☄️ Characterstics

Meaning

• First of all isolated & discovered by Buchner.
• Zymase (from yeast) was the first discovered enzyme (Buchner).
• The first purified and crystalized enzyme was urease (by J.B. Sumner) from Canavalia/Jack Bean (Lobia Plant).
• The word ‘enzyme’ was coined by W. Kuhne (1878) while working on fermentation.
• Greek:
  • En → In
  • Zyme → Living
• Enzymes are biocatalysts made up of proteins (except ribozyme), which increases the rate of chemical reactions by lowering down the activation energy.
• Thus enzymes alter the rate of reaction without affecting the equilibrium.
• First discovered ribozyme was L19 RNAase by T. Cech form rRNA of a protozoan Tetrahymena thermophile (Protozoa) and RNAase P or Ribonuclease P was discovered by Altman in prokaryotic cell. (Nobel Prize)

Characteristics of Enzymes

• Almost all enzymes are proteins, but all proteins are not enzymes.
• Enzymes are colloidal substances, which are very sensitive to pH & temperature. Optimum temperature for enzymes is 20-35 °C.
• Enzymes are macromolecules of amino acids, which are synthesized on ribosomes under the control of genes
• Enzymes are tertiary & globular proteins. Isoenzymes are quaternary protein.
• Enzymes are required in very minute amount for bio-chemical reactions. Their catalytic power is represented by Michaelis Menten constant or Km constant and turn over number.

“The number of substrate molecules converted into products per unit time by one molecule of the enzyme in favorable conditions is called turn over number.”

• The maximum turn over number is of Carbonic Anhydrase, is 360 lakh, for Catalase is 50 lakh, for Flavoprotein is 50 & for lysozyme is 30 per minute.
• Enzymes are very specific to their substrate or reactions.
• Enzymes lower down the activation energy of reactions and increase the rate of reaction.

  

Michaelis & Menten Constant

“Km constant of an enzyme, is the concentration of substrate at which rate of reaction of that enzyme attains half of its maximum velocity”.

• It is given by Michaelis & Menten.
• The value of Km should be lower for an enzyme.
• Km exhibits catalytic activity of an enzyme.

  

Activation Site

• Specific part of amino acid chain in enzyme structure at which specific substrate is to be binded and catalyzed, known as active site. Active site of enzyme is made up of very specific sequence of amino acids, determined by genetic codes.

Allosteric Site

• Besides the active site’s some enzyme poses additional sites, at which chemicals other than substrate (allosteric modulators) are bind.
• These sites are known as allosteric sites and enzyme with allosteric sites are called as allosteric enzymes. E.g. hexokinase, phosphofructokinase.

Types

(a) On the basis of structure

• Simple Enzymes: Such enzymes are wholly made up of proteins E.g. trypsin, pepsin, amylase, urease etc.
• Conjugated Enzymes: In such enzymes, non-protein parts are also the structural components. The protein part is called Apozyme and non-protein component is called Cofactor.

  

  

• Iso-enzymes: Such enzymes are different in molecular structure but are similar in function E.g. Lactic Acid Dehydrogenase (LAH).

(b) On the basis of presence

• Inducible Enzyme: Such enzymes do not occur in the absence of substrate e.g. Nitrate Reductase (Nitrate reductase is found in the nodules of leguminous crop).
• Non-Inducible Enzyme: Such enzymes are present at all the times e.g. most enzymes.

Mode of Action

• The action of the enzyme depends upon a combination of the enzyme and the substrate molecules to form an enzyme-substrate complex. The enzyme substrate relationship is often compared to as Lock and Key theory, proposed by Fischer (1894).
• Another theory called Induced-Fit hypothesis was proposed by Koshland (1959). According to the latter theory, the attachment of substrate to the enzyme brings about a three dimensional structural change in the enzyme.

Nomenclature and Classification

• Enzyme commission of IUB-1961 divides all enzymes into 6 major classes and also proposed an international code of 4 digitals for each enzyme.
• Transferases: Such enzymes catalyze the transfer of specific group (other than ‘H’) to another molecule e.g. Hexokinases, Transaminases etc.
• Hydrolases: Such enzymes catalyze the hydrolysis of the compound i.e. the addition of the water molecule e.g.

  C₆H₁₂O₁₁ + H₂O → C₆H₁₂O₁₁ + C₆H₁₂O₁₁

  • Proteases, Lipases, Carbohydrate, Invertase, amylase etc.
• Oxido-reductases: Involved in oxidation-reduction reactions.
  • Oxidases: Such enzymes catalyse the oxidation with molecular oxygen e.g. cytochrome oxidise, peroxidase.
  • Dehydrogenase: Catalyse the removal of 'H' through oxidation concerned with oxidative phosphorylation e.g. Alcohol dehydrogenase (i.e. for hydrogen transfer).
  • Reductases: These enzymes cause addition of 'H' or electron (e^-) and the removal of O₂ e.g. Nitrate reductase.
• Lyases: Split the substrate molecule without water. These enzymes split the specific covalent bonds without hydrolysis or H₂O addition leaving the double bond. Ex. hydrolysis, oxidation and reduction e.g. carboxylase, Fumerase, aldolase.
• Isomerases: These enzymes are the responsible for the isomeric changes through rearrangement.
• Ligases or Synthatases: Covalent bonding of two substrates to form a large molecule. e.g. Ligase, polymerase, RNA synthatase.

Nature of Enzyme Action

Enzyme + Substrate ⇌ ESC – EPC → Enzyme + Product

• The catalytic cycle of an enzyme action can be described in the following steps:
  • First, the substrate binds to the active site of the enzyme, fitting into the active site for enzyme-substare complex.
  • The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.
  • The active site of the enzyme, now in close proximity of the substrate breaks or form the chemical bonds of the substrate and the new enzyme-product complex is formed.
  • The enzyme releases the product of the reaction and the free enzyme is ready to bind to another molecule of the substrate and run through the catalytic cycle once again.

Nature and Properties of Enzymes

• Enzymes are specific in nature means specific in reaction with substrate.
• Enzymes are colloidal in nature and thus provide large surface area for reaction.
• Enzymes are made up of either only Apoenzyme or Apoenzyme + Co-Factor.
• Required in extremely small amounts.
• Remain unaffected in the reaction.
• Organic catalyst.
• pH regulated.
• Enzymes are amphoteric protein (means react with acidic and alkaline both substances).
• Thermolabile (means heat sensitive)
• Enzymic activity can be inhibited.
• Enzymic reactions are reversible but forward reaction is more.