Discovery: The discovery of enzymes dates back to 1850 when French microbiologist Louis Pasteur coined the term ferments for those factors responsible for fermentation of sugars in the presence of grounded yeast. The term enzyme was coined by Willy Kuhne (1878). Edward Buchner (1897) coined the term Zymase for the 'ferments' from the yeast.
What are enzymes?
Enzymes are organic catalysts present in plants and animals. Thus they are universal biological catalysts or middlemen. Physically enzymes are hydrophilic colloidals. The chemical nature of the enzymes first came to light by the work of J.B. Sumner. He prepared crystal of Urease (1926) extracted from Jack Bean and proposed that all enzymes are proteins. The work of John H. Northrop (1930) established and confirmed the protein nature of enzyme. He crystallised Pepsin, Trypsin. Dounce crystallised catalase. The chemical nature or Chemistry of the enzymes and fundamentals about them were first written in the form of an essay of JBS Haldene (1932). Dixon & Web (1964) defined the enzymes as ''Proteins with catalytic property due to their power of specific activation".
Do you know?
All enzymes are proteins. But Ribozyme is a catalytic RNA. It was discovered by Thomas Cech & S. Altman (1989) in Tetrahymena thermophila, a protozoan.
Do enzymes need nomenclature?
There are thousands of enzymes in plants and animals each with a particular function. To maintain their individuality they too need specific names as plants and animals need.
The substance on which enzyme acts is called substrate and that formed from it after action of the enzyme as product.
IUB - International Union of Biochemistry is associated with nomenclature of Enzymes. According to the rules of IUB.
1. The name of the enzyme must end with 'ase'. But there are some enzymes which do not obey to this rule. They are Pepsin, Trypsin and Renin, Casein.
2. Add ase as suffix to the major part of the name of the substrate.
e.g.:
Do you know?
Catalase is an enzyme that acts on H2O2 and converts it to H2O and O2.
There is no valid reason for its name 'Catalase', though ending with 'ase'.
3. Add ase as suffix to the name of the chemical reaction in which enzyme participates.
e.g.:
4. Incombination of substrate and chemical reaction 
What are Proenzymes?
The inactive form of enzymes or their precursors are called proenzymes or Zymogens.
e.g.: Pepsinogen - Pepsin
Trypsinogen - Trypsin
Where the enzymes are present?
Except cell wall, every part of the cell has specific enzymes.
Endoenzymes & Exoenzymes?
The enzymes which are produced inside the cell and act there are called endoenzymes.
Generally enzymes are endoenzymes. But the enzymes which are produced inside the cell but come out of the cell and act on the substrate are called exoenzymes. Action of such enzymes is extracellular or intracellular. Fungi secrete enzymes which digest the food outside their body. Insectivorous plants also secrete proteolytic enzymes to digest the proteins.
What are inducible enzymes?
The enzyme which is produced only in the presence of its substrate is called inducible enzyme.
e.g.: B galactosidase.
What are isoenzymes?
Structure of the enzyme is specific. Sometimes, an enzyme may exist in more than one form. One such enzyme is LDH (Lactic dehydrogenase). It exists in five forms. They differ slightly in their molecular weight but perform same function. Such enzymes are called isoenzymes or analogous enzymes.
General properties of enzymes
* Enzymes are universal biological catalysts or organic catalysts or biological middlemen.
* Enzymes of plants and animals are similar unlike hormones.
* Physically enzymes are hydrophilic colloids. The enzymes are soluble in water. Action of the enzymes increases with the availability of water. e.g.: Dry seeds when kept in soil germinate only in the presence of water. Digestion, respiration occur in the presence of water.
¤ All enzymes are proteins but all proteins are not enzymes. Only those proteins with catalytic property are enzymes. Exceptionally Ribozyme is non - proteinaceous. It is a catalytic RNA discovered in Tetrahymena thermophila, a protozoan. It was discovered by Thomas Cech & Sydney Altman for which they were awarded Nobel Prize 1989.
* They are produced in minute quantity.
* They are macro molecules. Enzymes are polymers of amino acids. So their molecular weight is very high. It is expressed in daltons. Mol wt of Catalase is 2,50,000 daltons; Urease is 4,83,000 daltons; Pyruvate dehydrogenase complex is 46,00,000 daltons.
* Action of the enzyme is specific i.e. the enzyme which acts on one substrate never acts on other substrate.
e.g.: Sucrase acts only on Sucrose to produce Glucose and Fructose. Sucrase do not acts on lipids or proteins.
* Structure of the enzyme is specific.
* Enzymes act at specific pH. Changes in the pH bring change in the solubility of the enzyme in water. Enzyme shows highest activity at Optimum pH. Activity declines both below and above the Optimum pH.
Optimum pH of certain enzymes are as follows
Pepsin - 2 Peroxidase - 5 Anylase - 7
Urease - 7 Catalase -7 Trypsin - 8
Arginase - 9.5
Extremes in pH also cause denaturation. However most of the enzymes work at
their best in neutral pH. The sensitivity of the enzyme can be shown as in the following
graph.
* Enzymes are thermolabile or heat sensitive. They are inactive at freezing temperatures. This is the reason why the vegetables, fruits, milk etc., kept in refrigerator remain fresh for longer time. The enzymes in microorganism & food are inactive.
As the temperature increases the action of the enzymes is doubled and reaches maximum at optimum temperature. Action of the enzyme is also reduced by half with decrease in temperature by 10 °C.
S0 Q10 = 2
At higher temperatures enzymes become denaturalised.
The enzymes at higher temperature in stored dry seeds are inactive. Enzymes in thermophilic bacteria are active at higher temperature where the enzymes of other organism show denaturation. Thus enzymes are heat sensitive because they are proteins. Optimum temperature of most of the enzymes is 25 °C - 30 °C.
The relation between enzyme activity and temperature can be shown as follows
Reversibility: Most of the enzymes participate in reversible reactions. An enzyme can speed up the reaction in forward and backward direction. They maintain chemical equilibrium.
Glyceraldehyde - 3 - Phosphate + Dihydroxy acetone phosphate
* Enzymes decrease activation energy or Gibb's free energy of activation. In the absence of enzyme, substrate requires high amounts of energy for activation.
Enzymes do not decrease reaction energy. It can be shown as follows
* Enzyme has numerous active sites suitable to a specific substrate. The capacity of the enzyme depends on the number of reaction sites. It is expressed in Turnover Number (TON). It is the number of molecules of substrates converted into products in one minute time by 1 molecule of enzyme.
TON also indicates the complexicity of the substrate. It is also called catalytic centre activity. It ranges from hundreds to thousands. Carbonic anhydrase is the fastest enzyme. The enzyme accelerates the reaction 10 million times.
* Action of the enzyme increases with increase in the concentration of the substrate. After reaching certain state, further increase in concentration of the substrate causes no change in the rate of reaction.
* Michaelis - Menten Constant (Km)
Substrate concentration required to reach half the maximal rate of enzyme action is called Michaelis Menten constant (Km).
Km values are more or less constant. They may vary with pH, temperature and also co-enzymes. Km values represent inverse measures of the affinity of the enzyme for the substrate ie., lower the Km value the more stable the enzyme - substrate complex Km also gives the concentration of the enzymes substrate in the cell.
Mechanism of the Enzyme action
Emil Fischer (1898) proposed lock and key hypothesis to explain the enzyme action.
Every enzyme has specific number of reaction sites. These sites are suitable to the shape of the substrate molecule. Substrate binds to the active site and intermediate or temporary transient stage i.e., Enzyme - Substrate complex is formed. Substrate undergoes changes (either bonds are broken or formed). Enzyme releases Product.
Activation energy makes the substrate to form complex with enzyme. Substrate is compared to lock to be opened. Key is compared to the enzyme. In a key the active site is important to open a lock.
Specific sequence of very few amino acids in the region of active site of the enzyme is concerned with functioning of enzyme. X - ray crystallography studies show that the active sites are 3 - D Clefts and Crevices formed due to the specific arrangement of amino acids.
The theory also states Enzyme specificity. It was later elaborated by Fields and Woods (1934).
Induced Fit Theory (1973)
It was proposed by Koshland according to it enzyme undergoes little changes to accommodate the substrate. The substrate induces the enzyme to alter the shape of active site so that it tightly binds to it. Thus the Geometry of the enzyme is slightly altered.
Inhibitors:
Any foreign substance that inhibits the action of the enzyme is called inhibitor. They may be inorganic (metal ions) or organic. They are divided into two types based on the effect of inhibitors.
1) Competitive Inhibitors:
These are organic in nature and resemble the substrate molecules. So they compete for the active site. They block the active sites of enzyme. The reaction do not occurs. Substrate molecules accumulate. Products are not formed.
A classical example for this is shown by Malonate. It resembles succinate. So malonate occupies the active site of Succinic dehydrogenase (Krebs Cycle). FADH2 & Fumaric Acid not formed.
Succinic dehydrogenase - Malonate Complex + FAD + Succinate
Another much more powerful inhibitor of succinate dehydrogenase is Oxaloacetate.
Non Competitive Inhibitors:
Toxins such as Cyanide and salts of heavy metals (Hg, Co etc.,) act as non competitive inhibitors. They sit on a site on the enzyme (other then active site) and bring a change in the structure of the active site. The altered active site then becomes not suitable to the substrate.
Allosteric modulation
Enzymes which combine with and respond negatively to product are called allosteric enzymes. The site where the product sits is called allosteric site (other than active site)
(allo = different, steric = site). Product sits in the allosteric site and brings a change in the active site. Thus the reaction is stopped.
A is the substrate, F = Product
B, C, D & E are intermediates
If the product is accumulated it acts on the enzyme X and stops the conversion of A to B. As a result, whole process is stopped and F is not produced.
e.g.: In Glycolysis Glucose is converted to Glucose - 6 - Phosphate in the presence of ATP and Hexokinase.
The term allosteric was coined by Jacob and Monad.
As the product inhibits the reaction, it is also called as Feed back inhibition or end - product inhibition.
It is an advantage to the cells (organisms). It prevents the over synthesis of end product. It occurs only after the product is synthesised to a level that is sufficient for cellular needs. When products become reduced and the cell is in need of them, the product dissociates from enzyme the reaction continues.
What is the difference between non competitive inhibitor and feed back inhibitors?
Non competitive inhibitor is metallic or toxins where as feed back inhibitor is a metabolic product.
Types of Enzymes
Enzymes are divided into two types basing on their structure or composition.
1. Simple Enzymes 2. Conjugate Enzymes or Holoenzymes
Simple Enzymes:
They are made of proteins. Most of the digestive enzymes are simple enzymes.
e.g.: Amylase, Lipase, Urease, Pepsin, Trypsin.
Conjugate Enzymes:
They have protein part and non protein part. Protein part is called Apo Enzyme and non protein part is called Co-factor. Thus
Conjugate Enzyme = Apo enzyme + Co-factor
Co - factors are 2 types = (1) Metallic Co-factors
(2) Organic Co-factors
Metallic Co-factors
These are tightly bound to Apoenzyme. The enzyme is not functional in the absence of the metallic co-factor. It acts as an activator. These enzymes are called Metallo enzymes.
e.g.: Cu2+ Cytochrome C oxidase, Ascorbic acid oxidase, Tyrosinase.
Fe2+ Catalase, Aconitase, Peroxidase
Mg2+ Hexokinase, Phosphotase
Mn2+ IAA oxidase, Arginase, Carbonic anhydrase
Mo2+ Nitrate reductase, Dinitrogenase
Zn2+ Carbonic anhydrase
Organic Co-factors
These are two types
1) Prosthetic group: Smaller, organic, non protein part tightly attached to protein by covalent bonds is called Prosthetic group.
e.g.: FMN - NADH - Ubiquinone oxidoreductase
FAD - Succinate Ubiquinone oxidoreductase
Haem - Peroxidase, Catalase
2) Coenzymes: Smaller, organic, non protein part loosely attached to protein (Apoenzyme) is called coenzyme.
e.g.: FAD, TPP, NAD, NADP, PP, CoA.
These are derived from water soluble vitamins.
Co enzyme Source
TPP (Thiamine Pyrophosphate) Vit B1 (Thiamine)
PP (Pyridoxal Phosphate) Vit B6 (Pyridoxine)
NAD & NADP Niacin (Nicotinic acid)
CoA Pantothenic acid
FAD is the coenzyme of succinate dehydrogenase and prosthetic group of Succinate Ubiquinone oxidoreductase.
Classification of Enzymes (IUB System 1964)
According to IUB, Enzymes are divided into 6 major classes. Each major class is divided into 4 - 13 sub classes. Each sub class is divided into sub-sub classes. Each sub-sub class has many enzymes with specific serial number.
Thus every enzyme has a 4 number enzyme code.
First No. → Major class Second No. → Sub class
Third No. → Sub-sub class Forth No. → Serial number
The six major classes are
(1) Oxidoreductases
(2) Transferases
(3) Hydrolases
(4) Lyases
(5) Isomerases
(6) Ligases or Synthetases
1) Oxidoreductases:
They participate in oxidation and reduction reactions which involves hydrogen transfer.
Its sub classes are
a) Dehydrogenases:
Transfer Hydrogen from the substrate to NAD or FAD
b) Reductases
They add Hydrogen to the substrate or remove Oxygen from substrate
c) Oxidases
They transfer Hydrogen from the substrate to Oxygen
2) Transferases
They catalyse a chemical group (other than Hydrogen) from one molecule to the other.
Some of the sub classes are
a) Transaminases
They transfer amino group from one substrate molecule to other substrate molecule.
b) Kinases:
3) Hydrolases: They break the bonds in the complex substrate molecule in the presence of water.
a) Phosphotases
b) Peptidases
They break peptide bonds.
4) Lyases: They are concerned with addition or removal of groups to form double bonds in the absence of water.
Glyceraldehyde - 3 - Phosphate + Dihydroxy acetone Phosphate
5) Isomerases:
They are concerned with intramolecular group transfer or rearrangement of atoms within a molecule. Inter conversion of optical, geometric or positional isomers.
6) Ligases or Synthetases
They are concerned with linking together of 2 compounds by forming new bonds. Energy from ATP is utilised in the joining of C _ O, C _ S, C _ N and P _ O etc.
Enzyme Code:
Every enzyme is known by its 4 member code.
e.g.: 2. 7. 1 . 2 → Glucose - 6 - Phosphotransferase
1. 1. 1. 1 →Alcoholic dehydrogenese.
