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Sometimes a small amount of a substance significantly influences the rate of a chemical reaction. The substance itself is not used up or changed at the end of the reaction. Such a substance is called a catalyst, and the phenomenon catalysis.  The latter may be positive if the reaction rate is enhanced; or negative if the reaction rate is reduced. Both organic and inorganic substances can play the role of catalysts. The rate and efficiency of the reaction in a cell depends on certain special molecules called enzymes which are synthesized by the cells. Enzymes are biological catalysts. The DNA in each cell has the necessary message (blue print) for the production of all the enzymes required by it. The cell uses this information as and when necessary to produce the enzymes required to catalyse specific reaction at any point of time.  Enzymes are synthesized by living cells. But they retain their catalytic ability even when extracted from cells. Rennet tablets containing the enzyme rennin from the calf's stomach have long been used for coagulating milk protein to obtain caesin (cheese from milk). Mostly enzymes are proteins but all proteins are not enzymes. Majority of the enzymes contain a non-protein part called the Prosthetic group. It is tightly bound to the enzyme. Some prosthetic groups are metal compounds. For example, iron-porphyrin complexes form the prosthetic groups of cytochromes. In addition, certain organic compounds and inorganic ions are required for enzyme activity. They are loosely bound to the enzyme and are called co-factors. Nicotinamide adenine dinucleotide (NAD), the precursor of nicotinic acid (niacin), and flavin adenine dinucleotide (FAD), the active form of vitamin B2 (riboflavin) are organic co-factors or Coenzymes of many oxidising enzymes of the mitochondria. Certain metals, especially those occurring in trace amounts also facilitate enzyme reaction. Iron (Fe++) is a co-factor responsible for the catalytic action of catalase. 

All enzymes have a specific three dimensional structure, a part of which is known as the Active site. An enzyme may have more than one action site. The active site serves as a 'lock' into which the reactant, (commonly referred to as the substrate) fits in like a key. The point where the substrate is bound on the action site is known as the substrate-binding site. Substrate binding causes a lowering of the activation energy and allows the reaction to proceed. Once the reaction is completed, the enzyme releases the products and is ready to catalyse again. How fast do enzymes act?

A single molecule of the enzyme carbonic anhydrates, the fastest enzyme known, hydrates 36 million (36 × 106) molecules of carbon dioxide per minute. The catalysed reaction is 10 million times faster than the non-catalysed reaction.

Properties of Enzymes

Each enzyme can catalyse the change of either a specific substrate or a specific group of substrates. The specificity for a substrate can easily be demonstrated.


Optimum Temperature
Enzymes generally function in a narrow range of temperature which usually corresponds to the body temperature of the organism. For instance, human enzymes work at the normal body temperature. Each enzyme shows its highest activity at a particular temperature called optimum temperature. Activity declines both above and below the optimum temperature. Low temperature preserves the enzyme in a temporarily inactive state. Food may be preserved for a long time in a frozen state because neither microbial enzymes nor enzymes in the food can act at low temperatures to cause its spoilage. High temperature destroys enzyme activity, because proteins are denatured by heat. For this reason, only a few cells can tolerate temperatures above 450C. Some heat resistant microorganisms living in hot springs at temperatures close to 1000C, possess heat resistant enzymes.

Optimum pH
Each enzyme shows its highest activity at a specific PH. This is called the optimum pH. Activity declines both above and below the optimum pH. Most intracellular enzymes function best around neutral pH. Some digestive enzymes have their optimum in the acidic or alkaline range. For example, the protein digesting enzyme pepsin found in the stomach has an optimum pH of 2.0. Another protein-digesting enzyme, trypsin, found in the duodenum, functions best in an alkaline pH of 8.5.

Enzyme Substrate Complex 
Each enzyme (E) has a substrate-binding site in its molecule. A highly reactive enzyme-substrate complex is consequently produced. The latter almost immediately dissociates into the product or product (P) and the unchanged enzyme. Formation of the enzyme-substance complex is essential for catalysis. The higher the affinity of the enzyme for its substrate, the greater is its catalytic activity.

Effect of substrate concentration       
With the increase in substrate concentration (S), the velocity V of the enzymatic reaction rises at first. The reaction ultimately reaches a maximum velocity, which is not exceeded by any further rise in substrate concentration. This happens because enzyme molecules are fewer than the substrate molecules. Any more increase in substrate concentration will saturate all the enzyme molecules. No enzyme is left free to bind with additional molecules of the substrate.

Inhibition of Enzyme Action
Enzyme action can be inhibited in four different ways. Inhibition of enzyme action by denaturation of proteins has already been mentioned. The other three ways in which enzymes can be inhibited are: 

Competitive inhibition
The action of an enzyme may be reduced or inhibited in the presence of a substance which closely resembles the substrate in its molecular structure. Such an inhibitor is called a competitive inhibitor of that enzyme. Due to its close structural similarity with the substrate, the inhibitor competes with the latter for the substrate-binding site of the enzyme. Consequently, the enzyme cannot participate in catalysis. As a result, the enzyme action declines, e.g., the inhibition of succinic dehydrogenase by malonate, which closely resembles succinate in structure. This may be compared to a lock jammed by a key similar to the original key. Such competitive inhibitors are often used in the control of bacterial pathogens. For instance, sulpha drugs are competitive inhibitors of folic acid synthesis in bacteria as they substitute for p-amino benzoic acid, thus preventing the next step in the synthesis. 

Non-competitive inhibition
Cyanide kills an animal by inhibiting cytochrome oxidase, a mitochondrial enzyme essential for cellular respiration. This is an example of non-competitive inhibition of an enzyme. Here the inhibitor (cyanide) has no structural similarity with the substrate (cytochrome c) and does not bind with the substrate-binding site but binds at some other site of the enzyme.  Thus in non-competitive inhibition substrate-binding site but binds at some other site of the enzyme. Therefore in non-competitive inhibition substrate binding takes place but no products are formed. 

Allosteric Modulation or Feedback Inhibition
The activities of some enzymes, particularly those which form a part of a chain of reaction (metabolic pathway), are regulated internally. Some specific low molecular weight substances, such as the product(s) of another enzyme in the chain, acts as the inhibitor. Such a modulator substance binds with a specific site of the enzyme different from its substrate-binding site. This binding increases or decreases the enzyme action. Such enzymes are called allosteric enzymes: e.g. hexokinase which changes glucose to glucose 6-phosphate in glycolysis. Decline in enzyme activity by the allosteric effect of the product is called feedback inhibition e.g., allosteric inhibition of hexokinase by glucose-6-phophate.

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