
• Non-competitive inhibitors bind at a different site
• Allosteric inhibitors change enzyme shape (non-competitive)

Enzyme Kinetics and Km

• Km reflects the enzyme's affinity for its particular substrate. A smaller Km suggests a higher affinity.
• Vmax represents the maximum reaction rate, limited by enzyme availability.

Km: A Clinical Example

• Enzyme Km values can vary between individuals, having clinical implications depending on the gene.

Regulation of Enzyme Activity

• Enzyme amount: Controlling enzyme synthesis and degradation
• Enzyme modification: Using covalent/non-covalent modifications and allosteric mechanisms
• Compartments: Enzyme localization in various cellular compartments (e.g. Mitochondria, cytosol)
• Isozymes: Enzymes with similar activity but differing enzyme structures

Phosphorylation/Dephosphorylation

• Post-translational modification such as phosphorylation/dephosphorylation (usually covalent) turns the enzyme "on" or "off"

Feed-Back Inhibition/Feed-forward activation

• The end-product of a chain of enzyme reactions can inhibit early enzyme in the chain
• To prevent the buildup of unused or unnecessary products
• To save energy
• Feed-forward activation is a regulatory mechanism where an intermediate in a metabolic pathway activates an enzyme further down the pathway.
• This allows for the coordinated regulation of metabolic processes.

Compartments

• Enzymes located in different cell compartments may have different activity based on location and the processes involved

Isozymes

• Enzymes with similar functions, but slightly different structures, enabling them to be useful in various aspects (e.g. heart and skeletal muscle)

This quiz covers the fundamental aspects of enzymes, including their properties, classifications, and factors affecting their activity. Additionally, it explores the role of enzymes in metabolic reactions like glycolysis and their impact on activation energy. Test your understanding of enzyme function and inhibition.