Biological catalysis was first recognized and described in the early 1800s, in studies of the digestion of meat by secretions of the stomach and the conversion of starch into sugar by saliva and various plant extracts. Louis Pasteur had concluded that fermentation of sugar into alcohol by yeast is catalyzed by ferments. Enzymes are bio-catalysts mostly made up of proteins (except ribozymes), which increases the rate of bio-chemical reactions by lowering down the activation energy. They were first isolated and discovered by the German chemist Eduard Buchner. The enzyme that he had discovered was zymase obtained from yeast. The term enzyme however, was given by the German physiologist Wilhelm Kühne. The first pure sample of an enzyme was urease, obtained from the Jack Bean (Canavalia ensiformis) plant by James Sumner. The first ribozyme was L19 RNase, discovered by American chemist Thomas Cech from rRNA of a protozoan Tetrahymena thermophila. Another well-known ribozyme was Ribonuclease P which was discovered by Sidney Altman in a prokaryotic cell.
Enzymes are everywhere in our food, tearing the microscopic world apart and building new structures from the scraps, recycling carbon and oxygen like an urban wrecking crew recycles steel and granite. As a general rule, polysaccharides, proteins and other massive molecules fail to register on our tongues and in our noses. Short-chain oligosaccharides can be sweet, and short peptides of a few amino acids can register as bitter, but above this general threshold molecules become too large to interact with the taste buds and lack the volatility required to float up to the receptors on our olfactory bulb. Gigantic polymer chains are useful for making sauces thick, ice creams smooth and jellies firm, but their contributions to deliciousness are limited to the world of texture.
Amylases and proteases have the ability to change that. Both amylases and proteases are catabolic enzymes – they hydrolyse the bonds linking each subunit in starch and protein chains, respectively. In the kitchen, they serve a singular purpose: liberating sweet sugars and savoury amino acids from the tasteless starch and protein that comprises a large portion of our food.CHEMISTRY WORLD
Characteristics of Enzymes
- Almost all enzymes are proteins, but all proteins are not enzymes.
- Enzymes are colloidal substances which are very sensitive to pH and temperature. Optimum temperature for most enzymes is 20-35°C.
- Enzymes are macro-molecules of amino acids, which are synthesized on ribosomes under the control of genes.
- Enzymes are tertiary and globular proteins. Isoenzymes are quarternary proteins.
- Enzymes are very specific to their substrate or reactions.
- Enzymes lower down the activation energy of reactions and increase rate of the reactions.
- Enzymes are required in very minute amount for bio-chemical reactions.
Catalytic power of an enzyme is represented by Km (Michaelis Menten constant) and turn over number.
The concentration of substrate at which the rate of reaction of that enzyme attains half of its maximum velocity. It exhibits the catalytic activity of an enzyme.MICHAELIS MENTEN CONSTANT
The number of substrate molecules converted into products per unit time by one molecule of the enzyme in favourable conditions.TURN OVER NUMBER
Enzymatic Sites of Amino Acids
- Catalytic Site: Specific part of an amino acid chain in enzyme structure at which specific substrate is to be bound and catalysed. It is made up of a very specific sequence of amino acids that is determined by the genetic codes.
- Allosteric Site: Additional sites where chemicals other than the substrate (allosteric modulators) are bound. Enzymes with allosteric sites are known as allosteric enzymes.
Nomenclature and Classification
Enzyme Commission of IUB 1961 divides all enzymes into six major classes and also proposed an international code of six digits for each enzyme.
- Oxidoreductases: These enzymes are involved in oxidation-reduction reactions. Eg Cytochrome oxidase.
- Transferases: These enzymes transfer specific chemical groups (other than H) from one substrate to another. Eg Transaminase, Hexokinase.
- Hydrolases: These enzymes are involved in hydrolysis reactions. Eg Proteases, Lipases.
- Lyases: These enzymes split specific covalent bonds of substrate molecules without hydrolysis or addition of water. Eg Aldolase.
- Isomerases: These enzymes are involved in the rearrangement of molecular structure to form isomers.
- Ligases: These enzymes are involved in the formation of large molecules by the covalent bonding of substrates. Eg Citrate synthase.
Nature of Enzyme Action
Enzyme + Substrate ⇌ E-S Complex ⇌ E-P Complex ⇌ Enzyme + Product
The catalytic cycle of an enzyme can be described in the following steps:
- Substrate binds to and fits into the catalytic site of the enzyme.
- Binding of substrate induces the enzyme to alter its shape and conformation, fitting more tightly around the substrate forming an enzyme substrate complex.
- Catalytic site, being in close proximity of the substrate, alters the chemical bonds of the substrate forming the enzyme product complex.
- Enzyme releases the products of the reaction and the free enzyme binds to another molecule of the substrate and runs through the catalytic cycle again.
Factors of Enzyme Action
Enzymes are very sensitive to pH and generally are effective only in a narrow range of pH values.
High temperatures can cause enzyme deterioration whereas low temperatures can cause temporary inactivation of enzymes. Generally, all enzymes perform better at the body temperature of the organism.
Increase in the substrate concentration, increases the activity of the enzyme. Increasing the same after saturation of the enzyme with substrate, however, there is negligible increase in activity of the enzyme.
- When substrate analogues bind to the catalytic site of enzymes, and inhibit the functioning of the enzyme, such an inhibition is termed as competitive inhibition.
- Succinic dehydrogenase is inhibited by malonate which is a substrate analogue of succinate.
- Sulpha drugs are analogues of p-amino benzoic acid which are used to inhibit folic acid synthesis in bacterial cells.
- It is overcome by increase in concentration of the substrate.
- Inhibitor substances like CO, CN and toxic metals, which bind simultaneously to an enzyme, other than it’s active site and cause destruction of the sulfhydryl (S-H) group of the enzyme cause an irreversible inhibition of the enzyme.
- Eg Cyanide poisoning of cytochrome oxidase.
- When a product of a bio-chemical reaction inhibits the enzyme action, it is known as feedback inhibition. Eg inhibition of hexokinase by glucose 6-phosphate.
- When the product of a reaction binds to the allosteric site of an allosteric enzyme and causes inhibition of the enzyme action, it is known as allosteric inhibition.
- When chemicals or products (allosteric modulators) fit into the allosteric sites of an allosteric enzyme and bring a change in the shape of the catalytic site of the enzyme, it is known as allosteric modulation. Eg phosphofructokinase (pacemaker enzyme of glycolysis) is modulated by ATP and AMP/ADP in different ways.
- All allosteric inhibitions & modulations are not feedback inhibitions.
Enzymes are one of the keys to understanding how cells survive and proliferate. Acting in organized sequences, they catalyze the hundreds of step-wise reactions in metabolic pathways by which nutrient molecules are degraded, chemical energy is conserved and transformed, and biological macro-molecules are made from simple precursors. Some of the many enzymes participating in metabolism are regulatory enzymes, which can respond to various metabolic signals by changing their catalytic activity accordingly. Through the action of regulatory enzymes, enzyme systems are highly coordinated to yield a harmonious interplay among the many different metabolic activities necessary to sustain life.