Propertiesof Enzyme:Proteins that work as biological catalysts, calledenzymes. Enzymes increase speed of metabolic reactions. Low contamination, lowtemperature and increased metabolism are only possible with enzymes.
Metabolismis increased, with the product made to a high degree of purity. General Properties: Enzymes have following general properties: Protein Nature of Enzymes: Most of enzymesare globular proteins. some enzymes like ribozymes are nucleic acid in nature.Globular structure of proteins is very important for proper works of enzymes.
• Composed of C, H, O and N. Sulphur may also bepresent. • One or more polypeptide chains – large number oflinked amino acids. • Formed on the ribosomes – translation of mRNAduring protein synthesis. • Destroyed by high temperature and unfavorable pH.
CatalyticProperties: Enzymes speed up the chemical reactions. Theyare not consumed in chemical reaction. An enzyme cannot start the reaction itcan only increase the speed of a reaction. The efficiency of an enzyme reactionis expressed form of turner number. The number for sucrose’s is 105and for catalases is 106. Folded Shape of Enzymes :• The polypeptide chains are folded into aspecific three-dimensional shape. • The correct foldedshape is important for enzyme action ‘tertiary structure’.
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• The shape gives theenzyme special areas known as active sites. • The compatiblesubstrate molecules bind to the complement active site. • Different enzymeshave a differently shaped active site.• Higher temperatures morekinetic energy, so more collisions Role of Enzymes in Living Things: Enzymes catalyze allmetabolic reactions. • They lower the activation energy – theenergy input needed to bring about the reaction. • Regulate thethousands of different metabolic reactions in a cell and in the organism. • The activity of a cell is determined bywhich enzymes are active in the cell at that time. • Cell activity is altered by removingspecific enzymes and/or synthesizing new enzymes.
Active Site Theory :”Lock and KeyHypothesis and Induced Fit” • The enzyme’s activesite has a shape complementary to the substrate. • The substrate locks into the active site ofthe enzyme. • The active site alters its shape holding thesubstrate more tightly and straining it. • An enzyme-substrate complex is formed. • The substrate undergoes a chemical change –a new substance is formed. • The product isreleased from the active site. • The free unaltered active site is ready toreceive fresh substrate.
Solubility: Enzymes as proteins are soluble in water ordilute salt solution Molecular weight :Enzymes have high Mw(varying from 10000 -several thousands)Enzymes have buffering capacity: They are amphoteric molecules i.e. behaveboth as acids and bases. At pKa they makethe most efficient buffer.Each enzyme has a specific isoelectricpH: It isthe pH at which the net charge on protein equal to zero –so they do not move inan electric field. It is the pH at which the protein molecule carries equal positiveand negative charges Above PI -negatively charged can move in an electric field Below PI -positive chargedand can move under an electric field.Denaturation: When proteins are heated , or subjected toextremes of temperature, high salt, organic solvents etc, the non-covalentbonds break, changing the native structure to random coil.
This unfolding ofprotein is due to loss of secondary, tertiary and quaternary structure. It doesnot affect primary structure. Effect of Denaturation: Loss of activity due to loss of shape andactive site.
Denaturing Factors: •Heat •Change in pH •Radiation •Heavy metals •Detergents •Digestive enzymes •Urea •Repeated freezing and the wing Chemical reaction:(i)Xanthroprotoicreaction:Proteins containingphenylalanine end tyrosine—give orange color.(ii)Mallon’sreaction: Proteins containingphenolic(-OH) group of tyrosine—give red color(iii)Sulfate reaction: Proteins containingsulfur amino acids (cysteine) —-give black or grey color(iv)Biuret’sTest:•This is a general testfor all proteins because it O is given by peptide linkage ( C -N) H•The reaction occursbetween protein, sodium hydroxide and copper sulfate giving violet complex.Specificity: The specific of enzyme is due to primaryamino acids sequence.
e.g. sucrose acts upon sucrose and lactase on milk sugar.
•Enzymes are highlyspecific. They are specific for: •the reactions they catalyze. •andin their course of reaction, which are called substrates.
(a)Absolute specificity: The enzyme can act onlyon one specific substrate.(b)Groupspecificity:•Broad specificity •Enzyme act on a groupof related substrates. •The substrates have a common group on whichthe enzyme acts: e.g. -esterase can act on different esters -proteases can acton different protein e.g. of proteases: chymotrypsin, trypsin, pepsin.
The specificity is dueto substrate binding site which lies on the enzyme surface -in specification isdue to the specific arrangement of amino acids in the active site thatparticipate in the bond making and bond breaking. Thespecification of an enzyme is determined by:(a)Workal groups of enzymes(b)Workal groups ofsubstrate During enzyme action,there is a temporary combination between enzyme and its substrate formingenzyme by relatively weak forces. This occur at the active site of the enzyme(most substrates are bound to the enzyme by relatively weak forces. E + S à ES complex Activationenergy: is minimum energy that isrequired to start a reaction. Enzymes lower the activation energy.
The reactioncan take place in the absence of enzyme. But it need higher amount ofactivation energy. ProductInhibition: The accumulation of products ofthe reaction can inhibit the enzyme activity. The effect is very important forcell. It maintain the concentration of the product in a cell.Allosterism: Some enzymes have specificallosteric site for binding of certain effecters.
The binding of effecterschange in the structure of enzyme. Allosteric is shown by many enzymes. Sensitivityof Enzyme: Enzymes are sensitive for following factors:a) HighTemperature: b) Enzymes are sensitive for heat. Activity ofenzymes increase with the increase of temperature of to 50 degree. The enzymesare denatured at 70 to 100 degree. Such conditions are present in spore and dryseeds.
c) b)Low Temperature: d) Enzymes are also sensitive for lowtemperature. They are inactivated but not destroyed at 0 degree. e) Radiation: f) Enzymes are also destroyed by radiation oflower wavelength such as ultraviolet rays and x-rays.pH: Each enzyme has optimum pH.
Its activityslows down with increase in pH. Catalases and Amylose show optimum activity inneutral solution. surcease and Ligase act in acidic medium.
Trypsin act inalkaline medium. Reversibility of enzyme:The majority of reaction catalyzedby enzymes are reversible. Thus an enzyme can speed up a reaction in bothdirections. Thus system state in equilibrium in a short time. In guard cell ofstomata the enzyme starch phosphorylase convert into starch and inorganicphosphate into glucose. The direction of reaction depend upon several factors.
It depend upon pH and chemical potential of both reactions. Synthesis of starchprotein and fats are irreversible.
