enzymes.pdf

ENZYMES  Enzymes: “The biological catalysts” An ENZYME is a biological catalyst with protienous nature which speed up the biochemical reaction. They are organic catalysts that increase the bio-chemical reaction without change. They accelerate the rate of biochemical react...

ENZYMES  Enzymes: “The biological catalysts” An ENZYME is a biological catalyst with protienous nature which speed up the biochemical reaction. They are organic catalysts that increase the bio-chemical reaction without change. They accelerate the rate of biochemical reaction without being consumed in the reaction.  Classification of enzymes 1- Hydrolases 2- Oxidoreductases (Oxidation reduction enzymes ) 3- Lyases 4-Transferases 5- Ligases 6-Isomerases  Hydrolases These enzymes catalyze the hydrolysis reaction by adding water to cleave the bond and hydrolyze the substrate. It subdivided into three classes: a)- Carbohydrase ex. Sucrase and diastase b)- Protease ex. Pepsin and trypsin c)- Esterase ex. Lipase and Chlorophyllase AB + H2O → AOH + BH  Oxidoreductases These enzymes catalyze oxidation/reduction reactions; transfer of H and O atoms or electrons from one substance to another. AH + B → A + BH (reduced) A + O → AO (oxidized) It subdivided into four classes a)- Oxidase b)- peroxidase c)-Dehydrogenase 2 H2O2 2H2O + O2 d)-Catalase Hydrogen Peroxide water + oxygen  Lyase Lyases are enzymes catalyze the breakage of bonds by removal of groups from substrates without addition of water. ex. Zymase is an enzyme complex that catalyzes the fermentation of sugar into ethanol and carbon dioxide. RCOCOOH → RCOH + CO2 ex. Aldolase (an enzyme in glycolysis) catalyzes the splitting of fructose-1, 6-disphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Aldolase Fructose- 1,6- diphosphate dihydroxyacetone phosphate + phospoglyceradehyde  Isomerase These enzymes catalyze geometric or structural changes within one molecule. They catalyze the formation of an isomer of a compound. Example: phosphoglucoisomerase catalyzes the conversion of glucose-6- phosphate to fructose-6-phosphate.  Ligases Ligases catalyze the association of two molecules. Join together two molecules by synthesis of new C-O, C-S, C-N or C-C bonds with simultaneous breakdown of ATP. X + Y+ ATP → XY + ADP + Pi Ex: Asparagine synthetase RNA Ligase  Transferases These catalyze transferring of the chemical group from one compound (generally regarded as donor) to another compound (generally regarded as acceptor).  examples are transaminase, which transfers an amino group from one molecule to another and Hexokinases, which transfers phosphate group from one molecule to another AB + C → A + BC  Enzyme Nomenclature There are many methods for naming enzymes: 1- The old trivial name as pepsin and trypsin. 1- according to the name of substrate by adding the suffix –ase to the end of the substrate for example; sucrose sucrase lactose lactase 2- according to the type of the reaction which the enzyme catalyse and adding also, the suffix – ase to the end of the reaction for example carboxylation carboxylase 3- According to Enzyme Commission Enzyme Code (EC): Each enzyme has a numerical code which is formed of four digits separated by dots: The first digit denotes the class (reaction type) of the enzyme. The second digit denotes the functional group upon which the enzyme acts. The third digit denotes the coenzyme. The fourth digit denotes the substrate. For example : E.C. 1.1.1.1 enzyme 1 means oxidoreductase, 1.1 means that the functional group is hydroxyl group (OH-), 1.1.1 means NAD is the coenzyme and 1.1.1.1 means alcohol. So, 1.1.1.1 means alcohol dehydrogenase enzyme  Structure of enzymes Protein enzymes are classified into 2 types: 1- Simple Protein enzymes: They are formed of protein only. 2- Complex (conjugated) Protein enzymes : They are formed of protein part and non protein part. Complex (conjugated) Protein enzymes : enzymes formed of two parts: 1) Protein part: called apoenzyme 2) Non- protein: called cofactor or co-enzyme The whole enzyme is called holoenzyme There are three kinds of cofactors present in enzymes: Prosthetic groups: These are cofactors tightly bound to an enzyme at all times. e.g. FAD Coenzyme: A coenzyme binds to an enzyme only during catalysis. At all other times, it is detached from the enzyme. e.g. NAD+ Activator: Is inorganic, firmly attached to enzyme. They are usually metal ions e.g. Zn2+, Ca2+ Substrate: reactant which binds to enzyme enzyme-substrate complex: temporary association Product: end result of reaction Active site: enzyme’s catalytic site; substrate fits into active site  Characteristics of Enzymes Enzymes are only needed in small amounts. The small quantity of enzymes catalyses the larger quantities of substances. Sensitivity (Enzymes are sensitive to temperature and pH) All enzymes are protein in nature. Specificity (Enzymes are highly specific in nature, i.e., a particular enzyme can catalyse a particular reaction) it can not be consumed during the biochemical reactions enzymes have a colloidal nature  Mechanism of enzyme action 1- The substrate (S) binds to the enzyme (E) at its active catalytic site to form activated intermediate enzyme substrate complex (ES). 2- The activated complex (ES) cleaved to the products (P) and the original enzyme (E)  Factors affecting on the rate of enzyme activity 1- Substrate concentration 2- Enzyme concentration 3- Temperature 4- pH 5- Inhibitors  1. Temperature The rate of an enzyme catalysed reactions increases with the increase in temperature up to a maximum and then falls.  2. Hydrogen ion concentration (pH) Enzyme activity is also affected by pH. Each enzyme has its unique optimum pH at which the rate of reaction is greatest. The optimum pH is the pH at which the activity of a particular enzyme is at a maximum. Many enzymes of higher organisms show optimum reaction rate around neutral pH (6-8). However, there are several exceptions such as pepsin (pH 1-2). Below and above the optimum pH, the enzyme activity is much lowered and at extreme pH, the enzyme becomes totally inactive.  3. Substrate concentration The substrate concentration also influences enzyme activity. As the substrate concentration increases the rate of reaction also increases. This is because the more substrate molecules will interact with enzyme molecules, the more products will be formed. However, after a certain concentration, further increase in substrate concentration will have no effect on the rate of reaction, since the substrate concentration will no longer be the limiting factor. At this stage, enzyme molecules become saturated and work at their maximum possible rate.  4. Enzyme concentration When enzyme concentration is low, the reaction is slower. As enzyme concentration increases, the reaction is faster up to a point when the amount of substrate available becomes limiting.  5. Inhibitors Enzymes’ activity can be inhibited in a number of ways: Competitive inhibitors – a molecule blocks the active site so that the substrate has to compete with the inhibitor to attach to the enzyme. Non-competitive inhibitors – a molecule binds to an enzyme somewhere other than the active site and reduces how effectively it works.