Topic 2.3: Enzymes

Topic 2.3: Enzymes 2.3.1 Enzymes are: 

globular proteins

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catalysts which speed up biological reactions

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unchanged by the reaction

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specific to their substrate

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affected by temperature and pH

2.3.2 Lock & Key Hypothesis 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:

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

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

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

2.2.3 The effects of temperature, pH and substrate concentration on enzyme activity( rate of reaction). The effect of temperature

(a)

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Increase Kinetic energy of substrate and enzyme.

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Increased chance of collision and reaction, therefore rate increases

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low temperatures has a low rate of reaction

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optimum temperature = maximum rate of reaction

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balance between enzyme stability and kinetic energy of reactants

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rapid decrease in the rate of reaction

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high temperatures destabilise the enzyme molecule

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enzyme is denatured

(b)

(c)

The effect of pH

(a) 

Decrease in pH (increase in H+)

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H+interact with exposed R groups on active site.

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Enzyme active site changes shape

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Specificity reduced

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Decrease in rate of reaction

(b) 

Optimum rate of reaction for the pH= (d).

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Active site structure and structure specific to the complementary shape of the substrate.

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Successful activated complex and therefore reactions occur.

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Increase in pH

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Decrease in H+ or increase in base concentration

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Enzyme active site changes shape

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Specificity reduced

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Decrease in rate of reaction

(c)

The effect of substrate concentration

(a) 

Increase conc of substrate molecules

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Increased chance of collision with enzyme

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Greater chance of forming activated complex

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Increase in rate of reaction

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Rate begins to level

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Active sites beginning to become saturated with substrate (fully occupied)

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New substrate must wait for previous reaction to complete and the product to exit the active site

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Full saturation of the active sites by substrate

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Rate becomes constant for further increases in substrate concentration.

(b)

(c)

2.3.4 Definition of denaturation ' a structural change in a protein that results in a loss (usually permanent) of its biological properties. '

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Enzymes are globular proteins

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Enzymes have tertiary structure

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Tertiary structure is maintained by hydrogen, ionic and covalent bonds

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Shape of the active site is maintained by hydrogen, ionic and covalent bonds

The bonds within enzymes (and proteins) has an increasing strength of:

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Hydrogen

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Ionic

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Covalent

Denaturation of an enzyme by temperature •

As the temperature increases the stability of the enzyme remains constant. The weakest hydrogen bonds will however break at higher temperatures.

•

The kinetic energy of both the enzyme and substrate increase. Therefore more activated complex's form and more product is formed. The rate increases.

BUT

•

(The KE of the enzymes constituent atoms has increased and the weakest bonds (hydrogen bonds)break. The shape of the active site is lost. There is a rapid loss of activity as the enzyme is denatured

Denaturation of an enzyme by a change in pH

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Enzymes have an optimum pH at which they achieve their maximum rate of reaction or Vmax

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Pepsin has an optimal pH of (a) and Vmax =(b)

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Amylase has an optimal pH of (c) and Vmax =(d)

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pH affects the charge of the amino acids of the active site

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This changes the properties of the active site

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e.g. carboxyl R group will be uncharged COOH at low pH but COO- at high pH.

2.3.5 Commercial applications of enzymes in biotechnology Biotechnology is the use of micro-organisms or parts of organism to produce a commercial product. In particular the use of enzymes can reduce production costs to a commercially viable level. Biotechnology has ancient origins in the production of fermentation products. There are however an increasing number of modern applications in industry. The follow is an outline of two such applications. Pectinase in fruit juice production

Proteases in biological washing powders

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Pectinase is naturally produced by fruit in the ripening process.

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Proteases hydrolyse polypeptide to amino acids.

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Pectinase hydrolyse the pectins(polysaccharides) in fruit cell walls. This softens the fruit.

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Washing powders contain detergents that can remove lipid stains.

Pectinases are also produced by fungi Aspergillus niger.

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Stains on clothing also include protein stains from food.

Fruit juice manufacturers want their fruit softened as much as possible so that maximum juice extraction can be achieved.

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The protease enzyme used in washing powders is selected to have a high optimal pH. (alkaline). These are the condition under which detergents work.

Pectinases from the fungi are added to increase juice production.

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Some of these same proteases are also thermostable for high washing temperatures. These proteases have been isolated from thermophilic bacteria.

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Equally they can have optimal temperatures that are lower than normal. This is for conditions in which warm water is not ordinarily used in washing clothes e.g. Thailand