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Biology Notes Class; F.Sc 1st Year By: Sabir Hussain Rana, M.D.(Biochemistry) -E N Z Y M E SEnzymes are defined as the most important group of globular proteins which are Biologically Active. OR as “Biocatalysts” which speed up the biochemical reactions (more specifically as catalysis) occurring in the living body by lowering their activation energy Importance of Enzyme: 1. They tremendously increase the efficiency of a biochemical reaction and are specific for each type of reaction 2. Without these enzymes the reaction would proceed at a very slow speed making life impossible 3. Many enzymes are simply dissolved in the cytoplasm while others are tightly bound to certain subcellular organelles 4. They are produced by living cells for use in or near the site of their production 5. Thus, enzymes important for photosynthesis are found in chloroplasts and enzymes involved in cellular respiration are found in the mitochondria 6. Some of the enzymes which are involved in the synthesis of proteins are integral part of ribosomes SOME IMPORTANT TERMS RELATED TO ENZYMES: Chemical Processes and Reactions occurring in the cell/organism is called Metabolism Metabolism is of two types, namely; catabolism and anabolism Break down of large molecules into smaller molecules is called Catabolism e.g. Respiration Small molecules may combine to form large molecules and the process is called Anabolism e.g. Photosynthesis The reactant of the enzyme or the substance at which an enzyme works is called its Substrate The site/part of the enzyme where normally a substrate attach or react is called Active Site* (see pg # 3) Enzymes Dr. Sabir Hussain (Chapter # 3) F.Sc Biology The non-protein part of an enzyme which is essential for its proper functioning and acts as a “bridge” between an enzyme and its substrate is called a Co-factor e.g. Mg2+, Fe2+, Cu2+, Zn2+ etc An inorganic detachable cofactor is known as Activator If the co-factor is covalently bonded to the enzyme, it is known as a Prosthetic Group If a co-factor is loosely attached to the protein part of enzyme, it is called Co-enzyme. Coenzymes are mostly vitamins which can also be used again and again like enzyme An enzyme with its co-enzyme or prosthetic group removed is designated as Apoenzyme An activated enzyme consisting of polypeptide chain and cofactor (co-enzyme and prosthetic group) is called Holoenzyme CHARACTERISTICS OF ENZYMES: Enzymes, the biochemical catalysts, possess the following important characteristics: 1. 2. 3. 4. 5. 6. 7. 8. All enzymes are globular proteins (except nucleases) They increase the rate of reaction without being used up They do not affect the nature or properties of end products Small amount of enzyme can accelerate chemical reactions They are very specific in their action; a single enzyme catalyzes a single chemical reactions or group of related reactions They are very sensitive to even the minor change in pH, temperature and substrate concentration Some enzymes require a co-factor for their proper functioning They lower the activation energy of the reactions Mechanism of Enzyme Action (CATALYSIS): An enzyme is a three dimensional globular protein that has specific chemical composition due to its component amino acid and a specific shape Every enzyme by virtue of its specificity recognizes and reacts with a special chemical substance called Substrate Any enzyme reacts only with its specific substrate and transforms it into the product(s) It is released unaltered after the catalysis and thus can be used again and again 2 Enzymes Dr. Sabir Hussain (Chapter # 3) F.Sc Biology An enzyme and its substrate react with each through a specific charge bearing site of an enzyme called active site The charge and shape of the active site is formed by some amino acids present in the polypeptide chain of the active site of the enzyme These amino acids are brought closer and are arranged in a specific way by coiling and folding of the polypeptide chain within the globular symmetry of the enzyme The active site* of the enzyme is made up of two definite regions i.e. the binding site and the catalytic site The binding site helps the enzyme in the recognition and binding of a proper substrate to produce ES complex and catalytic site catalyzes the transformation of the substrate into product(s) Enzyme detaches after catalysis from its products unchanged Enzyme requires aqueous medium for its activity Emil Fisher (1890) proposed Lock and Key Model to visualize substrate and enzyme interaction. o According to this model; as one specific key can open only a specific lock in the same manner a specific enzyme can transform only one substrate into product(s) o According to this model, the active site is a rigid structure and there is no modification or flexibility in the active site before, during or after the enzyme action and it is used as a template o Later study did not support this model in all reactions On the basis of new evidences Koshland (1959) proposed a modified form of enzyme and substrate interaction called Induce Fit Model o He argued that when substrate combines with an enzyme, it induces changes in the enzyme structure. o The change in structure enables the enzyme to perform its catalytic activity more effectively 3 Enzymes Dr. Sabir Hussain (Chapter # 3) F.Sc Biology FACTORS AFFECTING THE RATE OF ENZYME ACTION: Any factor that can alter the chemistry and shape of an enzyme can affect its rate of Catalysis. Some of the important factors that can affect the rate of enzyme action are: 1) 2) 3) 4) Enzyme Concentration Substrate Concentration Temperature pH of the medium 1. Enzyme Concentration: The rate of reaction depends directly on the amount of enzyme present at a specific time at unlimited substrate concentration If the amount of enzyme is increased by two fold the reaction rate is doubled No substrate is free for enzyme V is directly proportional to E By increasing the enzyme molecules an increase in the number of active sites take place More active sites will convert the substrate molecules into product(s) in a given period of time After a certain limiting concentration, the rate of reaction will no longer depend upon this increase 2. Substrate Concentration: At low concentration of substrate, the reaction rate is directly proportional to the substrate available 4 Enzymes Dr. Sabir Hussain (Chapter # 3) F.Sc Biology If the enzyme concentration is kept constant and the amount of substrate is increased, a point is reached where further increase in the substrate does not increase the rate of reaction any more The reason behind this constant rate of reaction is that at high substrate level all the active sites of the enzymes are occupied and further increase in the substrate does not increase the reaction rate 3. Temperature: The rate of enzyme controlled reaction may increase with increase in temperature but up to certain limit All enzymes can work at their maximum rate at a specific temperature called as optimum temperature For example, for human body optimum temperature is 37oC The reason behind this increase in rate of reaction is that heat provides activation energy and also supplies kinetic energy to the reacting molecules, causing them move rapidly due to which chances of their collision with each other increase However, further increase in heat also increases vibrations too violently of atoms which make up the enzyme molecule thus destroying the globular structure of enzyme (called denaturization) and reaction rate thus ultimately starts decreasing as shown below: 5 Enzymes Dr. Sabir Hussain 4. pH Value: Every enzyme (Chapter # 3) F.Sc Biology functions most effectively over a narrow range of pH known as the optimum pH A slight change in pH can change the ionization of the amino acids at the active site Moreover it may affect the ionization of substrate Under these Changed conditions enzyme activity is either retarded or blocked completely Extreme changes in pH causes bonds in the enzyme to break, resulting in the enzyme denaturation Enzyme Optimum pH Pepsin Sucrase Enterokinase Salivary Amilase Catalase Chymotrypsin Pacreatic Lipase 2.00 4.50 5.50 6.80 7.60 7.00-8.00 9.00 Enzyme Inhibitors An Inhibitor is a chemical substance other than substrate which can react with Enzyme in place of substrate but is not converted to the product(s) thus blocking the active site temporarily or permanently. For example, poisons like cyanide, antibiotics, anti-metabolites and some drugs. Types of Inhibitors: a) Irreversible Inhibitors b) Reversible Inhibitors Irreversible Inhibitors: They check the reaction rate permanently by occupying the active sites or destroying the globular structure 6 Enzymes Dr. Sabir Hussain (Chapter # 3) F.Sc Biology They block the enzyme’s active site physically or chemically by making covalent bond Reversible Inhibitors: They form weak linkages with enzyme An increase in substrate concentration can completely or partly neutralize their effect They are further of two major types: a) Competitive Inhibitors b) Non-competitive Inhibitors Competitive Inhibitors Because of structural similarity with substrate they may be selected by the binding sites but not able to activate active sites. Thus product(s) are not formed Non-competitive Inhibitors They form enzyme inhibitor complex at a point other than active site. They alter the structure of the enzyme in such a way that even if genuine substrate binds the active site, catalysis fails to take place The end These notes/handouts have been written keeping in view the course content of F.Sc premedical. All the necessary information accordingly has been provided omitting any unnecessary detail at this level. With Thanks, Sabir.H.N. 7