THE IMPORTANCE OF ACTIVE SITES OF ENZYMES

Have a look at the following figure:


A particular enzyme molecule has an active site which is usually complimentary to the shape of the substrate on which it acts on. Hence enzyme's specificity. Meaning, a particular enzyme can only act on a particular substrate but not with other substrates.

At temperature higher than the optimum temperature, the shape of the active site of an enzyme starts to become distorted making it difficult for the substrate molecule to fit in into it. We say that after the optimum temperature, the enzyme starts to denature. Because of this chances for the enzyme molecules and the substrate molecules to collide to form enzyme-substrate complexes are now becoming lower and lower despite the availability of more kinetic energy at higher temperature. This in turn, causes less products to be formed. Hence the rate of reaction becomes lower and lower at temperatures higher than the optimum temperature.

At extremely high temperature the active site of an enzyme is said to be totally DENATURED. This means that the active site is actually totally distorted. As a result, substrate molecule finds it difficult to fit in into the active site. No enzyme-substrate complexes formed. Meaning, no products formed. This in turn means no reaction occurs. Hence the rate of reaction is actually NIL at this temperature.

Is this REVERSIBLE?
Absolutely NOT reversible. Once an enzyme is denatured, it is actually damaged and cannot be used again even if one tries to lower back the temperature to the optimum temperature.

How about at extremely low temperature?
Enzyme is not DENATURED at a very low temperature. It is simply INACTIVATED. This means there is no kinetic energy available at this temperature to activate the enzyme. Once the temperature is increased, kinetic energy will start to become available and the enzyme and substrate molecules will start to collide with each other to form enzyme-substrate complexes which will eventually break/combine to form products. [Break if the reaction is a CATABOLIC reaction (as shown in the figure above)and Combine if the reaction is an ANABOLIC reaction]

HAPPY READING PEOPLE!!!

ENZYMES AND pH

pH is also a factor which affects the rate of an enzyme catalyzed reaction. The following graph shows how three different enzymes are affected by pH.

From the graph above, it can be seen that Enzyme #1 works well at pH 4. It means that the optimum pH of this particular enzyme is 4. Above or below this pH, the rate of the reaction slows down. The optimum pH for enzymes #2 and #3 are 6 and 9 respectively. This means that below or above these pH, the reactions catalyzed by these enzymes will slow down.

ENZYMES AND TEMPERATURE

One of the characteristics of enzymes is that they are very sensitive to temperature. The following graph shows how the rate of an enzyme catalyzed reaction is affected by temperature.

The graph above tells you that as the temperature is increased, the rate of the enzyme activity increases. As you can see from the graph, the rate of reaction doubles as the temperature is increased by 10 degree Celsius. The rate is at its optimum at the optimum temperature of about 40 degree Celsius. If the temperature is increased further the rate will decrease. This is due to the effect of high temperature on the active sites of the enzymes. At a very high temperature, the shape of the active sites changed. When this happened, the substrate molecules will not be able to fit in into the active site. As a result, the reaction stops. At this stage, the enzymes are said to be denatured and this is irreversible. Unlike at a very low temperature, though the enzymes are not active, they still can be activated if the temperature is increased.

ENZYMES

WHAT ARE ENZYMES?
Enzymes are biological catalysts made of protein. They alter (speed up) the rate of chemical reactions without themselves being chemically changed at the end of the reactions.

(Analogy: It is just like walking on foot or driving to KB. Walking on foot to KB (slower) is just like a reaction not catalyzed by an enzyme whereas driving to KB (faster) is just like an enzyme catalyzed reaction. Note that in this analogy the car is the enzyme which facilitates the journey just like an enzyme speeding up a chemical reaction).

How do enzymes really work? This can be explained by means of the LOCK AND KEY hypothesis. The following figures show this hypothesis.

Catabolic Reaction (a breaking down reaction) - In this type of reaction, an enzyme helps to facilitate the breakdown of a large molecule into smaller molecules. Digestion of starch into maltose is an example of a catabolic reaction. In this example, starch is broken down by an enzyme called amylase. Another example is the breakdown of maltose into glucose by an enzyme called, maltase.


Anabolic reaction (a building up reaction) - In this type of reaction, larger molecules are made from smaller molecules with the help of enzymes. For example, hundreds of glucose molecules might be joined together end to end to form a long molecules of starch and this process is catalysed by enzymes.

Note: In both types of reaction, the enzymes are not chemically changed and can be used over and over again. Hence a little enzyme can react with many substrates.

ENZYMES AND TEMPERATURE
A rise in temperature increases the rate of most chemical reactions and a fall slows them down. In many cases, a rise of 10 degree C will double the rate of reaction in a cell. This is equally true foe enzyme controlled reactions, but above 50 degree C, the enzymes, being proteins are denatured and stop working. When enzymes are denatured, substrates molecules will not be able to fit in into the active site of the enzymes. Hence reactions cannot take place. The graph below shows that enzymes will perform very well at optimum temperature. (Note: once denatured due to temperature, the enzymes will be permanently damaged and cannot be used again)

This is one of the reasons why organisms may be killed by prolonged exposure to high temperatures. The enzymes in their cells are denatured and the chemical reactions proceed too slowly to maintain life.

ENZYMES AND pH
Acid or alkaline conditions alter the chemical properties of proteins including enzymes. Most enzymes work best at a particular level of acidity or alkalinity (pH). Examples: Pepsin in the stomach works well at pH 2 and amylase in the saliva works well at pH 7. (The pH at which enzymes work best is called optimum pH)

Although changes in pH affect activity of enzymes, these effects are usually reversible (unlike the effect of temperature). Extremes of pH however may denature some enzymes irreversibly.

EFFECTS OF SUBSTRATE AND ENZYME CONCENTRATIONS ON RATE OF REACTION
From the following graph, it can be seen that as the substrate concentration increases, the rate of reaction increases initially until the point X is reached. However, further increase in substrate concentration will not increase the rate any further. This is because, after point X, all the enzyme molecules are being saturated or made use of. The same happened when enzyme concentration is increased, the rate of reaction can only increase so much (point Y) but after point Y the rate can no longer increase as all the enzyme molecules are being used in the reaction.

ENZYMES ARE VERY SPECIFIC IN NATURE
This means that a certain enzyme can only react with a certain substrate but not with other substrates. For example, amylase can only catalyse starch but not protein, and likewise, protease can only catalyse proteins but not starch. This is because of the active sites of the enzymes which have complimentary shape with the substrates they react with.