Module 2: Enzymes Flashcards
Enzymes introduction
Enzymes are globular proteins with specific tertiary structures.
The tertiary structure of the enzyme is crucial for creating the perfect shaped active site for the specific substrate molecule. This is where the reaction takes place.
Only a few amino acids contribute to the active site.
Extracellular and Intracellular enzymes
Extracellular enzymes:
- Catalyse outside cells
- Most digestive enzymes
Intracellular enzymes:
- Catalyse inside cells
- Catalyse within cytoplasm/ membranes
What is activation energy?
Activation energy is the amount of energy needed for a reaction to take place.
Enzymes reduce activation energy required by providing a different route for the reaction.
Metabolic reactions couldn’t happen fast enough without enzymes which allow reactions like the hydrolysis of maltose to happen quickly at lower temperatures.
Lock and key hypothesis
- Enzyme’s active site is complementary in shape to the substrate molecule.
- Substrate (key) can fit in an enzyme’s active site (lock).
Induced fit hypothesis
- As the substrate binds to the active site, the enzyme changes shape slightly.
- The active site is tighter around the substrate molecules.
- Oppositely charged groups on the substrate and active site interact and hold the substrate molecule in place. This is the enzyme substrate complex (ESC).
- The enzyme’s shape change puts strain on the bonds in the substrate which destabilises it, causing the reaction to occur more easily.
- The product(s) is formed (enzyme product complex) and because it is a different shape to the reactant it is released from the active site.
Cofactors
- Some enzymes can only work if another small non‐protein molecule is attached to them. These are called cofactors.
- A cofactor that is permanently bound, by covalent bonds, to an enzyme is called a Prosthetic group.
- eg‐ the enzyme carbonic anhydrase contains a zinc ion permanently bound as a prosthetic group.
- Some cofactors act as co‐substrates . They and the substrate together form the correct shape to bind to the active site of the enzyme.
- Some cofactors change the charge distribution on the surface of the substrate or in an active site and make the temporary bonds in the enzyme‐substrate complex easier to form.
- Coenzymes are small organic non‐protein molecules that bind temporarily to the active site of the enzyme molecule. The coenzyme is chemically changed during the reaction, and they need to be recycled to their original state, sometimes by a different enzyme.
Explaining the effect of temperature on enzyme activity
Up to and at optimum:
- As molecules are heated they gain kinetic energy and move around faster - this results in more frequent collisions.
- This results in more enzyme substrate complexes and therefore a higher (max) rate of reaction and more product formed.
Above optimum:
- Molecules have more kinetic energy.
- Enzymes vibrate too much and weaker bonds are broken (ionic and H).
- The tertiary structure of the enzymes are changed.
- This means the active site loses complementary shape.
- No ESCs can form as substrate doesn’t fit into active site.
- The enzymes have denatured.
- This is irreversible so reaction stops.
What is optimum temperature?
The optimum temperature is the temperature that gives the maximum rate of reaction – it is the balance between maximum collisions and the vibration of the enzyme molecules.
Explaining the effect of PH on enzyme activity
pH is a measure of H+ ions. More H+ ions = more acidic.
Large numbers of H+ ions will attract negatively charged parts of the enzyme and repel positively charged parts - this interferes with the hydrogen bonds and ionic bonds giving the enzyme its tertiary structure and therefore
specific active site.
Model answer - not at optimum:
- Change in pH or H+ ions alters distribution of charge on the enzyme molecule.
- This causes the hydrogen and ionic bonds to be disrupted.
- This means the enzyme loses its tertiary structure.
- This changes the shape of the active site of the enzyme.
- Substrates are no longer attracted to the active site because the H+ ions have altered its charge.
- Substrates can’t bind to the active site as it is no longer complementary.
- No ESCs can form = no product = no reaction
Enzymes are denatured at extremes of pH (for that enzyme).
Different enzymes have different optimum pHs. At their optimum, the conc of H+ ions gives the tertiary structure the best shape = most complementary active site.
Explaining the effect of concentration on enzyme activity
- no enzyme = no enzyme-substrate-complexes (ESCs)= no reaction.
- more enzymes = more active sites.
- more ESCs form so more product = higher rate of reaction.
- as long as the substrate is in excess, the rate will continue to rise with and increase in enzyme conc.
- after a point all substrate molecules are occupying active sites and a maximum rate will be reached.
- increasing the enzyme conc further will have no more effect on rate.
- At the point where increasing enzyme conc has no more effect on the rate, the substrate concentration is a limiting factor.
- When substrate conc becomes limiting, the rate will decrease as the substrate is used up.
- No substrate = no ESCs = no reaction.
- more substrate = more frequent collisions between enzyme and substrate = more ESCs = more product = higher rate.
- After a point, all active sites will be occupied - it is not possible for more ESCs to form at any one time = no more product = increasing the substrate conc further has no further effect on the rate.
- At the point where increasing substrate conc has no more effect on the rate, the enzyme concentration is a limiting factor.
What is an enzyme inhibitor?
Enzyme inhibitors are molecules that slow down the rate of an enzyme controlled reaction by affecting the enzyme molecule.
Competitive inhibitor
• Competitive inhibitors have similar shapes to an enzymes’ substrate.
• Their shape is complementary to the active site so they can bind with it and block it, forming an enzyme‐inhibitor‐complex.
• This prevents enzyme‐substrate complexes from forming and slows down the rate of reaction ‐ no products can be formed.
Most competitive inhibitors do not bind permanently to the active site ‐ they bind for a while and then leave. Their action is reversible.
Concentration of substrate and competitive inhibitors
- Rate of reaction depends on conc of substrate and competitive inhibitors.
- More substrate = less inhibition = higher rate of reaction because the substrate molecules are more likely to collide with an active site than an inhibitor.
Non-competitive inhibitors
- Non‐competitive inhibitors fit into the allosteric site on an enzyme.
- This alters the tertiary structure of the enzyme and changes the shape of the active site.
- This means the substrate can no longer fit into the active site, so no enzyme substrate complexes can form ‐ rate of reaction decreases.
- Non‐competitive inhibitors bind permanently to the enzymes ‐ their effect is irreversible ‐ the enzymes become useless.
Concentration of substrate and non-competitive inhibitors
- Increasing substrate conc has no effect on the rate with non‐competitive inhibitors because they bind irreversibly ‐ if all enzymes have a non competitive inhibitor bound, the reaction will stop.
- Changing the conc of inhibitors will have an effect.
