The ENZYMES

 iology XI Notes

Enzymes – Theory & Question Answers
Chapter # 03
Theory & Question Answers
ENZYMES

Introduction:
The term “enzyme” was coined by Friedrich Wilhelm Kuhne (1878). It was coined for the active ingredient in the yeast extract that promotes fermentation. Enzyme literally means in yeast, but it is now used as the collective name for the many hundreds of compounds that have since been extracted from cells and shown to have a catalytic action on specific chemical reactions.

Definition:
Enzymes may be defined as organic substances capable of catalyzing specific chemical reactions in the living system.
OR
Enzymes are organic catalysts which speed up chemical reactions in organisms.

Ribozymes:
Generally, enzymes are proteinaceous in nature, but Thomas Cech and Sidney Altman discovered that certain molecules of ribonucleic acid also function as enzymes. These molecules are called ribozymes.

Energy Of Activation:
Chemical transformation requires that certain covalent bonds be broken within the reactants. To do so the reactant, must contain sufficient kinetic energy (energy of motion) to overcome a barrier called energy of activation. Enzymes lower down the energy of activation. They can do this because they form a complex with their substrate (S) at the active site.

Characteristics Of Enzymes:
Nature:
Most of the enzymes are proteinaceous in nature. They may be entirely consist of protein e.g. amylase or pepsin or may contain, along with protein, a non-protein part e.g. holoenzyme.

React With Acidic And Alkaline Substances:
They react with both acidic and alkaline substances due to the presence of protein as their major part.

Endoenzyme And Exoenzyme:
Enzymes are produced in the protoplasm. They may act within the cell in which they are produced. These enzymes are called intra cellular enzymes or endoenzymes. Enzymes may diffuse out of the cell and act upon some outside medium called extracellular enzymes or exoenzymes.

Specificity Of Enzymes Action:
With a few exceptions, enzymes are specific in their action. A given enzyme can act only upon a particular substrate or a particular group of substrates, and each kind of metabolic reaction is catalyzed by a specific kind of enzyme e.g. sucrase acts upon sucrose (cane - sugar), lactase on lactose (milk sugar), lipase on fats.

Size Of Molecules:
Their molecules are much greater in size than the substrate.

Active Site:
The specificity of enzymes is due to their primary amino acid sequence; only a small part of which reacts with the substrates called the active site.

Catalytic Properties:
Enzymes influence the speed of chemical reactions. They are not utilized or consumed in the chemical reaction nor do they appear in the end products of the reaction. A very small quantity of the enzymes can catalyze the transformation of a very large quantity of the substrate and at a much greater speed than that of other chemically catalyzed organic reactions.

Enzyme Activators:
Enzymes activities can be accelerated by certain ions or salts called activators e.g. Mn, Ni, Mg, Cl.

Enzyme Inhibitors:
Enzyme activities can be inhibited by certain factors called inhibitors e.g. substrate concentration, enzyme concentration, pH.

Heat Sensitive – Enzymes Are Thermolabile:
Enzymes are inactivated by excessive heat. This property of enzymes relates to the fact that they are proteins. Enzymes are denatured at high temperatures.

PH Sensitive:
Enzymes are sensitive to pH. Every enzyme has its own range of pH in which it functions most efficiently e.g. pepsin only functions in an acidic medium. Trypsin functions in an alkaline medium.

Unchange:
They remain chemically unchanged during and after the chemical reactions.

Mode Of Action:
When an enzyme-controlled reaction takes place, the enzyme and substrate molecules combine with each other and form an enzyme — substrate complex. Action of enzyme is related to its structure which is complex and three-dimensional. Each enzyme has a dimple or groove of a specific shape called the active site, into which substrate can fit.

Lock & Key Theory:
An explanation of the specificity of enzymes, enzyme and substrate fitting together like a lock and key. Lock and key theory was proposed by Fischer in 1898, which was later improved by Paul Filder and D.D Woods. They proposed that a particular enzyme acts on a particular substrate like a particular lock can be unlocked by a particular key. Each enzyme has an active site of particular shape where the particular substrate is attached and produces the product after the catalytic action of enzyme.

Induce Fit Model:
According to the Kosh - Land in 1959 recent research on enzymes has suggested that the active site may not necessarily be exactly the right shape to begin with. It is believed that when the substrate combines with the enzyme it causes a small change to occur in the shape of the enzyme molecule, thereby enabling the substrate to fit more snugly into the active site.

Types Of Enzymes:
Enzymes are generally proteinaceous in nature. According to their composition they are divided into two types.

• Simple enzymes (Proteozyme)
• Conjugated enzymes (Holoenzyme)

Simple Enzymes (Proteozyme):
If an enzyme consists only of protein it is called simple enzyme or proteozyme e.g. amylase, peptidase, lipase.

Conjugated Enzyme (Holoenzyme):
If an enzyme contains another group with protein it is called conjugated enzyme. The protein part of a conjugated enzyme is called apoenzyme and non-protein part is called prosthetic group. Enler (1932) proposed that conjugated enzyme showing complete activity be called holoenzyme.

On the basis of the nature of prosthetic group conjugated enzymes or holoenzyme are of two types.

• Cofactor:
  The term cofactor is usually used for inorganic ions which are necessary to the functioning of an enzyme. Thus Cl⁻ is a cofactor for α - amylase. Role of Mg, Mn, Ca, K on enzymes like phosphatases, phosphorylase, amidase, peptidase, carboxylase are well known coenzyme.

• Coenzyme:
  The enzymes for which prosthetic group is organic substance called coenzyme. e.g. NAD Nicotinamide Adenine Dinucleotide ATPase Adenosine Triphosphate.

Factors Affecting Enzyme Activity:
Following are the factors which increase or decrease the activity of enzyme.

• Concentration Of Substrate:

At constant enzyme concentration, increase in substrate concentration causes an initial increase in the velocity of reaction till it reach a maximum and then remains constant. With further increase of substrate concentration the velocity of reaction decreases, indicating that at high substrate concentration enzyme activity is inhibited.

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

Effect Of pH:
Enzymes are also sensitive to acids, alkalis and certain salts. Each enzyme acts best in a certain hydrogen - ion concentration pH e.g. pepsin of stomach has an optimum of 1.4, catalase shows optimum activity in a neutral medium, sucrose and lipase in acid medium and trypsin in alkaline medium.

Co – Enzymes, Activators & Inhibitors:
Substances inhibited or accelerated the enzyme activity are called co - factor. Co - factors have been divided into three categories

• Co - Enzyme:
  The cofactor is a complex non protein organic molecule known as a coenzyme e.g. CoA, NAD, FAD etc. most enzymes are co - enzyme.

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

• Inhibitors:
  Inhibitors are substances which slow down the catalytic function of enzymes. They may be introduced from outside or they may be normally present in the cells. Their molecules react with groups at or near the active site of the enzyme protein, thus making them inactive. Inhibitors are of two types.

• Competitive Inhibitors:
  These inhibit only one enzyme or a group of closely related enzymes and are also called specific inhibitors. They have a molecular structure which is similar to that of the substrate. By virtue of this, the inhibitor molecule combines with active site of the enzyme forming a reversible complex which does not react to form end products. Thus the inhibitor competes with the substrate for the active site of enzyme and decreases the net amount of the enzyme available for reaction with the substrate. Examples of competitive inhibitors are malonic acid inhibits the activity of enzyme. Succinic dehydrogenase and Fluorocitric acid inhibits the activity of aconitic hydratase.

• Non-Competitive Inhibitors:
  These are general poisons like carbon monoxides, cyanides, and ions of heavy metals like Cu++, Zn++ and Hg++. They inhibit many enzymes and are also called nonspecific inhibitors. They obstruct enzymatic reactions by binding to a part of the enzyme away from the active site. This interaction causes the enzyme molecule to change its shape, rendering the active site unreceptive to the substrate or leaving the enzyme less effective at catalyzing the conversion of substrate to product. In noncompetitive inhibition, a molecule binds to an enzyme other than its active site, which is called allosteric site and inhibitor is called allosteric inhibitor.

• Feedback Inhibition:
  The activity of almost every enzyme in a cell is regulated by feedback inhibition, which is an example of negative feedback. When the product is in abundance, it binds competitively with its enzyme’s active site; once the product is used up, inhibition is reduced and more product can be produced.

• Effect Of Water:
  Water influences the rate of enzymatic activity. Most of enzymes perform their activity by hydrolytic mechanism. In seed germination, water activates enzymes, and germination proceeds.

• Radiation:
  Enzymes actively decrease rapidly by exposure to ultraviolet light and also to β, γ and X-rays.