Enzyme Mechanism PDF

MECHANISM intermediate This can Nowwillwe (enzyme) be simply unstable representeddiscuss ubstrate) how OF nzyme-substrate S ENZYME...

MECHANISM intermediate This can Nowwillwe (enzyme) be simply unstable representeddiscuss ubstrate) how OF nzyme-substrate S ENZYME by act. Precisely enzymes the enzyme plex) trate complexACTION following Unstable [ES] which equation: immediately speaking, there nzyme) breaks E is roduct) into formation P the product. of an NIYNES 7.17 Ihe threedimenstonal potein molecule of an enzyme posews specific sites, known as Mik Niten 0r ative centerr, on itn Nurface which bind the substrate molecule, Cenerally, umber of niten in one por polypeptide chain constituting a protein molecule. The enzyme much larger in slz than the Nubtrate and, therefore, the latter the enzyme Nurface which forms the catalytic nite consisting, of aoceupies only a hs The few amino acid ative nite iv varmarked intotwo Npecific areas: Binding nite-where the Nubstrale binds and A) Catalytic site where the enzyme catalysin takwN place. active in catalytie action of blochemical reaction, acomplex altor interactlons with the They act on substrate and enzyme is ahstrate forms a conmplex at the active centre, This called active center. The enzyme and astrale stable. The interaction between binding action makes both enzyme and hd hydrogen bonda or Van der Waal Nubstrate and enzyme may be either ionic bonds ups such av NIl, COOW, SH etc. which forceN, "The active sites of enzyme have some transitional (intermediate) compound calledbind the Nubstrate though above bonds tospecial form slhydryl (-S)groupN in the activecenters in enzyme-substrate complex (ES). Presence of etween an enzyme and its substrate is like acharacteristic of many enzymes, The relationship and turns the levers to open or lock and it's key. Just as a key fits close it, sO a substrate molecule fits into into a lock enzyme olecule to form an the active centre of ative site are tyrosine, histidine,enzyme-Nubstrate complex. The amino acids present at the cysteine, glutamic acid, aspartic acid, lysine aldolase, lysine iN preNent at the active site, In and serine. In has two carboxypeptidase, Preent at the active sile. two tyrosine residues RibonucleaNe Michaelis and Menten vstablished the histidines at the active site. to form the theory of combination of substrate on enzyme-Nubstrate which it acts to complex. Aceording to this, the enzyme enzyme with substrate form an lberated and the Nubstrate iN broken enzyme-substrate complex. combines with the E|Enzyme) + S down into the product of the Then, this enzyme is |Substrate) ES reaction. The ES complex Is also The called as 'M(Enzyme-Subst ichaelis Menten rate complex --E + Product opening binding causes of the bonda and transfornmation of the substrate complex' , the product is formation of new bonds. As molecule to the product through released, because it no longer fits into the these changes are returns to its original sOon as The vnzynme reaction can form. can now bind another accomplished, active site and the enzyme be substrate molecule at its activemolecule diagrammatically represented a in shown Fig. 7.3. centre. Substrate0  FUNDAMENTALS OF 7.18 BIOCHEMISTRY AND As the enzyme molecule remains unchanged after the can turn over a large amount of substrate to product. Howeve. reactions are theoretically reversible, the forward reaction can continue he reaction, asmall because tillall MICROBI of the product in the enzyme reaction mixture is so high that an at equilibrium, the rate of forward reaction equals to that of theequilibrium is Enzymes accelerate the rate of chemical reaction by four reverse at aine major erenzaycmte-icCooantn.denta 1. Proximity and Orientation: The enzyme binds to the substrate the susceptible bond is in close proximity to the catalytic group and in such viz. to it resulting in the catalysis. also mechanisms 2. Strain and Distortion or Induced Fit Model: Binding of the precisely conformational change in the enzyme molecule which strains the shape of substratacteivienduces also distorts the bounded substrate, thus substrate to the enzyme will bring about a bringing about the catalysis. Thethe enzyme molecule, which destabilizes the enzyme. distorts the substrate thereby forming the change in the tertiary or quaternary structureof In order to attain reaction product. be bi stability, the enzy nding 3. General Acid-Base good proton donors or proton Catalysis: The active site of the enzyme has 4. Covalent aminoof acids hat a acceptors, this result in acid-base catalysis covalently joined Catalysis: Some enzymes react with their substrates to the substate products. enzyme-substrate form very unstabe complexes, which undergo turther reaction to fom Like chemical catalysts, the activation energy. enzymes accelerate the rate of molecules to a higher This energy refers to the amountbiochemical reactions by lowern energy broken to yield the product level required to bring the reactat (activated state) where a chemical or products. bond may be formed r condition, are in a they are energy-rich and When the reactant molecules reach this activaed transition state. In enzyme reactions, the enzyme-substrate complex state. represents the transition Activation energy barrier system Activation Free energy of effect of catalysis:activation A and energy of thei uncatalyzed! reaction like substrate to chemical of reaction will take place when a product, energy number of substrate certain TActivation ienergy of the any instant, possess molecules at Free Tnftial state Icatalyzed to attain an enough energy reaction called the activated condition "transition which the probability of state" orin free Overall energy !i breaking a the productchemical bondmaking to change is very high. form energy of activation" is the "Free of energy required amount to bring all the Final state at molecules in one substrate gram mole at a given of a Progress of reaction equilibrium state temperature the Fig. 7.4. Free transition (Fig. 7.4). to energy of catalysis.activation and effect of ENZYMES 7.19 In presence of a catalyst, the substrate combines with it to produce a transient state ing a lower energy of activation than that of the substrate alone. This accelerates the reaction. Once the product is formed, the enzyme (catalyst) is free to combine with another loule of the substrate and repeat the process. Though there is a change in the free energy nolecule dactivation in presence of an enzyme, the overall free energy change of the reaction remains Ae same whether the reaction is catalyzed by an enzyme or not. The enzyme functions to lower the amount of energy required to bring the substrate to the transition state as shown in Fig.84. An example may be cited to explain the significance dactivation energy. Amixture of hydrogen and oxygen will remain unchanged indefinitely, although they have the ability to combine producing water. An electric spark will bring them o the transition state and they combine with release of energy. Here, the electric spark provides the activation energy. The same reaction is catalyzed by an enzyme,called hydrogenase, atordinary temperature, because the enzyme lowers activation energy.