# Enzyme Activity: pH ## The Optimum pH - Every enzyme has an **optimum pH** at which it operates most effectively - This is the pH value where the enzyme catalyses reactions at its maximum rate - The optimum pH varies between different enzymes depending on where they function - Enzymes working in acidic environments have low optimum pH values - Enzymes working in alkaline environments have high optimum pH values - Many enzymes in the human body have an optimum pH around 7 (neutral)

Optimum pH is the specific pH at which an enzyme achieves its maximum rate of catalysis

## pH and Enzyme Structure - **Hydrogen bonds** and **ionic bonds** maintain the tertiary structure of enzyme proteins - These bonds hold the enzyme molecule in its precise three-dimensional shape - The active site shape depends on this tertiary structure being maintained ## How pH Affects Enzyme Activity ### pH changes below and above the optimum - Solutions with excess $\text{H}^+$ ions (acidic) or $\text{OH}^-$ ions (alkaline) disrupt enzyme structure - These ions interfere with the hydrogen and ionic bonds in the enzyme - The bonds begin to **break** when pH moves away from the optimum - This causes the **active site to change shape** - The altered active site becomes less complementary to the substrate - Enzyme-substrate complexes form **less easily** as pH deviates further from optimum ### Complete denaturation - At extreme pH values, the active site shape changes so much that it is no longer complementary to the substrate - The substrate can **no longer bind** to the active site - **Enzyme-substrate complexes cannot form at all** - At this point, **complete denaturation** has occurred

Denaturation at extreme pH is permanent and irreversible

## Examples of Enzyme Optimum pH | Enzyme | Location | Environment pH | Optimum pH | |--------|----------|----------------|------------| | Pepsin | Stomach | pH 2 (acidic due to HCl) | pH 2 | | Urease | Certain bacteria | pH 7 (neutral) | pH 7 | | Trypsin | Small intestine | pH 8 (slightly alkaline) | pH 8 | - The location where an enzyme naturally functions indicates its optimal working conditions - For instance, pepsin must work in the highly acidic stomach environment, so it has evolved to function best at pH 2 - If you transferred pepsin to the small intestine (pH 8), it would rapidly denature ## Graphical Representation - When plotting rate of reaction against pH, the graph shows a **bell-shaped curve** - The peak of the curve represents the **optimum pH** - Either side of the optimum, the rate decreases as the enzyme becomes less effective - At extreme pH values, the rate drops to zero as denaturation occurs

Different enzymes have different optimum pH values, each adapted to their specific working environment

## Buffer Solutions in pH Investigations ### Why use buffers? - When investigating pH effects on enzymes, you need to maintain **specific, constant pH values** - Some enzyme-catalysed reactions naturally produce or consume $\text{H}^+$ ions, which would change the pH - Buffer solutions **resist pH changes** even when acids or bases are produced during reactions ### Using buffers experimentally - Each buffer solution maintains one **specific pH value** - A measured volume of buffer is added to the reaction mixture - The **same volume** of each different buffer should be used for each pH being investigated - This ensures pH is the only variable changing between different experimental runs ## Investigating pH Effects: The Amylase-Starch Experiment ### Background - **Amylase** is a digestive enzyme that breaks down starch (a polysaccharide) into maltose (a disaccharide) - This reaction can be monitored using the **iodine test for starch** - Iodine solution is orange-brown normally but turns **blue-black** in the presence of starch - When starch has been completely digested, iodine remains **orange-brown** ### Apparatus - Test tubes - Buffer solutions at different pH levels - Amylase solution - Starch solution - Iodine solution - Spotting tile - Pipettes and syringes - Stopwatch - Water bath (for temperature control) - Safety equipment: goggles and gloves

Enzymes can cause allergic reactions on direct skin contact—always wear gloves

### Method 1. **Prepare spotting tile** - Place single drops of iodine solution in rows on the spotting tile - Iodine appears orange-brown initially 2. **Set up reaction mixture** - Label a test tube with the pH being tested - Add $2 \text{ cm}^3$ of amylase solution using a syringe - Add $1 \text{ cm}^3$ of buffer solution to the test tube - Equal volumes and concentrations of enzyme must be used so these variables are controlled 3. **Start the reaction** - In a separate test tube, measure $2 \text{ cm}^3$ of starch solution - Add the starch to the amylase-buffer mixture - **Start the stopwatch immediately** while mixing with a pipette - Mixing ensures even distribution of enzyme and substrate molecules 4. **Test for starch at regular intervals** - After **10 seconds**, use a pipette to transfer one drop of the reaction mixture onto the first iodine drop - The iodine should turn **blue-black** if starch is still present - Wait another 10 seconds and test again on the next iodine drop - **Repeat every 10 seconds** until the iodine solution **remains orange-brown** 5. **Record results** - When iodine stays orange-brown, all starch has been digested - Record the **time taken** for complete starch breakdown - This indicates the reaction rate: shorter time = faster rate = better enzyme activity at that pH 6. **Repeat with different pH buffers** - Clean all equipment thoroughly - Use fresh solutions - Test a range of pH values (e.g., pH 5, 6, 7, 8, 9) - The pH giving the **shortest completion time** is closest to the optimum ### Controlling temperature - All solutions (starch, amylase, pH buffers) should be placed in a water bath at $35°\text{C}$ - Allow solutions to **reach the water bath temperature** before mixing - Use a thermometer to verify temperature - This ensures temperature remains constant and only pH varies ## Alternative: Using a Colorimeter - A **colorimeter** measures how much light passes through (transmission) or is absorbed by (absorbance) a solution - For the starch-amylase reaction, a colorimeter can track the **colour change** more precisely than visual observation - As starch is digested, the mixture becomes lighter (when iodine is added) - This increases **transmission** or decreases **absorbance** ### Advantages of colorimetry - Provides **quantitative data** rather than qualitative observations - More **accurate** than judging colour change by eye - Can measure reaction progress **continuously** rather than at discrete time intervals - Reduces **human error** in detecting the endpoint ## pH vs Temperature: Key Difference

pH does not affect the kinetic energy or collision rate of molecules—it only disrupts enzyme structure

- Temperature affects how fast molecules move (kinetic energy) and therefore collision frequency - pH affects only the **shape of the enzyme's active site** through bond disruption - Both can cause denaturation, but through different mechanisms - pH-induced denaturation occurs through **breaking ionic and hydrogen bonds** - Temperature-induced denaturation occurs through **excessive molecular vibration** ## Why Homeostasis Matters - Organisms must maintain stable internal pH (among other factors) through **homeostasis** - Even small pH deviations can reduce enzyme efficiency - Large pH changes cause irreversible enzyme denaturation - Since enzymes are essential for virtually all metabolic reactions, pH control is critical for survival - Many buffer systems exist in organisms (e.g., carbonic acid-bicarbonate in blood) to prevent pH fluctuations