AS Module 1
10.5 Enzymes are proteins which control biochemical reactions in cells.
By the end of this tutorial you should be able to:
| Explain how enzymes catalyse biochemical reactions. |
| Describe an enzyme substrate complex. |
| Explain the lock and key and induced fit models of enzyme action. |
| Describe and explain the effects of temperature, pH, substrate and enzyme concentration and competitive and non-competitive inhibitors on enzyme activity. |
Enzymes
Enzymes are organic catalysts which speed up chemical reactions in organisms. Enzymes are always proteins, and their characteristics therefor reflect the properties of proteins. Their main properties are as follows:
| They work very rapidly, or have a "high turnover". |
| Enzymes are not destroyed by the reactions they catalyse. Therefor they can be used again. |
| An enzyme can work in either direction. Thus metabolic reactions are reversible. |
| Enzymes are inactivated by excessive heat (denatured). |
| Enzymes are sensitive to pH. |
| Enzymes are generally specific to only one reaction or type of reaction. |
Enzymes and activation energies
Chemical reactions involve breaking and remaking chemical bonds. In order to react, molecules first need sufficient energy- they have got to get over an "energy hill", known as the activation energy for the reaction. Imagine pushing a large boulder up a steep hill. Once the boulder gets to the top (achieves activation energy), it will roll down the side quite easily.
Enzymes lower the activation energy needed for the reaction to take place (they make the hill smaller)
How enzymes lower activation energy
All enzymes are globular proteins. Within these large molecules is an active site which is the part responsible for the functioning of the enzyme. In order to lower an activation energy, the enzyme forms a complex with a substrate. The substrate is the chemical molecule which needs to be broken down to release the useful products:
Substrate+enzyme=enzyme/substrate = enzyme+ products
complex
The graph below shows the effect an enzyme has on the activation energy of a reaction. You can see that the energy required for the reaction to be successful is substantially less with the addition of an enzyme.
Once the products of the reaction are formed, they are released and the enzyme is free to form a new complex with another substrate molecule.
The lock and key model of enzyme activity
This model forms the basis of our understanding in enzyme reactions. The active site is made up of amino acids and has a very specific shape. This shape means that the enzyme can only be used for one specific substrate, or type of substrate which fits into the site. The enzyme and substrate slot together to form a complex, as a key slots into a lock. This complex reduces the activation energy for the reaction. This may be due to the bonds within the substrate being deformed and stressed by the complex, making them more likely to react. Once the reaction has been catalysed, then the products no longer fit into the active site and are released allowing another substrate in.
Click
here for an animation of a lock and key enzyme reaction
The induced fit model
The Induced Fit model is a modification of the lock and key model. Instead of having a ridged substrate molecule that will only fit into a specific active site, we have an active site that can stretch to accept a range of molecular shapes for the substrate analogous to how gloves stretch to fit more than one size hand. This model explains more adequately how active site blockers (substrate imposters) can block the active site and inhibit the enzyme molecule.
Click here for an animation of an induced fit reaction.
Factors which effect enzyme activity
Temperature and Enzyme Function
Chemical reactions speed up as temperature is increased, so, in general, catalysis will increase at higher temperatures. However, each enzyme has a temperature optimum, and beyond this point the enzyme's functional shape is lost (denatured). Boiling temperatures will denature most enzymes.
pH and Enzyme Function
Each enzyme functions best within a certain pH range. For example, the enzyme pepsin, which works in your stomach, functions best in a strongly acidic environment. Lipase, an enzyme found in your small intestine, works best in a basic environment.
When the pH changes, the active site progressively distorts and affects enzyme function. What happens to catalysis when an enzyme is subjected to a pH far from its optimum range?
Click here to see an animation of the effect of pH on enzymes
Enzymes are affected by changes in pH. The most favorable pH value - the point where the enzyme is most active - is known as the optimum pH. This is graphically illustrated in Figure 14.
Extremely high or low pH values generally result in complete loss of activity for most enzymes. pH is also a factor in the stability of enzymes. As with activity, for each enzyme there is also a region of pH optimal stability.
Effect of enzyme concentration
The amount of enzyme present in a reaction is measured by the activity it catalyzes. The relationship between activity and concentration is affected by many factors such as temperature, pH, etc. An enzyme assay must be designed so that the observed activity is proportional to the amount of enzyme present in order that the enzyme concentration is the only limiting factor. It is satisfied only when the reaction is zero order.
Figure 5 shows that in the area from A to B on the concentration axis, the enzymes active sites are continually full and thus working to their optimum level. However, in the area from B to C, there is not enough substrate left to keep all the active sites full. Thus the output is reduced due to a lack of usable substrate. At point C there is no more substrate left to react so activity stops.
Effect of substrate concentration
It has been shown experimentally that if the amount of the enzyme is kept constant and the substrate concentration is then gradually increased, the reaction velocity will increase until it reaches a maximum. After this point, increases in substrate concentration will not increase the velocity. This is represented graphically in Figure 8. This is because all of the available active sites are full. Thus, there will be no effect by increasing substrate concentration because the molecules will have to wait until a site becomes available, keeping the reaction rate the same. This point is represented on the graph by Vmax.
Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. There are three common types of enzyme inhibition - competitive, non-competitive and substrate inhibition.
Most theories concerning inhibition mechanisms are based on the existence of the enzyme-substrate complex ES. As mentioned earlier, the existence of temporary ES structures has been verified in the laboratory.
Competitive inhibition
Competitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. The theory of the "lock and key" of enzyme catalysts can be used to explain why inhibition occurs.
The lock and key theory utilizes the concept of an "active site." The concept holds that one particular portion of the enzyme surface has a strong affinity for the substrate. The substrate is held in such a way that its conversion to the reaction products is more favorable. If we consider the enzyme as the lock and the substrate the key (Figure 9) - the key is inserted in the lock, is turned, and the door is opened and the reaction proceeds. However, when an inhibitor which resembles the substrate is present, it will compete with the substrate for the position in the enzyme lock. When the inhibitor wins, it gains the lock position but is unable to open the lock. Hence, the observed reaction is slowed down because some of the available enzyme sites are occupied by the inhibitor. If a dissimilar substance which does not fit the site is present, the enzyme rejects it, accepts the substrate, and the reaction proceeds normally.
Non-competitive inhibition
Non-competitive inhibitors are considered to be substances which when added to the enzyme alter the enzyme in a way that it cannot accept the substrate. Figure 10.
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