Biology XI Notes
Chapter # 03
Short Questions Answers
Section II – Unity Of Life
THE ENZYMES
Q.1: What are enzymes and what are the characters of enzymes?
Ans: ENZYMES:
Enzymes are the organic proteinaceous substances which catalyze chemical reactions in the living organisms. These are considered as bio-catalysts. They increase the rate of chemical reaction but are not consumed in the process:
The term enzyme was used by a scientist, Friedrich Wilhelm Kuhn in 1878.
Characteristics Of Enzymes:
- Enzymes are made up of proteins. They are big molecules with higher molecular weight.
- They can react with both acidic and alkaline substances due to the presence of proteins.
- In a biochemical reaction, only a small amount of enzyme is required as compared to the substrate.
- Enzymes are not consumed during the reaction. They remain unaffected and can be used again and again.
- The enzymes catalyze only specific reactions, that is, each individual enzyme is restricted in its catalytic activity to one particular reaction or one group of related chemical reactions.
- Their activities can be accelerated by certain ions or salts, called activators, such as Ni, Mn, Mg, Cl, etc.
- They do not initiate the reaction, but only increase its rate by lowering the energy of activation.
- Some enzymes contain a non-proteinaceous part, called Prosthetic group.
- When many different enzymes catalyze the same chemical reaction, they are known as isoenzymes.
- The enzymes have a specific active centre which is attached to the substrate. If this active centre is bonded with another substrate, then the enzyme loses its activity.
- Enzymes are sensitive to change in temperature and pH. Heat, alcohol and concentrated solution of inorganic acids stop the activity of enzymes.
Q.2: What are the types of enzymes?
OR
Describe the structure and composition of Enzymes?
Ans: STRUCTURE AND COMPOSITION OF ENZYMES: (TYPES OF ENZYMES)
The enzymes are basically proteinaceous in nature. They have relatively high molecular weight of 40,000 and Catalase has a molecular weight of 250,000.
Certain enzymes consist only of protein; they are called simple protein enzymes. Many enzymes have an attached non-protein group; they are known as conjugated protein enzymes or holoenzymes. It was proposed by Euler in 1932. The holoenzyme consists of two parts:
- The protein part, called apoenzyme.
- The non-protein part, called prosthetic group.
On the basis of prosthetic group, the holoenzymes are of two types:
- With inorganic ions: When the prosthetic group is an inorganic ion, it is called co-factor, such as Mg, Ca, K, Mn, etc. e.g. Phosphatase, carboxylase, Peptidase, Amidase, etc.
- With organic compound: When the prosthetic group is an organic compound, this organic compound is called co-enzyme. It has only 1% part of whole enzyme.
e.g. - NAD (Nicotine amide adenine dinucleotide)
- NADP (Nicotine amide adenine dinucleotide phosphate)
- ATP (Adenosine triphosphate)
Q.3: Describe the function or mode of action of enzymes?
Ans: MODE OF ACTION:
The action of enzyme depends upon the structure of enzyme. It has three-dimensional structure. It has an active site, which is of a particular size and shape. This active site is attached with substrate.
Fischer (1898) proposed a theory about the action of enzyme, called key-lock theory. This theory was improved by Paul Filder and D.D. Woods. According to the theory, each enzyme can react with a specific substrate in the manner of a lock and key, just like a lock can be unlocked by a particular key.
The site of enzyme which is active and attached with substrate is called active site. This site helps in the catalytic action. Sometimes other molecules are connected with active site, but there is no bond formation and no chemical reaction.
Koshland in 1959 proposed another theory about the action of enzymes, called Induce Fit Model Theory. According to this theory, when enzyme combines with a substrate, some changes occur in the structure of enzyme, due to this change the enzyme performs its catalytic function in more effective manner.
Q.4: Describe the different factors of enzymes?
Ans: FACTORS AFFECTING ENZYME ACTIVITIES:
Following factors affect the enzyme activity.
- Temperature
- Substrate concentration
- pH
- Co-enzyme, activators, and inhibitors
- Water
- Radiation
Temperature:
Enzymes are sensitive to heat. They lose their activity at high temperature. The enzymes are denatured by heat, i.e., they are destroyed. The optimum temperature for most of the enzymes is 30°C to 37°C. At freezing point, they become inactive but are not destroyed; at 100°C, the enzymes are completely destroyed.
Substrate Concentration:
With the increase of substrate concentration, the rate of reaction is also increased. The enzyme molecule is much larger than its substrate. When concentration of substrate is low, the active sites on the enzyme molecule may not be occupied; thus, the enzyme does not work. When enzyme is saturated with substrate concentration, then the rate of reaction becomes independent of concentration, and further increase in substrate has no effect.
pH:
Each enzyme has an optimum pH at which the enzyme shows maximum activity. Change in pH can cause loss of its activity. It can be destroyed. When pH scale is shifted to the alkaline or acidic side, its activity is dropped. The optimum pH of pepsin is 1.6 (acidic), while pH of trypsin is 8.2 (alkaline).
Co-Enzymes, Activators, And Inhibitors:
Other groups, such as co-enzymes and co-factors, increase or decrease the enzyme action. These groups are of three types:
Co-Enzymes:
The organic molecule of enzyme is called co-enzyme. Its presence increases the activity of certain enzymes. Without these co-enzymes, their activity is stopped. e.g., COA, NAD, FAD, etc.Activators:
Activators are the inorganic substances which increase the activity of enzyme; for example, Phosphatase enzyme has Mg⁺² as an activator, and Zn⁺² is the activator of enzyme carbonic anhydrase.Inhibitors:
Inhibitors are the substances which decrease the activity of enzyme. These inhibitors either attach directly with enzyme or its activator, then the activity of enzyme is stopped. The inhibitors are of two types:- Competitive inhibitors
- Non-competitive inhibitors
Competitive inhibitors:
These inhibitors resemble the normal substrate, these are attached with the active site of enzyme, so take part to reduce the function of enzyme. If the reaction is reversible, it can be controlled by increasing the concentration of substrate; in this way the substrate gets site of enzyme for attachment and reaction is proceeded.Non-Competitive Inhibitors:
When certain substances are attached to a part of enzyme, away from active site, its activity is affected; such inhibitors are called non-competitive inhibitors. By the attachment of these substances, the enzyme becomes less active. The binding site other than active site is called Allosteric site.Water:
Water also plays an important role to affect the activity of enzymes. In germinating seeds, water enters the body and helps to produce enzymes. The enzymes become active during germination.Radiation:
Ultraviolet rays, n-rays, γ-rays, and x-rays destroy the activity of enzymes.
Q.5: Write a note on Feedback inhibition?
Ans: FEEDBACK INHIBITION:
When product is formed in a chemical reaction, it binds with its enzyme; it is called feedback inhibition. It regulates the activity of enzymes. When the product is in more amount, it is attached competitively with enzyme's active site. The product is used, then inhibition is reduced, as a result of which more product is formed.
In some enzymatic reactions, the end product binds non-competitively at allosteric site on the first enzyme of the reaction. This binding stops the reaction and no more product is formed.
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