5.1 Metabolism
Metabolism refers to the sum total of all chemical reactions that occur within a living organism to sustain life.
Types of Metabolism
- Anabolism: The metabolic process of synthesizing complex molecules from simpler ones.
- Requires energy input (endergonic).
- Example: Photosynthesis (synthesis of glucose from carbon dioxide and water), protein synthesis from amino acids, glycogen synthesis from glucose.
- Catabolism: The metabolic process of breaking down complex molecules into simpler units.
- Releases energy (exergonic).
- Example: Cellular respiration (breakdown of glucose into carbon dioxide, water, and ATP energy), hydrolysis of starch into glucose.
5.2 Enzymes
An enzyme is a biological catalyst, mostly composed of proteins, that speeds up the rate of biochemical reactions without being consumed or chemically altered at the end of the reaction.
Mechanism of Enzyme Action (Lock and Key Hypothesis)
- Each enzyme has a specific 3D spatial conformation with an active site.
- The substrate represents the "key" while the active site of the enzyme represents the "lock".
- The substrate binds to the specific active site to form an unstable enzyme-substrate complex.
- The reaction proceeds, lowering the activation energy ($E_a$) required for the reaction to occur.
- The enzyme converts the substrate into products and detaches unchanged to catalyze further reactions.
General Characteristics of Enzymes
- Highly specific in action; each enzyme catalyzes only one specific reaction due to active site geometry.
- Required in small quantities because they remain chemically unchanged and can be reused.
- Reversible: Catalyze reactions in both forward and reverse directions depending on substrate/product concentrations.
- Sensitive to temperature and pH: Activity is easily affected or destroyed (denatured) by extreme environmental conditions.
- Some require non-protein helpers called cofactors (e.g., inorganic ions like $Mg^{2+}$, $Zn^{2+}$, or organic coenzymes like Vitamin B complex) to function effectively.
- Activity can be stopped or slowed down by enzyme inhibitors (e.g., cyanide, heavy metals like mercury and lead).
Factors Affecting the Rate of Enzyme Activity
- Temperature:
- At low temperatures, kinetic energy is low; collision frequency between enzyme and substrate molecules is low.
- As temperature increases, kinetic energy increases, raising collision frequency and the rate of enzyme-substrate complex formation.
- Optimum Temperature: Maximum reaction rate (typically around $37^\circ\text{C}-40^\circ\text{C}$ for human enzymes).
- Above optimum temperature (exceeding $60^\circ\text{C}$), thermal energy breaks hydrogen and ionic bonds, changing the 3D shape of the active site. The enzyme becomes denatured and permanently loses activity.
- pH:
- Each enzyme operates at an optimum pH (e.g., Pepsin in the stomach at $\text{pH } 1.5-2.0$; Salivary Amylase at $\text{pH } 6.8$; Trypsin in the duodenum at $\text{pH } 8.0-8.5$).
- Deviations from optimum pH alter ionic charges on the active site and substrate, breaking hydrogen/ionic bonds, leading to denaturation.
- Substrate Concentration:
- As substrate concentration increases, the rate of reaction increases because more active sites are occupied.
- Beyond the maximum rate ($V_{\text{max}}$), the rate levels off because the enzyme concentration becomes the limiting factor (all active sites are saturated).
- Enzyme Concentration:
- As enzyme concentration increases, reaction rate increases linearly provided excess substrate is present.
- If substrate supply becomes limited, the rate levels off because substrate concentration becomes the limiting factor.
5.3 Application of Enzymes in Daily Life
Immobilized enzymes and extracted commercial enzymes are widely utilized across various industries:
Industrial Applications
- Protease:
- Biological Washing Powders: Breaks down protein stains (blood, egg, meat juices).
- Food Industry: Tenderizes meat and removes hair/hides in leather processing.
- Amylase: Converts starch into glucose/maltose syrup in baking, brewing, and food processing industries.
- Lipase: Breaks down fat stains in laundry detergents and enhances flavor in cheese production.
- Cellulase: Breaks down plant cellulose fibers in denim softening (stonewashing) and fruit juice extraction (clarification).
- Lactase: Hydrolyzes lactose into glucose and galactose to produce lactose-free milk products for lactose-intolerant individuals.
- Zymase (Yeast): Converts glucose into ethanol and carbon dioxide during bread making and alcoholic beverage brewing.
- Pectinase: Breaks down pectin in plant cell walls to clarify and increase yield in fruit juice manufacturing.
- Rennet / Chymosin: Coagulates milk proteins (casein) during cheese production.