AS Biology OCR (Enzymes)

Enzymes

Globular proteins, with a specific tertiary structure, which catalyse metabolic reactions in living organisms.

Catalyst

A molecule that speeds up a chemical reaction without being used up itself.

Substrate

A substance that is used up in an enzyme controlled reaction, leading to the formation of a product.

Substrate complex

Intermediate structure formed when a substrate molecule binds to an enzyme's active site.

Product complex

Intermediate structure in which product molecules are bound to an enzyme's active site.

Primary Structure

The sequence of amino acids found in a protein molecule.

Tertiary Structure

The 3D shape of a protein molecule held together by hydrogen bonds, disulphide bridges, ionic bonds and hydrophobic interactions.

Active site

An area on an enzyme molecule which the substrate binds.

Allosteric site

Another part on a enzyme where a substrate could bind, away from the active site.

Carbohydrase

Enzyme that catalyses the hydrolysis of glycosidic bonds.

Lipase

Enzyme that catalyses the hydrolysis of ester bonds.

Protease

Enzyme that catalyses the hydrolysis of peptide bonds.

Activation Energy

The level of energy required to enable a reaction to take place.

Lock and Key

The theory in which the enzyme active site is complementary to the substrate molecule.

Induced Fit

The theory in which the enzyme active site changes shape to fit the substrate more closely as it binds to it.

Denature

When an irreversible change has been made to the tertiary structure of a protein molecule, leading to loss of function.

Optimum

Condition that gives the fastest rate of reaction in enzyme controlled reactions.

Metabolism

The chemical processes that occur within a living organism in order to maintain life.

How does the role of enzymes affect metabolism at a cellular level?

Allows products in a metabolic pathway to be formed quicker.

Intracellular enzymes

1. Remain within cell walls

2. Reduce the energy used inside the cell for the reaction.

Extracellular enzymes

1. Work outside the cell

2. This makes it easier for the cell to bring it into the cell for further processing.

What is the role of catalase in catalysing intracellular reactions?

This ensures hydrogen peroxide is broken down to harmless water and oxygen.

What is the role of amylase and trypsin in catalysing extracellular reactions?

1. Amylase breaks down starches (carbohydrates) into sugars.

2. Trypsin breaks down proteins into amino acids.

Specificity

The active site of an enzyme is a specific shape, depending on the reaction that it catalyses, meaning that other molecules won't fit into the active site.

Lock and key hypothesis

The theory of enzyme action in which the enzyme active site is complementary to the substrate molecule, like a lock and key.

Induced fit hypothesis

The theory of enzyme action in which the enzyme molecule changes shape to fit the substrate molecule more closely as it binds to it.

Enzyme substrate complex

The intermediary formed when a substrate molecule binds to an enzyme molecule.

Enzyme product complex

The intermediate structure in which product molecules are bound to an enzyme molecule lowering.

Lowering the activation energy

Enzymes reduce the activation enthalpy so the reaction can proceed at a much lower temperature

What are the effects of pH on enzyme activity?

1. Low pH means theres lots of H+ ions

2. Either extreme of H+ ion concentration can interfere with the hydrogen and ionic bonds holding the tertiary structure together.

3. The pH affects the charge of the amino acids at the active site, so the properties of the active site change and the substrate can no longer bind.

What are the effects of temperature on enzyme activity?

1. Up to a certain point, increasing temperature will increase the rate of reaction.

2. There will be more collisions between enzymes and the substrate.

3.Heat puts strain on the inter-molecular bonds, and some of the weaker bonds (H bond and ionic bonds) may break.

3. Many bonds hold together the tertiary structure and as these bonds break, the structure would disintegrates.

4. Denatures at its optimum temperature.

What is Q10?

The temperature coefficient (Q10) of a reaction is a measure of how much the rate of reaction increases with a 10 degree increase in temperature. However, this no longer applies when the enzymes have denatured.

What are the effects of enzyme concentration on enzyme activity?

1. As enzyme concentration increases, the rate of reaction increases as there are more active sites available

2. This carries on until the substrate concentration becomes a limiting factor and the rate stops increasing.

What are the effects of substrate concentration on enzyme activity?

1. As the substrate concentration rises, the rate of reaction rises because there are more substrate molecules to react.

2. At higher concentrations, all of the active sites become filled, so the rate of reaction remains the same.

Give an example on how you could measure the rate of an enzyme controlled reaction.

Measure how fast oxygen is given off when catalase is used to catalyse the break down of hydrogen peroxide into water and oxygen.

Coenzymes

Small, organic, non-protein molecules that bind for a short period of time to the active site.

Cofactors

Ions that increase the rate of enzyme-controlled reactions.

Prosthetic group

A (co-factor) non-protein group combined with a protein.

Why are co-enzymes needed in some enzyme controlled reactions?

They carry chemical groups between enzymes so they link together enzyme-controlled reactions that need to take place in sequence.

Why are cofactors needed in some enzyme controlled reactions?

They help enzymes and substrates to bind together which helps to speed up the rate of enzyme controlled reactions.

Why are prosthetic group needed in some enzyme controlled reactions?

They are cofactors that bind tightly to proteins or enzymes and help to catalyse some enzyme controlled reactions.

Zn2+

A prosthetic group for carbonic anhydrase.

Cl-

Cofactor for amylase.

Vitaimins

A source of co-enzymes.

What is the effect of a competitive inhibitor on the rate of enzyme controlled reactions?

1. Have a similar shape to that of the substrate molecule so can occupy the active site, forming enzyme-inhibitor complexes.

2. As the number of substrate molecules is increased, the level of inhibition decreases because a substrate molecule is more likely than an inhibitor molecule to collide with the active site.

3.Most temporarily bind to the active site.

4. Reversible reaction

What is the effect of a non competitive inhibitor on the rate of enzyme controlled reactions?

1. Attach to the enzyme in a region away from the active site (known as the allosteric site).

2. This distorts the tertiary structure of the enzyme molecule, leading to the shape of the active site changing. 3. Substrate can no longer fits into the active site so the enzyme-substrate complexes cannot form and the reaction rate decreases.

4. Most bind permanently to the enzyme molecule.

5. Irreversible reaction

Reversible inhibitors

When the bonds are weak H or ionic bonds and the inhibitor can be removed.

Irreversible inhibitors

When the bonds are strong and covalent and the inhibitor cant be removed easily.

Metabolic pathway

1. A series of connected metabolic reactions (the product of the first reaction takes part in the second reaction)

2. Each reaction is catalysed by a different enzyme.

Product inhibiton

Where enzymes are inhibited by the product of the reaction they catalyse.

End product inhibiton

When the final product in a metabolic pathway inhibitis an enzyme that acts earlier on in the path way.

What is the importance of end product inhibition?

Controls the amount of end product that gets made. Eg. 1. Phosphofructokinase is involved in breaking down glucose to make ATP.

2. ATP inhibits the action of phosphofructokinase.

3. Therefore, a high level of ATP stops more ATP from being made.

How does enzyme inhibition help to protect cells?

1. Enzymes can be synthesised as inactive precursors in a metabolic pathway.

2. This prevents damage to cells.

3. Part of the precursor molecule inhibits its action as an enzyme.

4. Once this part is removed (by a chemical reaction) the enzyme becomes active.