Review Questions

Enzymes:

Proteins that speed up chemical reactions inside living organisms, by minimizing the activation energy are known as enzymes.

Functions of Enzymes:

Enzymes are proteins that catalyze i.e. speed up) biochemical reactions and are not changed during the reaction. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products. Almost all processes in a cell need enzymes to occur at significant rates.

Characteristics of Enzymes:

1. Almost all enzymes are proteins i.e. they are made of amino acids.
2. Most enzyme reaction rates are millions of times faster than those of comparable uncatalyzed reactions. As with all catalysts, enzymes are not consumed by the reactions they catalyze.
3. Enzymes are usually very specific for the type of reaction and the nature of their substrates
4. The activities of enzymes are determined by a small portion of the enzyme molecule (around 34 amino acids) which is directly involved in catalysis. This catalytic region, known as the active site, recognizes and binds the substrate, and then carries out the reaction.
5. Since enzymes are extremely selective for their substrates and speed up only a few reactions, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.
6. Enzymes can be categorized based on the site where they work i.e., they may be intracellular enzymes (e.g., enzymes of glycolysis working in the cytoplasm) or maybe extracellular enzymes (e.g., pepsin enzyme working in the stomach cavity).

7. The enzyme activity is controlled in the cell in many ways. Enzyme production can be enhanced or diminished by a cell in response to changes in the cell's environment. Enzyme activity can also be regulated by inhibitors and activators. 

8. Some enzymes do not need any additional components to show full activity However, others require non-protein molecules or ions called cofactors for activity.

Types of Cofactors: Cofactors can be either inorganic (e.g. metal ions) or organic (e.g. flavin and heme).

Coenzymes:

If organic cofactors are tightly bound to the enzymes, they are called prosthetic groups, but if they are loosely attached to the enzyme, they are called coenzymes. Coenzymes are small organic molecules that transport chemical groups from one enzyme to another. Some important coenzymes are vitamins (e.g. riboflavin, thiamine, and folic acid).

9. Several enzymes can work together in a specific order, creating metabolic pathways. In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme.

Activation Energy:

The minimum amount of energy required for effective collusion during a chemical reaction is called activation energy.

Significance of Activation Energy:

All chemical reactions require activation energy to break chemical bonds and begin the reaction. The need for activation energy acts as a barrier to the beginning of the reaction.

Reference of Activation Energy in Enzymes:

Enzymes lower such barriers by decreasing the requirement of activation energy of enzymes, reactions proceed at a faster rate Enzymes lower the activation energy in several ways. They do so by:

1. Altering the shape of the substrates and reducing the amount of energy required to complete the transition.
2. Disrupting the charge distribution.
3. Bringing substrate in the correct orientation to react.

Temperature increases will speed up the rate of enzyme-catalyzed reactions, but only to a certain limit.

Optimum Temperature:

Every enzyme works at its maximum rate at a specific temperature called the optimum temperature for that enzyme. When the temperature rises to a certain limit, the heat adds in the activation energy and also provides kinetic energy and so reactions are accelerated.

 Denaturation of Enzyme:

When the temperature is raised well above the optimum temperature, the heat energy increases the vibrations of atoms of enzyme molecules and the globular structure of the enzyme is lost. This is known as the denaturation of the enzyme. It results in a rapid decrease in the rate of enzyme action and it may be blocked completely.

Conclusion: Thus above 35°C and below 0°C Enzyme activity slows down and eventually stops.

pH: All enzymes work at their maximum rate at a narrow range of pH, called the optimum pH. A slight change (increase or decrease) in this pH causes retardation in enzyme activity or blocks it completely. Every enzyme has its specific optimum pH value.

For example, Pepsin (working in the stomach) is active in an acidic medium (low pH) while trypsin (working in the small intestine) shows its activity in an alkaline medium (high pH). A change in pH can affect the ionization of the amino acids at the active site.

Substrate concentration:

If there are enzyme molecules with vacant active sites, an increase in substrate concentration will increase the rate of reaction. If the enzyme concentration is kept constant and the amount of substrate is increased, a point is reached where any further increase in substrate does not increase the rate of reaction anymore. When all the active sites of the enzymes are occupied (at high substrate concentration), any more substrate molecules do not find free active sites. This state is called saturation of active sites and the reaction rate does not increase.

Factors Affecting the Rate of Enzyme Action:

1. Temperature:

Temperature increases will speed up the rate of enzyme-catalyzed reactions, but only to a certain limit.

Optimum Temperature:

Every enzyme works at its maximum rate at a specific temperature called the optimum temperature for that enzyme. When the temperature rises to a certain limit, the heat adds in the activation energy and also provides kinetic energy and so reactions are accelerated.

Denaturation of Enzyme:

When the temperature is raised well above the optimum temperature, the heat energy increases the vibrations of atoms of enzyme molecules and the globular structure of the enzyme is lost. This is known as the denaturation of the enzyme. It results in a rapid decrease in the rate of enzyme action and it may be blocked completely

1. Substrate concentration:

If there are enzyme molecules with vacant active sites, an increase in substrate concentration will increase the rate of reaction. If the enzyme concentration is kept constant and the amount of substrate is increased, a point is reached where any further increase in substrate does not increase the rate of reaction anymore. When all the active sites of the enzymes are occupied (at high substrate concentration) any more substrate molecules do not find free active sites. This state is called saturation of active sites and the reaction rate does not increase.

1. pH:

All enzymes work at their maximum rate at a narrow range of pH, called the optimum pH. A slight change (increase or decrease) in this pH causes retardation in enzyme activity or blocks it completely. Every enzyme has its specific optimum pH value. For example, Pepsin (working in the stomach) is active in an acidic medium (low pH) while trypsin (working in the small intestine) shows its activity in an alkaline medium (high pH) Change in pH can affect the ionization of the amino acids at the active site.

Mechanism of Enzyme Action:

When the enzyme attaches to the substrate, a temporary enzyme-substrate (ES) complex is formed. The enzyme catalyzes the reaction and the substrate is transformed into a product. The ES complex breaks and enzymes and products are released.

Lock and Key Mechanism of Enzyme Action:

To explain the mechanism of enzyme action German chemist Emil Fischer, 1894, proposed the lock and key model. According to this model, both the and the substrate possess specific complementary geometric shapes that fit exactly into one another. This model explains enzyme specificity.

Modification of the Lock and Key Model: ("induced fit model").

The "induced fit model" is more acceptable than the "lock and key model. In 1958 an American biologist Daniel Koshland suggested a modification to the lock and key model and proposed the induced-fit model. He said that enzymes are flexible structures and their active site is reshaped as the substrate interacts with the enzyme. According to this model the active site is not a rigid structure rather it is molded into the precise position to perform its function.

Coenzyme:

The organic cofactors of enzymes that are loosely attached to enzymes; transport chemical groups from one enzyme to another

Cofactor:

The non-protein molecules or ions required by enzymes for activity e.g., metal ions and organic molecules.

Paper industry:

Enzymes degrade starch to lower its viscosity which aids in making paper.