Question

Properties of Enzymes

1. The concentration of an enzyme within a cell is extremely low relative to other material present, yet functions effectively in its metabolic role. How can an enzyme function and never seem to run out?

2.Name 4 factors which affect the activity of an enzyme.

3. A chemist found that a reaction did not take place under any set of conditions. If the chemist added a catalyst, would the reaction then occur? Explain your answer.

4.The rate of an enzyme-catalyzed reaction usually increases with temperature, but then decreases after reaching the “optimal temperature”. Why does this take place?

5.Why does saliva hydrolyze starch in the mouth but not in the stomach?

6.For a given enzyme-catalyzed reaction, on a molar basis, substrate concentration increases from two to four times the enzyme concentration (double). Would the rate of the reaction also double? Explain your answer.

Answer 1

1.

The following is an explanation of how enzymes function. For two molecules to react they must collide with one another. They must collide in the right direction (orientation) and with sufficient energy. Sufficient energy means that between them they have enough energy to overcome the energy barrier to reaction. This is called the activation energy.

Enzymes have an active site. This is part of the molecule that has just the right shape and functional groups to bind to one of the reacting molecules. The reacting molecule that binds to the enzyme is called the substrate. An enzyme-catalysed reaction takes a different 'route'. The enzyme and substrate form a reaction intermediate. Its formation has a lower activation energy than the reaction between reactants without a catalyst.

Route A

              reactant 1 + reactant 2 --> product

Route B

              reactant 1 + enzyme --> intermediate

              intermediate + reactant 2 --> product + enzyme

So the enzyme is used to form a reaction intermediate, but when this reacts with another reactant the enzyme reforms: this is why it never runs out.

2.

4 factors which affect the activity of an enzyme are:

1. temperature
2. pH
3. enzyme concentration
4. substrate concentration

3.

Catalysts increase the rate of a reaction by lowering its activation energy. Some enzymes can make their conversion of substrate to product occur many millions of times faster. An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds. Therefore if the chemist finds a catalyst which can indeed speed up the reaction by a huge factor and adds it to the reaction, the reaction would occur.

4.

As the temperature rises, reacting molecules have more and more kinetic energy. This increases the chances of a successful collision and so the rate increases. There is a certain temperature at which an enzyme's catalytic activity is at its greatest. Above this temperature the enzyme structure begins to break down or denature since at higher temperatures intra- and intermolecular bonds are broken as the enzyme molecules gain even more kinetic energy.

5.

Saliva hydrolyzes starch in the mouth but not in the stomach because salivary amylase, which begins the digestive process by breaking down starch, converting it into maltose, a smaller carbohydrate. However, the food and salivary enzymes continue the digestion process until the secretion of stomach acid causes the pH to drop below 3.0, which is the activity range of plant enzymes. Before food arrives, the stomach normally has a pH between 5.0 and 6.0. In young and healthy adults it takes about 45 minutes before enough acid is generated to drop the pH to 3.0.

6.

When the substrate concentration is two times the enzyme concentration, the enzyme becomes saturated with substrate. As soon as the catalytic site is empty, more substrate is available to bind and undergo reaction. The rate of formation of product now depends on the activity of the enzyme itself, and adding more substrate will not affect the rate of the reaction to any significant effect. Therefore doubling the substrate concentration does not double the rate of reaction; instead the change in rate will not be significant.