"Q: What's the difference between an enzyme and a hormone? A: You can't hear an enzyme"
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Enzymes are organic catalyst usually proteinaceous in nature, which promotes or speed up rate of chemical change in living cells but they themselves are not used up in the process. It is important to note that all enzymes are proteins and not all proteins are enzymes. Enzymes performs their work by accelerating metabolic reactions without changing their composition in the process.
There are two major types of enzymes;
• Intracellular Enzymes : These enzymes functions inside the cell of living organisms. Example are the enzymes that catalyse cell respiration inside the mitochondria.
• *Extracellular Enzymes" : These are enzymes which carry out their function outside the cell. Example includes the digestive enzymes.
Classes Of Enzymes
The new method of classifying enzymes is base on the nature of chemical change brought on substrates. These changes may involve oxidation in which the enzymes will be called an oxidase
Classes Of Enzymes and their action on substrates
• Hydrolases : It catalyses the hydrolytic cleavage of breaking up of carbon-oxygen or carbon-nitrogen bond.
• Oxido-reductases They catalyse the transfer of electron from one molecule, the reductant, also called the electron donor, to another, the oxidant, also called the electron acceptor.
• Isomerases : They work on isomers and usually change the spatial configuration of a molecule in a living cell.
• Ligases : They catalyses the joining together of two molecules with accompanying hydrolysis of a high energy bond.
• Transferases : They catalyse the transfer of specific functional groups from one molecule to another.
• Lyases : They catalyse the breaking up of bond by elimination or addition reaction.
Some enzymes needs additional components to show full activity. Others do not, additional components are nonprotein molecules called cofactors. Cofactors can either be organic (flavin and heme) or inorganic (metal ions). They serve many purposes, for instance, metal ion helps to stabilize nucleophilic species within active sites. Organic cofactors can either be coenzymes, which are released from the enzyme's active site during the reaction or prosthetic groups, which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bounded as in the case of biotin in enzymes such as pyruvate carboxylase.
Characteristics Of Enzymes
• All enzymes are proteins and can function outside organism producing them.
• Enzymes are usually involved in most reversible reactions and do not lose their chemical composition at the end of a reaction.
• Enzymes are affected by the pH of their surroundings.
• Enzymes accelerates the rate of chemical reactions and like any other catalyst acts in a very small quantities and catalyses large amount of substrate.
• Each enzymes are specific in nature and bind specifically to their substrates and they all have a specific temperature range below or above which they become inactive or denatured.
Enzyme Inhibition
Enzymes reactions rates can be decrease by various types of enzyme inhibitors.
Types Of Inhibition
• Competitive Inhibition: A competitive inhibitor and substrate cannot bind to the enzyme at the same time. Often competitive inhibitors strongly resembles the real substrate of the enzyme. This type of inhibition can be overcome with high substrate concentration.
• Non-competitive Inhibition : Inhibitors of these kind bind to a site other than where the substrate binds. The substrate still binds with it's usual affinity, however the inhibitor reduces the catalytic efficiency of the enzyme.
• Uncompetitive Inhibition : An uncompetitive inhibitor cannot bind to the free enzyme, only to the enzyme-substrate complex; hence, these types of inhibitors are most effective at high substrate concentration. In the presence of the inhibitor, the enzyme-substrate complex is inactive.
• Mixed Inhibition : Mixed inhibitors binds to an allosteric site and the binding of the substrate and the inhibitor affect each other. The enzyme function is reduced but not eliminated when bound to the inhibitor.
• Irreversible Inhibition : An irreversible inhibitor inactivate the enzyme permanently, usually by forming a covalent bond to the protein. Penicillin is a common antibiotics that act in this manner.
Biological Functions Of Enzymes
• They are indispensable for signal transduction and cell regulations
• They also generate movement, with myosin hydrolysing ATP to generate muscle contraction.
• They transport cargo around the cell as part of the cytoskeleton.
• They help in the breakdown of protein to amino acids, fat and oil into fatty acids and glycerol and carbohydrates into glucose, fructose and galactose that can be absorbed into the blood stream.
• Virus can also contain enzymes for infecting cells such as the HIV integrase and reverse transcriptase or for viral release from cells.
Enzymes are also used in the chemical industry and other industrial applications when extremely specific catalyst are required. Enzymes in general are limited in the number of reactions they have evolve to catalyse and also by their lack of stability in organic solvents and at high temperatures.
References
Biochemistry by: Donald Voet , Judith G. Voet Edition: 3rd edition
Enzymes
