Mechanism of Enzyme
Enzymes must bind their substrate before they can have any chemical reaction. To understand the mechanism of enzyme action two models have been proposed.
Lock and Key Model:
This model was developed by German chemist Emil Fischer in 1894.
The specific action of the enzyme with a single substrate can be explained using a lock and key analogy. In this analogy, the lock is the enzyme and the key is the substrate.
Only the correctly sized key is the substrate fits into the keyhole which is the active site of the lock that is enzyme. The same enzyme can be used to catalyze hundreds of the same substrates. The enzymes that work on this mechanism are called nonregulatory enzymes e.g., lipase, amylase, etc.
This model explains the specificity of enzymes but does not say anything about the change in the active site. Molecules are in an inactive form. To become active, enzymes must undergo slight conformational changes in the structure to accommodate the substrate.
A suitable analogy would be that of hand and gloves. The hand corresponds to the substrate and glove asthe enzyme is shaped bythe insertion of the hand. Enzymes that follow the induced-fit mechanism are called regulatory or allosteric enzymes e.g. hexokinase.
3.3 Factors Affecting
The Rate of Enzyme Action 9980. The activity of enzymes is affected by the following factors.
Temperature:
Heat increases molecular motion. As the temperature rises from "zero" reacting molecules of substrate and enzyme will get more and more kinetic energy. This increases the chance of a successful collision and so the rate of reaction increases. For every 10°C rise in temperature, the rate of enzyme is approximately.
There is a specific temperactivity doubles at which an enzyme catalytic activity is fastest and
this is known as optimum temperature. The optimum temperature for enzymes found inhumansn is 37°C.
After this point, the rate of enzyme activity will decrease and at 45-50°C the enzyme activity will be stopped, as ethe nzyme binding site will denature at this temperature. Some bacteria live in hot springs so the optimum temperature for their enzymes is more than 37°C.
Such enzymes have been used in biological washing powders and detergents. That is why cloth washing needs lukewarm water, not too hot.
pH:
Every enzyme needs a specific pH for its proper functioning. The pH at which an enzyme works maximum is called its optimum pH. Some enzymes work best in an acidic medium e.g., pepsin, some ina neutral medium e.g., amylase, and others in an alkaline medium e.g., lipase.
However, most enzymes in our body work in the range of pH 6-8. Some enzymes may work on both acidic and alkaline media e.g., papain enzyme in green papaya. Change in pH alters the ionic charge of acidic and basidic groups as a result ionic bonding is disrupted.
This ionic bonding is needed to maintain the specific shape of ethe nzyme. Thus the change in pH may change the shape of ethe enzyme as well as denature the active site.
Enzyme Concentration:
Enzyme concentration is directly proportional to enzyme activity. If substrate concentration is maintained at a high level, and other conditions such as pH andtemperaturesare kept constant then with the increase of enzyme concentration the activity of the enzyme will also increase and with the decrease of enzyme concentration, the activity of the enzyme will also decrease.
Usually in natural conditions, the substrate concentration is higher than enzymes. However, when the enzyme concentration becomes saturated as compared to the substrate, then the rate of reaction will not increase further, this maximum rate (Vmax value) is never obtained.
Substrate Concentration:
Like enzymes the substrate concentration is also directly proportional to enzyme activity up to optimum level. If we keep the other conditions such as temperature, pH, and enzyme concentration at a constant state and change the amount of substrate then we find that with an increase in substrate concentration, the reaction rate will increase only up to the maximum level.
This is because more substrate molecules will be colliding with enzyme molecules so moreproductst will be formed. However, at a certain concentration substrate becomes saturated then any further increase will have no effect on the rate of reaction because at this point all the active sites of the enzyme are occupied, and the maximum rate (V- max).
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