Unit 5: Enzymes

1. Introduction to Enzymology

Enzymology is the study of enzymes, their kinetics, structure, and function, as well as their relationship to each other.

1.1 What are Enzymes?

• Definition: Enzymes are biological catalysts that accelerate the rate of biochemical reactions without being consumed or permanently altered in the process.
• Composition: The vast majority of enzymes are globular proteins. However, some RNA molecules, called ribozymes, can also act as catalysts (e.g., in protein synthesis at the ribosome).
• Key Terminology:
  • Substrate (S): The reactant molecule that an enzyme acts upon.
  • Product (P): The molecule(s) resulting from the enzymatic reaction.
  • Active Site: A specific three-dimensional cleft or pocket on the enzyme's surface where the substrate binds and catalysis occurs. The amino acid residues in the active site are responsible for both binding the substrate and catalyzing the reaction.
  • Enzyme-Substrate Complex (ES): A transient complex formed when a substrate binds to the active site of an enzyme.

1.2 Properties of Enzymes

1. Catalytic Power: Enzymes can increase reaction rates by factors of 10⁶ to 10¹² compared to the uncatalyzed reaction. They achieve this by lowering the activation energy of the reaction.
2. High Specificity: Enzymes are highly specific for their substrates. This specificity arises from the precise 3D structure of the active site, which is complementary to the shape of the substrate.
   • Absolute Specificity: The enzyme acts on only one specific substrate (e.g., urease acts only on urea).
   • Group Specificity: The enzyme acts on a group of related molecules (e.g., hexokinase phosphorylates various hexose sugars like glucose and fructose).
   • Stereospecificity: The enzyme acts on only one stereoisomer of a substrate (e.g., L-amino acid oxidase will not act on D-amino acids).
3. Regulation: The activity of enzymes in a cell can be regulated (increased or decreased) to meet the physiological needs of the organism. This is crucial for controlling metabolic pathways.
4. Mild Reaction Conditions: Enzymes function optimally under mild physiological conditions of temperature (usually 25-40°C), pH (usually around 7.4), and atmospheric pressure. This is a major advantage over industrial chemical catalysts which often require extreme conditions.

2. Classification of Enzymes

Enzymes are systematically classified and named by the International Union of Biochemistry and Molecular Biology (IUBMB). Each enzyme is assigned a four-part EC (Enzyme Commission) number. There are six major classes based on the type of reaction they catalyze.

3. Mechanism of Enzymatic Action

Enzymes accelerate reactions by providing an alternative reaction pathway with a lower activation energy (Ea). Activation energy is the minimum energy required for reactants to transform into products.

(Image Description: A reaction coordinate diagram showing that the activation energy for an enzyme-catalyzed reaction is significantly lower than for an uncatalyzed reaction, while the overall free energy change (ΔG) between reactants and products remains the same.)

3.1 Models of Enzyme-Substrate Interaction

1. Lock and Key Model (Emil Fischer, 1894):

   • This early model proposed that the active site of the enzyme has a rigid, pre-formed shape that is perfectly complementary to the shape of the substrate.
   • Analogy: A specific key (substrate) fits into a specific lock (enzyme).
   • Limitation: This model fails to explain the flexibility of some enzymes and the stabilization of the transition state.

2. Induced Fit Model (Daniel Koshland, 1958):

   • This is the more widely accepted model. It proposes that the active site is flexible and not perfectly complementary to the substrate initially.
   • The binding of the substrate induces a conformational change in the enzyme, causing the active site to mold itself around the substrate for a more precise fit.
   • Analogy: A hand (substrate) fitting into a glove (enzyme). The glove changes shape as the hand enters it.
   • This induced fit aligns the catalytic groups of the active site correctly and can also strain the bonds of the substrate, pushing it towards the transition state.

3.2 The Catalytic Cycle

The general mechanism of an enzyme-catalyzed reaction can be described in a few steps:

1. Binding: The substrate (S) binds to the active site of the enzyme (E), forming the transient enzyme-substrate (ES) complex.
   E + S ⇌ ES
2. Catalysis: The enzyme converts the bound substrate into the product (P). This involves the formation of an even more transient enzyme-product (EP) complex. The enzyme stabilizes the transition state of the reaction, lowering the activation energy.
   ES ⇌ EP
3. Release: The product(s) are released from the active site. The enzyme is now free and in its original conformation, ready to bind to another substrate molecule.
   EP ⇌ E + P

The overall reaction is: E + S ⇌ ES ⇌ EP ⇌ E + P

4. Role of Prosthetic Group, Co-factor and Co-enzymes

Many enzymes require a non-protein chemical component for their catalytic activity. These components are broadly called co-factors.

• Apoenzyme: The inactive protein part of an enzyme.
• Holoenzyme: The complete, catalytically active enzyme, consisting of the apoenzyme and its co-factor.

Holoenzyme = Apoenzyme + Co-factor

Co-factors can be classified into two main groups:

4.1 Inorganic Ions (Metal Ion Activators)

• These are metal ions such as Mg²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Mn²⁺, and K⁺.
• Functions:
  • They can help bind the substrate to the active site.
  • They can stabilize the enzyme's folded structure.
  • They can participate directly in the catalytic reaction, often by acting as an electrophile or by facilitating redox reactions.
• Example: Zn²⁺ is a co-factor for the enzyme carbonic anhydrase, which is essential for transporting CO₂ in the blood.

4.2 Organic Molecules (Co-enzymes)

• These are small, organic, non-protein molecules. Many co-enzymes are derived from vitamins.

• They are further divided based on how they bind to the apoenzyme:

  • Co-enzymes (loosely bound):

    • They bind transiently (non-covalently) to the enzyme's active site.
    • They act as carriers, transporting chemical groups, atoms, or electrons from one reaction to another. They are often modified during the reaction and must be regenerated in a separate reaction.
    • Examples:
      • NAD⁺ / NADH (from Niacin, Vitamin B3): Carries electrons in redox reactions (e.g., in cellular respiration).
      • FAD / FADH₂ (from Riboflavin, Vitamin B2): Carries electrons in redox reactions.
      • Coenzyme A (CoA) (from Pantothenic Acid, Vitamin B5): Carries acyl groups.

  • Prosthetic Groups (tightly bound):

    • They are tightly or covalently bound to the apoenzyme.
    • They remain associated with the enzyme throughout the catalytic cycle.
    • Examples:
      • Heme group: A porphyrin ring containing an iron atom, found in cytochromes and catalase. It is involved in electron transfer and catalysis.
      • Biotin (Vitamin B7): Covalently attached to carboxylase enzymes involved in CO₂ transfer.

5. Role of Enzymes in Biological Systems

Enzymes are fundamental to virtually every process in a living organism. They play critical roles in:

1. Metabolism: All metabolic pathways (e.g., glycolysis, Krebs cycle, gluconeogenesis) are sequences of enzyme-catalyzed reactions. Enzymes control the flow of energy and matter through these pathways.

   • Example: Hexokinase catalyzes the first step of glycolysis, phosphorylating glucose.

2. Digestion: Digestive enzymes break down large food macromolecules into smaller molecules that can be absorbed by the body.

   • Example: Amylase in saliva and pancreatic juice breaks down starch into sugars. Trypsin in the small intestine breaks down proteins into peptides.

3. DNA Replication and Repair: The integrity and inheritance of genetic information depend on enzymes.

   • Example: DNA Polymerase synthesizes new DNA strands during replication. DNA Ligase joins DNA fragments together.

4. Signal Transduction: Enzymes like kinases and phosphatases act as molecular switches in cell signaling pathways, controlling cellular responses to external stimuli.

   • Example: Protein Kinase A (PKA) is activated by cyclic AMP and phosphorylates target proteins to regulate various cellular processes.

5. Detoxification: Enzymes, particularly in the liver, metabolize and detoxify foreign substances (xenobiotics) like drugs and pollutants.

   • Example: Cytochrome P450 oxidases are a large family of enzymes that detoxify a wide range of compounds.

6. Muscle Contraction: The movement of muscles is powered by the hydrolysis of ATP.

   • Example: Myosin ATPase is an enzyme in muscle fibers that hydrolyzes ATP to provide the energy for contraction.

6. Enzymes Used in Industry

The specificity, efficiency, and ability of enzymes to work under mild conditions make them ideal catalysts for a wide range of industrial applications. This field is known as Industrial Biotechnology or Applied Enzymology.

6.1 Food and Beverage Industry

• Baking: Amylases break down starch in flour into simple sugars, which are then fermented by yeast to produce CO₂ for leavening.
• Brewing: Amylases and proteases from malted barley are used to break down starches and proteins during the mashing process to produce fermentable sugars and amino acids.
• Cheese Making: Rennin (or Chymosin), a protease, is used to coagulate milk protein (casein) to form curds.
• Fruit Juice Clarification: Pectinases break down pectin, a complex polysaccharide that causes cloudiness in fruit juices, resulting in a clear product.
• High-Fructose Corn Syrup (HFCS): Glucose isomerase converts glucose (from corn starch) into the much sweeter fructose.

6.2 Detergent Industry

Enzymes are key ingredients in "biological" laundry detergents for stain removal at lower temperatures.

• Proteases (e.g., Subtilisin): Break down protein-based stains like blood, grass, and egg.
• Amylases: Break down starch-based stains like gravy, pasta, and chocolate.
• Lipases: Break down fat and oil-based stains.
• Cellulases: Break down microfibrils on cotton fabrics, which helps in "bio-polishing" (restoring color and softness) and removing particulate soil.

6.3 Textile Industry

• Desizing: Amylases are used to remove starch sizing agents applied to yarn to prevent breaking during weaving.
• Stone-Washing: Cellulases are used to give denim a faded, worn-out look and soft feel, replacing the abrasive and environmentally damaging process of using pumice stones.

6.4 Biofuels Industry

• The production of bioethanol from biomass (like corn or sugarcane) involves breaking down complex carbohydrates into simple sugars for fermentation.
• Amylases and glucoamylases hydrolyze starch into glucose.
• Cellulases and hemicellulases are crucial for breaking down lignocellulosic biomass (wood, agricultural waste) into fermentable sugars, a key area of research for second-generation biofuels.

6.5 Pharmaceutical and Medical Industry

• Diagnostics: Enzymes are used as markers in diagnostic tests. For example, glucose oxidase is used in blood glucose sensors for diabetics. Enzymes like horseradish peroxidase (HRP) are used in ELISA tests.
• Therapeutics (as drugs):
  • Streptokinase and Urokinase are thrombolytic (clot-busting) enzymes used to treat heart attacks and strokes.
  • Asparaginase is used as an anti-cancer drug to treat certain types of leukemia by depleting the amino acid asparagine, which the cancer cells need to grow.
• Contact Lens Cleaners: Proteases are used to remove protein deposits from contact lenses.

6.6 Pulp and Paper Industry

• Bio-bleaching: Xylanases are used to pre-bleach wood pulp, which reduces the amount of chlorine-based chemicals needed for final bleaching, making the process more environmentally friendly.