3.6 & 7.6 Enzymes

16/10/2012 § 1 Comment

Okay, kids, it’s time to talk about enzymes.

Enzymes are globular proteins that act as catalysts of chemical reactions – they speed up the rate of chemical reactions without changing. Cells can make some enzymes and not others to control the chemical reactions occurring in their cytoplasm. Enzymes take substances and change them, making two products. These substances are called substrates and the process of a general enzyme-catalyzed reaction is as follows:

substrate –––> (enzyme!) –––> product

There are thousands of different kinds of cells purely because there are thousands of of different reactions that need to be catalyzed – and usually a single enzyme is specialized to only do one or few of those reactions. This is called enzyme-substrate specificity. The substrates bind themselves to a special region on the surface of the enzyme called the active site. The shape of the active site and substrate fit together; they match like a lock and key model, which is useful to explain the substrate specificity of enzymes.

Energy changes during chemical reactions

Now we know: chemical reactions convert reactants (substrates) into products. Before a molecule of a reactant can go through its conversion though, it needs energy – activation energy. As the substrate binds, the conformation of the protein will alter and the shape complements that of the reactant. The bonds of the substrate weaken and convert it into products, which then unbind (or dissociate) from the active site and then the enzyme returns to its original shape.

Factors affecting enzyme activity

Three factors that affect enzyme activity are pH, temperature, and the substrate concentration. Some factors affect the enzyme by denaturation or by damaging or altering the enzyme irreversibly.

pH: The pH of a solution measures its acidity (or alkalinity). A low pH indicates a higher acidity (or a low alkalinity). Acidity is caused by a presence of hydrogen ions so a higher acidity means a higher content of hydrogen ions. Enzymes are particularly sensitive to pH and have an optimum pH at which their chemical reactions are highest – this optimum pH is usually 7 for all enzymes. When the pH shifts from that optimum level and either increases or decreases, the enzyme’s activity will decrease and eventually stop all catalysis (denaturation). The graph below illustrates this effect of pH on the rate of activity.

temperature: As enzymes heat up, the particles are given more kinetic energy and activity increases along with the temperature. Sometimes activity doubles with every 10°C increase. This is caused by an increase in collisions between substrates and the active sites due to faster motion (given by the increase kinetic energy). At very high temperatures, the bonds that make the enzyme will break because of the heat and the enzyme denatures. The graph below illustrates this effect of temperature on the rate of activity.

substrate concentration: This one’s pretty logical. If the concentration of the enzyme’s special substrate increases around the enzyme, then the rate at which activity occurs also increases because there are more substrates present for the enzyme to catalyze. However, there is a fixed number of enzymes and active sites for the substrates to bind to, so at a certain point, even if the substrate concentration increases, the active sites are still occupied at any given moment and are blocked. Therefore, chemical activity doesn’t increase. The graph below illustrates this effect of temperature on the rate of activity.

Milk and lactase and lactose intolerant peoples

Lactose is a sugar naturally present in milk. The enzyme lactase can convert it into glucose and galactose, like:

lactose –––> (lactase enzyme!!) –––> glucose + galactose

Biotechnology companies extract lactase from Kluveromyces lactis, a yeast that grows naturally in milk, for multiple reasons. These are:

  • people who lactose intolerant can’t drink more than 250ml of milk a day unless it is lactose-free or reduced
  • galactose and glucose are sweeter than lactose; less sugar needs to be added to sweet foods with milk (like milk shakes, fruit yoghurt)
  • used in ice cream because while lactose crystallizes and makes the ice cream gritty, lactase is soluble and makes the ice cream smoother
  • bacteria can ferment glucose and galactose quicker than lactose; production of goods like yoghurt and cheese is faster

Lactase is used in two different ways during food processing:

  1. can be added to the milk; the final product will hold the enzyme
  2. can be immobilized on a surface or in beads of porous material; milk flows past immobilized lactase and avoids contaminating the product with the lactase


Essay Questions

  1. Outline the thermal, cohesive, and solvent properties of water. (5 marks)
  2. Describe the significance of water to living organisms. (6 marks)
  3. Describe the use of carbohydrates and lipids for energy storage in animals. (5 marks)
  4. List three functions of lipids. (3 marks)
  5. Describe the significance of polar and non-polar amino acids. (5 marks)
  6. Outline the role of condensation and hydrolysis in the relationship between amino acids and dipeptides. (4 marks)
  7. Describe the structure of proteins. (9 marks)
  8. List four functions of proteins, giving an example of each. (4 marks)
  9. Distinguish between fibrous and globular proteins with reference to one example of each protein type. (6 marks)
  10. option i – Lactase is widely used in food processing. Explain three reasons for converting lactose to glucose and galactose during food processing (3 marks) // option ii – Simple laboratory experiments show that when the enzyme lactase is mixed with lactose, the initial rate of reaction is highest at 48°C. In food processing, lactase is used at a much lower temperature, often at 5°C. Suggest reasons for using lactase at relatively low temperatures. (2 marks)
  11. Outline how enzymes catalyze reactions. (7 marks)
  12. Explain the effect of pH on enzyme activity. (3 marks)
  13. Compare the induced fit model of enzyme activity with the lock and key model. (4 marks)
  14. Draw graphs to show the effect of enzymes on the activation energy of chemical reactions. (5 marks)
  15. Explain, using one named example, the effect of a competitive inhibitor on enzyme activity. (6 marks)

Look, we’ve got two more left! Also, I think that #14 may have been answered but I wasn’t sure so I left that out. A [few] questions I definitely need to look over is #8, #10, #11 and, hahaha, #3 and #4. Hey, lay off, it’s a learning process.



Page 81 biosynthesis of glycogen

1. Explain why two different enzymes are needed for the synthesis of glycogen from glucose phosphate.

Glycogen is made of glucose phosphate; it is a polysaccharide, and because of the multiple present molecules, multiple enzymes will be needed to synthesize these. One enzyme cannot do all the work for the entire polysaccharide.

2. The formation of side-branches increases the rate at which glucose phosphate molecules can be linked on to a growing glycogen molecule. Explain the reason for this.

The increasing presence of more side-branches increases the chances that glucose phosphate molecules can link themselves to the growing glycogen molecules. This is reminiscent of the effect of substrate concentration on enzyme activity.

3. Curve A was obtained using heat-treated enzymes. Explain the shape of curve A.

The conversions of glucose phosphate into glycogen remains very low, below 5% definitely, because the heat gave so much energy to the enzymes that the bonds broke and they denatured. 

4. Curve B was obtained using enzymes that had not been heat-treated.

a) Describe the shape of Curve B. —> The shape of curve b increases exponentially up until about 35 minutes and starts a decelerated increase all the way until about 43 minutes. 

b) Explain the shape of Curve B. —> This shape is similar to the shape of a temperature-enzyme activity graph. This means that as the temperature rises, so will the rate of enzyme activity. At a certain point, the temperature starts to destroy all of the bonds of the enzymes and enzyme activity starts to decrease (here around 35 minutes) until it plateaus (43 minutes) and later stops all activity.

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