2.5 - Enzymes

Question 1

Active sight

Answer 1

where substrate binds – reaction is catalysed

Question 2

Product

Answer 2

compound produced by reaction

Question 3

Substrate

Answer 3

compound which binds to active sight of enzyme

Question 4

Enzyme substrate complex:

Answer 4

enzyme + substrate

Question 5

Cofactor

Answer 5

binds to enzyme at allosteric sight, activates enzyme

Question 6

Allosteric sight:

Answer 6

not the active site

Question 7

Competitive inhibitor

Answer 7

binds to enzyme at active sight, stops substrate from binding

Question 8

Non-competitive inhibitor

Answer 8

binds to enzyme at allosteric site, results in change to active sight stopping enzyme substrate complex from forming

Question 9

Lock and key fit:

Answer 9

enzyme shape exactly matches substrate at active sight

Question 10

Induced fit

Answer 10

enzyme substrate complex induces change in the enzyme shape

Question 11

Points about enzymes

Answer 11

• The active site of a particular enzyme is specific for its matching substrate
• Whilst an enzyme catalyses a reaction is not used up in the reaction, is available to catalyse subsequent reactions
• Successful reactions only occur if the substrate + the active site of the enzyme are correctly aligned and they collide with sufficient kinetic energy
• In biological systems enzyme catalysed reactions occur in a liquid environment. Collisions between substrate + enzymes are random
• Enzymes are proteins – their active site is determined by the R-groups of individual amino acids and the tertiary structure

Question 12

Factors affecting enzyme activity

Answer 12

Temperature
PH
Substrate concentration

Question 13

Temperature as a factor affecting enzyme activity

Answer 13

increasing temperature increases the number of possible collisions between substrate and active site. This results in a faster rate of reaction
At high temperatures enzymes are denatured as the increased kinetic energy breaks down the bonds maintaining the structure of the enzyme. The active site is changed and the structure can no longer bind to the active site

Question 14

PH as a factor affecting enzyme activity

Answer 14

enzymes will have a range of PH environments in which they can still function
Changing the PH away from the optimum will decrease the rate of reaction
Changing the PH will alter the change of the enzyme which will then change its shape, solubility and its conformation shape of the active site

Question 15

Substrate concentration as a factor affecting enzyme activity

Answer 15

increasing substrate concentration increases the rate of reaction as particles interact more frequently with enzymes
At the optimum concentration of substrate molecules all active sites are occupied and working at maximum efficiency
Any increase in substrate concentration beyond the optimum will have no increasing effect as there are more substrates than free active sites to be utilised

Question 16

Enzymes have an active site to which specific substrates bind

Answer 16

• Enzymes are globular proteins that work as catalysts by speeding up chemical reactions and not being altered themselves
• The substrates that enzymes convert into products in these reactions are called substrates
• Example equation = substrate (enzyme) product
• Enzymes are found in all living cells
• Many different enzymes are needed, as enzymes only catalyse one biochemical reaction and thousands of reactions take place in cells. This is known as enzyme-substrate specificity
• The mechanism by which enzymes speed up reactions involves a substrate(s) binding to an active site.
• The shape and chemical properties of the active site and the substrate match each other, this allows the substrate to bind but not other substances
• Substrates are converted into products while they are bound to an active site and the products are then released, freeing the active site to catalyse another reaction.

Question 17

Enzyme catalysis involves molecular motion and the collision of substrates with the active site

Answer 17

• There are three stages to enzyme activity as a catalysis of a reaction by an enzyme
• Stage 1: the substrate binds to the active site of the enzyme, some enzymes have to substrates that bind to different parts of the active site.
• Stage 2: while the substrates are bound to the active site they change into different chemical substances, which are the products of the reaction
• Stage 3: the products separate from the active site, leaving it vacant for substrates to bind again
• A substrate can only bind to an active site if it moves very close to it
• The coming together of a substrate molecule and an active site is known as a collision
• With most reactions the substrates are dissolved in water around the enzyme, due to the liquid state all particles are in contact and are in continual motion
• Through this movement collisions between substrates and active sites occur.
• The substrate may be at any angle to the active site when the collision occurs, successful collisions are ones in which the substrate and active site are correctly aligned to allow binding to take place

Question 18

Temperature, substrate concentration and pH affect the rate of activity of enzymes

Answer 18

• Temperature: In liquids particles are in continual random motion and when liquid is heated the particles move with more kinetic energy, therefore, enzymes and substrate molecules are moving faster and the chance of a collision is higher. Enzyme activity therefore increases
• Temperature: When enzymes are heated bonds in the enzyme vibrate more the chance of bonds breaking is increased, when bonds in the enzyme break the structure of the enzyme changes including the active site, this is permanent and is called denaturation. Denatured enzymes cannot catalyse reactions, as more and more enzymes become denatured enzyme activity decreases eventually stopping.
• pH: Low pH means high hydrogen ion concentration; high pH means low hydrogen ion concentration. Most enzymes have an optimum pH at which their activity is highest. If pH is increased or decreased from the optimum enzyme activity decreases and eventually stops. When the hydrogen ion concentration is altered relative to the level at which the enzyme works, the enzyme structure is changed, including the active site.
• Substrate concentration: If the concentration of substrates is increased, substrate-active site collisions will take place more frequently and the rate at which enzyme catalyses its reaction increases. However, as substrate concentration increases, substrate-active site collisions increase meaning that more active sites are occupied meaning more and more substrates are blocked from colliding. For this reason, the increases in the rate at which enzymes catalyse reactions gets smaller and smaller as substrate concentration rises.

Question 19

Enzymes can be denatured

Answer 19

• Low temperatures, high temperatures or changes in pH level can denature enzymes.
• Denatured enzymes have had their structure irreversibly changed including their active site
• Due to the alteration of the active site substrates can no longer bind to the enzyme
• In many cases denaturation causes enzymes that were dissolved in water to become insoluble and form a precipitate.

Question 20

Immobilised enzymes are widely used in industry

Answer 20

• An immobilised enzyme is an enzyme attached to another material or into an aggregation, so that the movement of an enzyme is restricted
• Enzyme immobilisation has advantages
• The enzyme can easily be separated from the products of the reaction, stopping the reaction at the ideal time and preventing contamination of the products
• After being retrieved from the reaction mixture the enzymes may be recycled, giving useful cost savings, especially as many enzymes are very expensive
• Increases the stability of enzymes to change in temperature and pH, reducing the rate at which they are degraded and have been replaced
• Substrates can be exposed to higher enzyme concentrations than with dissolved enzymes, speeding up reaction rates

Question 21

How is lactose free milk produced

Answer 21

Lactase is obtained from Kluveromyces lactis, a type of yeast that grows naturally in milk. Biotechnology companies culture the yeast and purify it for sale to food manufacturing companies

Question 22

Advantages of lactose free milk

Answer 22

• Some people are lactose intolerant and cannot drink more than 250mL of milk per day, unless it is lactose free milk
• Galactose and glucose are sweeter than lactose, so less sugar needs to be added to sweet foods containing milk, such as milk shakes or fruit yoghurt
• Lactose tends to crystallise during the production of ice cream, giving a gritty texture. Because glucose and galactose are more soluble than lactose they remain dissolved, giving a smoother texture
• Bacteria ferment glucose and galactose more quickly than lactose, so the production of yoghurt and cottage cheese is faster