Enzymes

What Are Enzymes?

• Enzymes are biological catalysts that speed up chemical reactions without being changed or used up.
• They are proteins made in living cells.
• Enzymes are essential for life because they maintain the speed of metabolic reactions at a rate that can sustain life.

Which of the following best describes what an enzyme is?

• A catalyst that speeds up reactions in cells, being used up in the process
• A protein that speeds up reactions in cells, being used up in the process
• A carbohydrate that functions as a biological catalyst
• A protein that functions as a biological catalyst

Enzyme Action & Specificity

• Enzymes are specific to one particular substrate because the active site has a complementary shape to the substrate.
• This is called the lock and key hypothesis.
• When the substrate binds to the active site, an enzyme-substrate complex forms.
• After the reaction, products leave the active site and the enzyme is free to catalyse another reaction.

Which of these are true about enzyme action?

• The active site has a complementary shape to the substrate.
• Enzymes can catalyse any reaction.
• The enzyme-substrate complex forms when substrate binds to the active site.
• After the reaction, the enzyme is destroyed.

Enzymes & Temperature

• Enzymes work fastest at their optimum temperature (e.g., 37°C in the human body).
• Increasing temperature from 0°C to optimum increases kinetic energy, leading to more collisions and a faster rate.
• High temperatures break bonds holding the enzyme's shape, causing denaturation (irreversible loss of active site shape).
• Low temperatures do not denature enzymes; they just slow down the reaction.

Which graph shows the correct effect of temperature on enzyme activity?

• A: Rate increases steadily then drops sharply after optimum
• B: Rate decreases steadily with temperature
• C: Rate increases linearly without drop
• D: Rate is constant at all temperatures

Enzymes & pH

• Most enzymes have an optimum pH of 7 (neutral).
• Some enzymes, like those in the stomach, have a lower optimum pH (around 2).
• Others, like those in the duodenum, have a higher optimum pH (8 or 9).
• Extreme pH changes can denature the enzyme by destroying bonds that hold its shape.

The optimum pH for most enzymes is ____. Some enzymes in the stomach work best at pH ____. If the pH is too high or too low, the enzyme may become ____.

Investigating the Effect of Temperature on Amylase

• Heat starch solution to a set temperature.
• Add iodine to wells of a spotting tile.
• Add amylase to the starch solution and mix.
• Every minute, add a drop of the mixture to a new iodine well until the iodine stops turning blue-black.
• Record the time taken; repeat at different temperatures.
• The quicker the reaction, the faster the enzyme is working.

In the amylase temperature investigation, what indicates that all starch has been digested?

• The iodine solution turns blue-black
• The iodine solution remains orange-brown
• The mixture becomes cloudy
• The mixture changes colour to green

Investigating the Effect of pH on Amylase

• Place drops of iodine solution in rows on a tile.
• Label a test tube with the pH to be tested.
• Add $2 cm^{3}$ of amylase and $1 cm^{3}$ of buffer solution.
• Add $2 cm^{3}$ of starch solution, start stopwatch, and mix.
• Every 10 seconds, place a drop of the mixture on a new iodine drop until it remains orange-brown.
• Repeat at different pH values; less time indicates better enzyme activity at that pH.

Which of these are true about enzyme investigations?

• Iodine is used to test for the presence of starch.
• Buffer solutions are used to maintain a constant pH.
• The rate of reaction is measured by the time taken for the iodine to turn blue-black.
• Temperature and pH are both variables that can affect enzyme activity.

Enzyme Action & Specificity (Extended)

• Enzymes are specific to one substrate because the active site has a complementary shape.
• The lock and key hypothesis describes how the substrate fits into the active site.
• The enzyme-substrate complex forms, then products are released.
• The enzyme is unchanged and can catalyse further reactions.

The shape of the active site is complementary to the ____. This is known as the ____ hypothesis. After the reaction, the ____ leave the active site.

Enzymes & Temperature (Extended)

• Enzymes are proteins with a specific shape held by bonds.
• The active site shape is crucial for substrate binding.
• Optimum temperature for human enzymes is 37°C.
• High temperatures denature enzymes irreversibly.
• Low temperatures slow reactions but do not denature.

What happens to an enzyme when it is heated above its optimum temperature?

• It works faster
• It becomes denatured and loses its shape
• It changes pH
• It becomes more specific

Enzymes & pH (Extended)

• Optimum pH varies: most enzymes pH 7, stomach enzymes pH 2, duodenum enzymes pH 8-9.
• Extreme pH can denature enzymes by destroying bonds.
• Denaturation changes the active site shape, reducing or stopping activity.

The pH of an enzyme-controlled reaction is decreased by 2. How does this affect the rate?

• It has no effect
• It always decreases the rate
• It always increases the rate
• It may increase or decrease the rate

Summary: Key Points

• Enzymes are protein catalysts that speed up reactions without being used up.
• They are specific due to complementary active site shape (lock and key).
• Temperature and pH affect activity; extremes cause denaturation.
• Investigations use iodine to detect starch digestion by amylase.