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Mamoona is Qualified author in HSC Pre Medical.She is interested in such topics.


All reaction would occur very slowly at low temperature and atmospheric pressures, the conditions under which living cells carry on their life process. But in living system these reactions proceed at extremely high rates. This is due to the presence of some specialized substances or biocatalysts which are synthesized inside the living cells. These biocatalyst are called enzymes(Gr: En=In; zyme= yeast). The term 'enzyme' was coined by Friedrich Wilhelm Kuhn e (1878). Enzymes may be defined as organic substances capable of catalyzing specific chemical reactions in the living system.
Just a few years ago, it was considered that all enzymes were proteins. During the 1980s, however, Thomas Cech and Sidney Altman discovered that certain molecules of ribonucleic acid also function as enzymes. These molecules are called ribozymes; which catalyze reactions involved in processing genetic information to be used by a cell. But generally enzymes are proteinaceous in nature.

Energy Of Activation:

Chemical transformation requires that certain covalent bonds to be broken within the reactants. To do so the reactants must contain sufficient kinetic energy (energy of motion) to overcome a barrier called Energy of activation or Activation energy. The important role-played by the enzymes during reactions is that they lower the activation energy of the reaction. The enzyme reacts with the energy rich and energy poor molecules to form an intermediate complex. This complex again breaks into product and enzyme. If activation energy of this complex is low, many molecules can participate in reaction. In this way activation energy is lowered by the enzyme but in this action equilibrium (ratio of concentration of reactant and product) is never altered, it remains the same.


Mode Of Action:

Action of enzyme is related to its structure which is complex and three-dimensional. Each enzyme has as dimple or groove of a specific shape called the active site, into which substrate can fit.

To explain the mode of action of enzyme. Fischer(1898) proposed a “Key-Lock” theory which was later improved by Paul Filder and D. D Woods. They proposed that a particular enzyme acts on a particular substrate like particular lock can be unlocked by a particular key. This theory depends upon physical contact between substrate and enzyme molecules.

The active site of each enzyme has a distinct shape and distribution of charge which is complementary to its substrate, like lock and key, where a lock allows very few keys to fit in. Similarly, enzymes allow a few complementary molecules to fit in and react while rejecting even fairly similar molecules.

On the other hand, some molecules may be able to fit in the active site of an enzyme but do not have chemical bond upon which the enzyme can act, so no reaction occurs.

Koshland(1959) proposed Induce Fit Model. He stated that when a substrate combines with an enzyme, it induces changes in the enzyme structure, this change enables the enzyme to perform its catalytic activity more effectively.

The term 'enzyme' was coined by Friedrich Wilhelm Kuhn e (1878).

— Professor

Characteristics Of Enzymes

Enzymes are biocatalyst produced in the protoplasm. They are synthesized in the cell. The basic properties of enzymes are:

  • Most of the enzymes are proteinaceous in nature. They are macro molecules of globular proteins with higher molecular weight. They may entirely consist of protein e.g amylase or pepsin or may contain along with protein, a non protein part e.g holoenzyme.
  • They react with both acidic and alkaline substances due to the presence of protein as their major part.
  • Enzymes generally act within the living cell where they have been produced but sometimes they diffuse out of the cell and perform catalytic function outside the cell or in other cells. An enzyme which acts within the same cell is called intracellular enzymes or endoenzyme and the enzyme which acts outside the cell is called exoenzyme.
  • They are specific in their nature and their action.
  • Their molecules are much greater than the substrate.
  • They have particular sites to react with the substrates called active site.
  • They have bio catalyst, which speed up the rate of reaction. They are required in very small quantities which are capable of bringing about a change in large amounts of substrates.

Types of Enzymes:

Enzymes are generally proteinaceous in nature. They may entirely consist of protein e.g amylase or may contain a non protein part with protein. If an enzyme consists only of protein it is called simple enzyme (proteozyme) and if it contains another group with protein it is called conjugated enzymes. Euler (1932) proposed that conjugated enzyme showing complete activity be called holoenzyme. It contains two parts, the protein part of enzyme is called apoenzyme and the non protein part is called prosthetic group.
On the basis of the nature of prosthetic group, conjugated enzymes or holoenzymes are of two types:

1. The holenzymes in which prosthetic group is an inorganic ion are known as co-factor. Role of magnesium, manganese, calcium and potassium on enzymes like phosphates, phosphorylase, amidase, peptides, carboxylases are well-known.

2. The holoenzymes in which prosthetic group is an organic compound, although inorganic ions may also be present in it are called co-enzymes. A co-enzyme constitutes about 1% portion of the entire enzyme molecule. This part of enzyme is more or less easily separable, usually heat-resistant. Some co-enzymes of oxidation and reduction processes are NAD (Nicotinamide adenine dinucleotide). NADP (Nicotinamide adenine dinucleotide phosphate)., FMN (Flavin mono nucleotide), ATP (Adenosine triphosphate) etc.


Factors Affecting Enzyme Activity

1. Concentration of substrate:

The rate of reaction increases with an increase in the concentration of substrate until the available enzyme becomes saturated with substrate. There is no increase in the enzymatic activity to a certain higher level of substrate concentration. At a very high concentration the substrate exerts a retarding effect upon enzyme action.

This may be due to two reasons:

a. Higher quality of substrate than enzyme.

b. Accumulation of end product in high quantity.

Hence, substrate and enzyme concentration are directly proportional up to a certain maximum velocity after which further increase in substrate concentration has no effect on the rate of reaction.

2. Effect of Temperature:

Enzymes are sensitive to temperature. Each enzyme has its optimum temperature for its maximum activity, above and below this temperature its rate of reaction decreases. Most of the enzymes are highly active at about 37 °C and all an completely destroyed at 100 °C, whereas at minimum i.e. 0 °C, activity is reduced to minimum but enzymes are not destroyed.

3. Effect of pH:

The activity of enzyme varies considerably with pH and there is generally a marked optimum pH for each enzyme e.g pepsin of stomach has an optimum pH of 1.4. It is inactive in neutral or alkaline solution.

Co-enzymes, activators and inhibitors

Co-factors have been divided into three categories.


If the co-factor is an organic molecule, its is called co-enzyme. Without co-enzyme certain enzymes are unable to function e.g CoA, NAD, FAD etc. Most vitamins are co-enzymes or raw materials from which co-enzymes are made.

Inorganic substances which increase the activity of an enzyme are called activators. Magnesium Mg is an inorganic activators for the enzyme phosphatase and Zinc ion Zn is an activator for the most enzyme carbonic anhydrase.

Substances which decrease the activity of an enzyme are called inhibitors. The inhibitors may act by combining directly with the enzyme, or they may react with the activator therefore, activator does not remain available to enzyme for activation.







© 2019 Mamoona Rasheed


Abdullah Rao from Multan on January 28, 2019:

Wow .... Now I know where to get my enzyme chapter done ...