Click4Biology: 3.6: Enzymes

3.6.1 Define enzyme and active site.(1)

Define means to give the precise meaning of a word, phrase or physical quantity.

• globular proteins

• catalysts which speed up biological reactions

• unchanged by the reaction

• specific to their substrate

• active site is the position on the enzyme occupied by the substrate

• affected by temperature and pH

 

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3.6.2 Explain enzyme–substrate specificity. (3)

Explain means to give a detailed account of causes, reasons or mechanisms.

 

a) Large globular protein enzyme

b) Active Site where the substrate combines to the enzyme

c)Substrate which fits the active site

d) Activated complex. The substrate is weakened to allow the reaction.

e)Unchanged enzyme/ re-used at low concentrations

f) Product of the reaction

 

 

 

other keypoints from the hypothesis:

• The active site is often composed of open loops of polar amino acids on the exterior of the enzyme molecule.

• Enzyme specificity is due to the complementary shape of the active site and the substrate.

• Enzymes work at low concentrations because they are unaffected by the reaction and can return for more substrate.

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3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.(3)

Explain means to give a detailed account of causes, reasons or mechanisms.

Effect of temperature on the rate of an enzyme catalysed reaction:

(a) As the temperature increases enzyme stability decreases. The kinetic energy of the enzyme atoms increases causing vibrations in the enzyme molecule that lead to the hydrogen bonds to breaking, shape changes in the active site.

(b) As the temperature increases the kinetic energy of the substrate and enzyme molecules also increases. Therefore more collisions of the substrate with the active site and the formation of activated complex's and product. The rate of reaction is increasing.

(c) The optimal temperature (X) is the highest rate of reaction. Compromise between decreasing enzyme stability and kinetic energy of the reactants.

(d) Higher temperature increases the kinetic energy of the enzyme atoms so much that they break bonds, change shape of the active site.

 

 

 

 

 

The main diagram is often simplified to this diagram which still shows the three key stages in the reaction.

 

 

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The effect of pH on the rate of an enzyme catalysed reaction:

 

• pH also affects the rate of reaction of an enzyme catalysed reaction.

• At the optimal pH (a) or (b) the maximum rate of reaction is achieved.

• Above or below the optimal pH the rate decreases.

• The change in rate is because bonds are made and broken which change the shape of the active site and therefore decrease the rate of reaction.

• The two enzyme shown in the image illustrate the fact that different enzymes can have very different optimal pH.

• e.g. Blue curve = pepsin (a)= pH3, Red curve =salivary amylase (b)= pH 7.2

 

 

 

Effect of substrate concentration on the rate of an enzyme catalysed reaction:

(a) As the substate concentration is increased the rate of reaction increases.

There are more collisions between the substrate and the enzyme such that more activated complex's are formed and therefore product per unit time.

(b) Further increases in substrate also increase the rate but proportionately less than previously.

The number of occupied active site is increasing and there is competition for the active site.

(c) The rate is constant.

The enzyme active site is fully saturated with substrate such that adding more substrate does not increase the rate of reaction. The enzymes molecules are fully occupied converting substrate to product and any substrate must await a free active site before conversion to product.

 

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3.6.4 Define denaturation.(1)

Define means to give the precise meaning of a word, phrase or physical quantity.

Denaturation is a structural change in a protein that results in the loss (usually permanent) of its biological properties.

Temperature:(see section 3.6.3)

• Temperature rises cause the average kinetic energy of the enzyme atoms to increase.

• This vibration breaks the weakest bonds first, which in the enzyme are the hydrogen bonds.

• The breaking of bonds, changes the shape of the enzyme.

• Change the shape of the enzyme changes the shape of the active site.

• Change the shape of the active site prevents substrate from entering.

• The rate of reaction reduces or stops.

pH: (see section 3.6.3)

• At pH lower than the optimal pH the concentration of H⁺ in the solution will be higher than normal.

• The hydrogens will tend to be attracted to electronegative regions of the enzyme protein.

• Bonds are formed or changed as a consequence of the additional H⁺ which changes the shape of the enzyme molecule.

• Changes in shape, change the active site shape.

• Changes in active site shape reduces the ability of the substrate to bind with the active site.

• This reduces the rate of reaction that changes substrate to product.

• The rate of reaction reduces.

• For pH values above the optimum breaks bonds in the same way and have the same reductions in the rate of reaction

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3.6.5 Explain the use of lactase in the production of lactose-free milk.(3)

Explain means to give a detailed account of causes, reasons or mechanisms.

• Lactose is a disaccharide (glucose + Galactose) milk sugar

• Around 90% of all humans show some kind of lactose intolerance.

• People who are lactose intolerant can drink milk if it is lactose free.

• Lactase is an enzyme extracted from yeast that can digest the milk sugar to glucose and galactose.

Enzyme Immobolisation:

It is possible to make the process more efficient by immobilising the lactose on a recoverable surface such as alginate.

 

• First the Lactase is immobilised in alginate beads.

• Next the beads are placed in a container over which milk can be passed.

• The milk is collected and re-circulated (pump) to convert any remaining lactose to glucose and galactose.

• The circulation is maintained until all lactose has been converted.

• This model of an industrial process allow the lactase to be recovered and re-used (cheaper).

• Efficient conversion of lactose to glucose and galactose.

• High % lactose conversion is achieved.

• All these factors reduce cost particularly on the downstream processing and purification.

 

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