Cellular Biology

Enzymeuse of Enzymechemical Reactionactivation Energy



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Enzymes are the catalysts or chemical substances synthesized in living cell and responsible for metabolic activities within the organism. Subatances which react with the help of enzymes are called "Substrates" and materials produced as the result of a chemical reaction are called "Products". Active sites are present on the surface of enzyme to combine with a substrate through hydrogen bonds or ionic forces. Substrates and enzyme combine to form enzyme-substrate complex. It involves a "lock and key" machanism in which some part of substrate establishes a close fit into some part of enzymes.

Enzymes are essentially proteins. But some of them consists of protein part called "apoenzyme" ans a non-protein part called "prosthetic group". Prosthetic group is divided into two catagories i-e activators and coenzymes/cofactors. Acticator is the inorganic metal portion attached to apoenzyme to help in binding a substrate to the enzyme. Loss of activator causes the inactivity of an enzyme. Some known acticators are copper, calcium, cobalt, iron etc. Coenzyme is the organic prosthetic group of enzyme. Coenzymes have loose association with enzyme acts as donors or acceptors of atoms in forming enzyme-substrate complex. Some coenzymes are NAD (nicotinamide adenine dinucleotide), NADP (nicotinamide adenine dinucleotide phosphate), FAD (flavin adenine dinucleotide) and COA(coenzyme A). Certain vitamins also act as coenzymes.

Some specific features of enzymes differentiate them from inorganic catalysts as enzymes are specific in their action associated with particular chemical reaction, these are not as passive during reaction as catalysts as sometimes molecules of enzymes get destroyed during reaction and velocity of a reaction is not always proportional to the concentration of the enzyme. But usually enzymes retain themselves during the reaction and can be reused for some other reaction. The chemical reactions involving enzymes are reversible if product get accumulated for a long time.

Enzymes catalyses a reaction by reducing the its energy of activation to make molecules more reactive. It brings the substrates closer either binding them in closer association or putting them under stress or strain. Activation energy is the energy required to start a chemical reaction. Enzymes can bring about those reactions at low temperature which only occur at high temperature. So it also act in the place of "thermal agitation".

Certain factors affect the activity of enzymes including temperatue, pH and poisons. Rise in temperature causes rise in enzyme activaty. But at very high temperature denaturation of apoprotein results in the inactivation of enzyme due to the breaking of its hydrogen bonds present in enzyme. Change in pH causes changes in ionic state of a substrate results in the formation of charged particles which may not correspond with the ionic groups present in the active sites of enzymes. As a result no enzyme-substrate complex formed. Certain poison or inhibitors mask the active sites of enzymes forming the enzyme-inhibitor complex. In this situation substrate have to compete for its active site so it is called "competitive inhibition".It can be removed by increasing the concentration of substrate. The conformational change in the enzyme due to the formation of enzyme-inhibitor complex is called "allosteric effect".

Enzyme nomenclature follows some rules. Some enzymes are named after the name of their substrate by adding a suffix 'ase'. For example lipase reacts with lipids (fats), Carbohydrase reacts with carbohydrates and proteinase reacts with proteins. Sometime the name of enzyme refers to the reaction rather than the substrate e.g decarboxylase, dehydrogenase, mutase and oxidase etc. In this case the name of substrate is added to the name of enzyme such as pyruvic acid decarboxylase, succinic acid dehydrogenase, glucomutase and cytochrome oxidase. Old names of enzymes formulated before the nomenclature have been retained.

Enzymes can be classified into following groups.

1-Hydrolyzing & Oxidizing Enzymes : Hydrolyzing enzymes break down the substrate by the addition of water. e.g Carbohydrases(acts on carbohydrates), proteinases (acts on proteins) and eaterases (acts on easters). Carbohydrases include amylase (converts starch into maltose), maltase ( converts maltose into d-glucose) ,sucrase ( converts sucrose into glucose ) etc. Easterases include lipases (converts fats into fatty acid and glycerols) and phosphatases (converts phosphoric acid esters into phosphoric acid) etc.

Oxidizing enzymes help in oxidation-reduction process by transfer of electrons or hydrogen. These are classified as oxidases and dehydrogenases. Oxidases use molecular oxygen as hydrogen acceptor to form water e.g cytochrome oxidase. While hydrogenases remove hydrogen from the substrate and transfer it to the oxygen or to reducible substance. Flavoproteins (yellow enzymes) also accept hydrogen from substrates.

2-Intracellular & Extracellular Enzymes : Intracellular enzymes are produced and used in the same cell while extracellular enzymes are produced in one cell and used in other cells in other part of body.

3-Desmolases & Phosphorylases: Desmolases catalyse the reaction by breaking carbon to carbon bonds e.g aldolase and carboxylase. while phosphorylases acts by adding phosphate group to the substrate. This process is called phosphorylation Its reversible process is called dephosphorylation. Some known phosphorylases are amylo-phosphorylases, trans-phosphorylases, phospho-isomarase and phospho-mutase etc.

Enzymes are utilized in variety of ways. They play a major role in all metabolic activities going on in a living organism. In digestive process they help in breaking up of food particles to release the energy stored in them. Salivary amylase present in saliva converts starch into maltose. In stomach, pepsin present in gastric juice acts on proteins to converts it into polypeptides and peptones. Proteases,amylase and lipase present in pancreatic juice acts on protein,starch and lipids respectively to convert them into digestible food particles. In infants rennin is present in place of pepsin to digest the milk.

In food industry a variety of enzymes are used for processing of various foods e.g in the production of various types of syrups, in dairy industry, in fruit juice and brewing industries. Enzymes are also present in our food as a raw material. Foods that are high in enzymes include raw meat, fresh fruits, unpasteurized milk, vegetables, fermented foods and nuts etc. Breast milk is also enzyme rich food in its uncooked state, as in this state it contains natural enzymes to help in its digestion.

Commercial uses of enzymes includes detergents, leather, antibiotics and fructose production. Infact detergents are the first large scale application for microbial enzymes. Bacterial proteinases is considered an important detergent enzymes. Lipases decompose fats into more water-soluble compounds by hydrolysing the ester bonds between the glycerol and fatty acid. Amylases are also used in detergents to remove starch based stains.

Enzymes are also responsible for metabolic functions in plants cells. Oxidases and de-hydrogenases are found in the matrix of the mitochondria. Many phosphorylases have been discovered to be attached on the inner membrane of mitochondria. Enzymes necessary for carbon dioxide fixation are present in the stroma. Ribosomes also have enzymes which mediate the production of peptide chains of proteins. Deoxyribonuclease present in nucleus helps in hydrolysis of DNA.

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