Edexcel iGCSE Biology-2.13 changes in pH and enzyme function- Study Notes- New Syllabus

Effect of pH on Enzyme Function

📝 Introduction

• Enzymes only work properly under certain pH conditions.
• The pH scale measures how acidic or alkaline a solution is.
• Changes in pH can affect the structure of an enzyme.
• If the pH becomes too high or too low, the enzyme may stop working properly.
• This happens because pH can change the shape of the enzyme’s active site.
• Every enzyme is adapted to work best at a particular pH, called its optimum pH.

🧬 What is pH?

• pH tells us whether a solution is acidic, neutral, or alkaline.
• The pH scale ranges from 0 to 14.
• pH 7 is neutral.
• Below pH 7 is acidic.
• Above pH 7 is alkaline.

🔑 Why Does pH Affect Enzymes?

• Enzymes are made of proteins that have a fixed three-dimensional shape.
• Their active site must keep the correct shape for the substrate to fit.
• Changes in pH can affect the bonds that help maintain this shape.
• When these bonds are disturbed, the active site changes shape.
• If the active site changes, the substrate cannot bind properly.
• As a result, the enzyme cannot carry out the reaction efficiently.

🌟 Optimum pH

What is Optimum pH?

• The optimum pH is the pH at which an enzyme works most efficiently.
• At this pH, the enzyme has the correct shape and the active site fits the substrate perfectly.
• The rate of reaction is at its highest.

Important Points

• Different enzymes have different optimum pH values.
• An enzyme’s optimum pH depends on where it normally works inside the organism.
• Many enzymes inside the human body work best around pH 7, but not all enzymes have the same optimum pH.

⬇️ Effect of Low pH (Highly Acidic Conditions)

What happens?

• The solution contains a large number of hydrogen ions.
• These ions interfere with the bonds that maintain the enzyme’s shape.
• The active site begins to change.

Effect on Enzyme Activity

• The substrate cannot fit into the active site as easily.
• Fewer enzyme-substrate complexes are formed.
• The reaction becomes slower.

If the pH becomes extremely low

• The enzyme may become denatured.
• The active site changes permanently.
• The reaction stops completely.

⬆️ Effect of High pH (Highly Alkaline Conditions)

What happens?

• The alkaline conditions also disturb the bonds inside the enzyme.
• The enzyme gradually loses its normal shape.

Effect on Enzyme Activity

• The active site no longer matches the substrate.
• Enzyme activity decreases.
• Fewer products are formed.

If the pH becomes too high

• The enzyme becomes denatured.
• The active site changes permanently.
• The substrate can no longer bind.

🚫 Denaturation by pH

• Very acidic or very alkaline conditions can cause denaturation.

During denaturation:

• The enzyme loses its original shape.
• The active site changes.
• The substrate cannot fit into the active site.
• The enzyme stops working.

Important Points

• Mild changes in pH may only slow the reaction.
• Extreme changes in pH can permanently damage the enzyme.
• Once an enzyme is permanently denatured, it usually cannot return to its original shape.

🔬 Examples of Enzymes with Different Optimum pH

🍽️ Pepsin

• Pepsin is a digestive enzyme found in the stomach.
• The stomach contains hydrochloric acid.
• Pepsin works best in strongly acidic conditions.
• Its optimum pH is about 2.
• This allows it to digest proteins efficiently inside the stomach.

🍞 Amylase

• Amylase is found in saliva.
• It begins the digestion of starch in the mouth.
• The mouth has a nearly neutral pH.
• Amylase works best at about pH 7.

🥩 Trypsin

• Trypsin is a digestive enzyme found in the small intestine.
• The small intestine is slightly alkaline.
• Trypsin works best at about pH 8.
• This alkaline environment allows it to digest proteins effectively.

📈 pH and Enzyme Activity Graph

• The graph showing enzyme activity against pH forms a bell-shaped curve.

At Low pH

• Enzyme activity is low.
• The active site begins to lose its correct shape.

At the Optimum pH

• Enzyme activity reaches its highest point.
• The active site has the correct shape.
• The substrate fits perfectly.

At High pH

• Enzyme activity decreases again.
• The active site becomes altered.
• The reaction slows or stops.

Remember

• The graph rises towards the optimum pH.
• It falls on both sides of the optimum.
• This gives the graph a bell-shaped appearance.

💡 Important Points

• Every enzyme has its own optimum pH.
• Different enzymes do not all work best at the same pH.
• pH affects the shape of the active site.
• The correct pH helps the substrate bind easily.
• Very acidic or very alkaline conditions reduce enzyme activity.
• Extreme pH values can denature enzymes permanently.
• Enzymes are adapted to the pH of the place where they normally work.

📊 Summary Table – Effect of pH on Enzymes

📋 Optimum pH of Common Enzymes

⚡ Quick Recap
✔ pH measures how acidic or alkaline a solution is.
✔ Every enzyme has a specific optimum pH where it works best.
✔ At the optimum pH, the active site has the correct shape and the reaction is fastest.
✔ Low or high pH can change the shape of the active site.
✔ If the substrate cannot fit into the active site, the enzyme cannot work properly.
✔ Very acidic or very alkaline conditions can denature the enzyme permanently.
✔ Pepsin works best at pH 2, Amylase at pH 7, and Trypsin at pH 8.
✔ Enzyme activity against pH forms a bell-shaped curve, with the highest point at the optimum pH.