Enzymology PDF

Enzymology Enzymes: The biological catalysts”  They are organic thermo-labile catalysts that increase the chemical reaction without change.  They accelerate the rate of chemical reaction without being consumed in the reaction. Chemical Nature of Enzymes  All enzymes are protein in natu...

Enzymology Enzymes: The biological catalysts”  They are organic thermo-labile catalysts that increase the chemical reaction without change.  They accelerate the rate of chemical reaction without being consumed in the reaction. Chemical Nature of Enzymes  All enzymes are protein in nature except ribozymes (RNA in nature).  Protein enzymes are classified into 2 types: 1- Simple Protein enzymes: They are formed of protein only. 2- Complex (conjugated) Protein : They are formed of protein part and non protein part. 2- Complex (conjugated) Protein : enzymes formed of two parts: 1) Protein part: called apoenzyme 2) Non- protein: called cofactor  The whole enzyme is called holoenzyme. The cofactor may be coenzyme or prosthetic group  Coenzyme: Is organic, thermo-labile , loosely attached to enzyme.  They are mainly vitamin B derivatives e.g. FAD, NAD.  Prosthetic group: Is inorganic, thermo- stable, firmly attached to enzyme.  They are usually metal ions e.g. Ca, Zn Enzymes vocabulary 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 Active Site(catalytic site)  A restricted region of an enzyme molecule which binds to the substrate. Active Site It is formed from Amino acids sequences in the polypeptide chain. 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) Enzyme action 1- Enzymes increase the rate of reaction by decreasing the activation energy of reaction. 2- The activation energy is the energy barrier between reactants and products. Enzymes increase the rate of reaction by:  It decreases the energy needed for activation (activation energy).  It decreases the energy barrier between reactants and end products. Factors Affect Rate of Enzyme Action:  Enzyme concentration  Substrate concentration  Temperature  pH  Concentration of coenzymes  Concentration of ion activators   Time  Inhibitors 1- Effect of enzyme concentration The rate of enzyme action is directly proportional to the concentration of enzyme provided that there are sufficient supply of substrate & constant conditions. 2- Effect of substrate concentration -The rate of reaction increases as the substrate concentration increases up to certain point at which the reaction rate is maximal (Vmax.) At Vmax, the enzyme is completely saturated with the substrate any increase in substrate concentration doesn't affect the reaction rate. Michaelis constant (Km)  It is the substrate concentration that produces half maximum velocity of enzyme  Enzymes with low Km: have high affinity to the substrate i.e. they act at maximal velocity at low substrate concentration  E.g. Hexokinase acts on glucose at low concentration (fasting state) Enzymes with high Km: they have low affinity to the substrate i.e. they act at maximal velocity at high substrate concentration  E.g. Glucokinase enzyme acts on glucose at high concentration (fed state) 3- Effects of temperature - Rate of reaction increases gradually with the rise in temperature until reach a maximum at a certain temperature, called optimum temperature - The optimum temperature is 37-40 C in The effect of temperature on reaction rate is due to: 1- Increase of temperature increase the initial energy of substrate and thus decrease the activation energy 2- Increase of collision of molecules: more molecules become in the bond forming or bond breaking distance. After the optimum temperature, the rate of reaction decrease due to denaturation of the enzyme (60-65 C). 4- Effect of PH - Each enzyme has an optimum PH at which its activity is maximal  E.g. Optimum PH of pepsin = 1.5 - 2  Optimum PH of pancreatic lipase = 7.5 - 8  Optimum PH of salivary amylase = 6.8 “Change of PH above or below optimum PH decrease rate of enzyme action due to: 1- The enzyme activity depends on the ionization state of both enzyme and substrate which is affected by PH. 2- Marked change in PH will cause denaturation of enzyme. 5- Concentration of coenzymes: In the conjugated enzymes that need coenzymes, the increase in the coenzyme concentration will increase the reaction rate. 6- Concentration of ion activators:  The increase in metal ion activator increase the reaction rate Enzymes are activated by ions: 1- Chloride ion activate salivary amylase 2- Calcium ion activate thromobokinase enzyme. 7- Effect of time:  In an enzymatic reaction, the rate of reaction is decreased by time.  This is due to: 1- The decrease in substrate concentration. 2- The accumulation of the end products. 3- The change in PH than optimum PH. 8- Presence of enzymes inhibitor:  presence of enzyme inhibitor decreases or stops the enzyme activity. Enzyme inhibitors may be: 1- Competitive inhibitors. 2- Non competitive inhibitors Competitive inhibitors Competitive inhibitors are molecules which are very similar to the enzymes natural substrate, and thus compete for the active site. As a result, the the inhibitor binds to the active site and remains their, preventing further reactions. The enzyme may react with the inhibitor and release the products as it would usually do to its substrate, thus the inhibitor and substrate compete for the active site. Non-Competitive inhibitors bind to an allosteric site of the enzyme (A site on the enzyme which is not the active one). This results in a conformational change of the protein, distorting the active site and thus is unable to bind the substrate. So long as the non-competitive inhibitor is bound, the enzyme remains inactive. Biomedical importance of Enzymes