Enzymes

Life processes at the cellular level

What are enzymes?

Enzymes are biological catalysts that speed up reactions by lowering activation energy. They are proteins made of amino acids that fold into a specific 3D shape, including a substrate-specific active site.

Why are enzymes specific?

The enzyme's active site has a specific shape that is complementary to the substrate. Only substrates with the correct shape can bind.

Are enzymes used up?

No. The enzyme is unchanged after the reaction, so it can be reused to catalyse another reaction.

How does an enzyme catalyse a reaction?

1. Substrate enters the active site.

2. An enzyme-substrate complex forms.

3. The active site may change shape slightly (induced fit).

4. The enzyme helps convert substrate → products by lowering activation energy.

5. Products leave the active site.

6. The enzyme remains unchanged and can be reused.

Why are enzymes important to organisms?

They allow biological reactions to occur fast enough to maintain metabolism at normal cellular temperatures.

What is the link between enzymes and cellular respiration?

Enzymes are required at each step of cellular respiration. They help break down glucose and produce ATP, which provides energy for cellular processes.

What happens if enzyme activity decreases?

Decreased enzyme activity → slower cellular respiration → less ATP produced → less energy available for metabolism, active transport, growth and movement → reduced cell/organism function and potentially survival.

What happens to enzyme activity at low temperatures?

Molecules have less kinetic energy, so they move more slowly. This causes fewer successful collisions between enzymes and substrates, so fewer enzyme-substrate complexes form and the reaction slows down.

Does low temperature denature an enzyme?

No. Low temperature reduces enzyme activity but generally does not permanently change the enzyme's shape. When temperature increases, activity can increase again.

What happens as temperature increases?

Molecules gain kinetic energy, causing more frequent and successful collisions between enzymes and substrates. Enzyme activity therefore increases until the optimum temperature.

What is the optimum temperature?

The temperature at which an enzyme works at its highest efficiency, producing the fastest reaction rate.

What happens when temperature goes above the optimum?

High temperatures can denature the enzyme, permanently changing its 3D structure and active site shape.

How does denaturation affect enzyme function?

High temperature disrupts bonds holding the enzyme's shape together. The active site changes shape, so the substrate can no longer fit → fewer/no enzyme-substrate complexes → reaction slows or stops.

How does high temperature affect cellular respiration?

High temperature → enzyme denaturation → active site changes shape → substrate cannot bind → enzyme-controlled respiration reactions slow/stop → less ATP produced → less energy for cellular processes → organism may be seriously affected or die.

What is optimum pH?

The pH at which an enzyme's active site has the best shape for binding its substrate, allowing the enzyme to work most efficiently.

What happens when pH moves too far from the optimum?

The enzyme can become denatured. Changes in pH can disrupt bonds in the enzyme's protein structure, changing the active site shape.

How does extreme pH affect enzyme activity?

Extreme pH → bonds disrupted → enzyme shape changes → active site changes shape → substrate cannot bind effectively → fewer enzyme-substrate complexes → reaction slows/stops.

How can extreme pH affect cellular respiration?

Extreme pH → enzyme denaturation → active site changes shape → substrate cannot bind → respiration reactions slow/stop → less ATP produced → less energy available for metabolism, active transport, growth and movement → survival negatively affected.

What is a cofactor?

A molecule, mineral or metal ion that binds to an enzyme and helps it achieve the correct shape so its active site can bind the substrate effectively.

How do cofactors affect enzyme activity?

Cofactors can help the enzyme's active site bind the substrate correctly and can help lower activation energy, increasing enzyme activity.

What happens if an enzyme doesn't have its required cofactor?

The enzyme may not have the correct shape, meaning the substrate cannot bind effectively and the enzyme cannot catalyse the reaction properly.

What is an enzyme inhibitor?

A substance that reduces or stops enzyme activity by interfering with the enzyme or its active site.

What is a competitive inhibitor?

A competitive inhibitor binds to the active site, preventing the substrate from binding.

What is a non-competitive inhibitor?

It binds to a different part of the enzyme, changing the enzyme's shape and therefore changing the shape of the active site.

How can heavy metals such as mercury and cadmium affect enzymes?

They can bind to enzymes and block or alter the active site, preventing substrates from attaching. This can prevent product formation and may be irreversible.

How does enzyme action affect cellular respiration and survival?

Enzymes control the rate of cellular respiration because they are required at each step of the pathway.

At optimum temperature and pH, with enough substrate, oxygen and cofactors, the active site fits the substrate and reactions occur quickly → more respiration → more ATP.

If temperature is too low, molecules have less kinetic energy → fewer successful collisions → fewer enzyme-substrate complexes → respiration slows → less ATP.

If temperature or pH becomes too extreme, enzymes denature → active site changes shape → substrate cannot bind → respiration slows/stops → less ATP.

If inhibitors such as cadmium or mercury block/change the active site → substrate cannot bind → products aren't formed efficiently → respiration decreases → less ATP.

Less ATP means less energy for metabolism, active transport, growth, movement and other cellular processes, which can negatively affect survival.