CatalyticMechanismofEnzymeReaction：酶反应的催化机理.doc

Catalytic Mechanism of Enzyme ReactionRemarkable properties of Enzymes as catalysts1. Catalytic power2. Specificity3. RegulationAd1. Catalytic Power- enzymes increase the rate of reaction by as much as 1012 fold- not many examples to compare between the rates of an enzyme-catalyzed reaction and the reaction occurring under similar conditions (temperature, pH etc) but in the absence of enzyme as it is too low to be measured- Comparison between enzymatic and non-enzymatic catalysts also difficult. As enzymatic catalysts occur :o Much higher rateso Lower temperatureIt is well illustrated by the process of nitrogen fixation (N2 ammonia) catalyzes by nitrogenase complex (300 K ,pH neutral)Industrial synthesis of ammonia from nitrogen and hydrogen temp 700-900 K, pressure 100-900 atm, with iron catalystAd2. Specificity- enzyme highly specific both in :o nature of substrateo the reaction they catalyze- the range of specificity varies between enzymeso low specificity the specificity base on bond specificity ex. Peptidase, phoshatase, esterase utilize a wide range of substrates which contain the required chemical bondmostly for degradative enzymes but not biosynthetic enzymes.o intermediate specificity group specificityex. Hexokinase catalyse the phosphorylation of a variety of sugars provided they are aldohexoseo absolute / near absolute specificity. only catalyse a reaction with a single substrate at an appreciable rateAd3. Regulation- The catalytic activity regulated by small ions or other molecules- Ex.The breakdown of glycogen in skeletal muscleCarbohydrate reserved degraded to generate ATP required for muscle contractionThe muscle contraction is triggered by release of Ca2+ from the sarcoplasmic reticulum also ensure the continuation of ATP productionFeedback inhibition phenomena common in many biosynthetic pathwayEx. Biosynthetic pathway leading to the synthesis of pyrimidine nucleotide the end product UTP and CTP block the first enzyme they are able to to limit the flow of metabolites into the pathway and regulate their own biosynthesisHOW ENZYME WORKING A catalyst works simply by lowering the energy barrier of a reaction, G Catalysts provide an alternative way the catalyst increases the fraction of molecules that have enough energy to attain the transition state, thus making the reaction go faster in both directions. The position of the equilibrium (the amount of product versus reactant) is unchanged by a catalyst. Catalysts lower the energy barrier in two ways:o The catalyst binds a substrate in an intermediate conformation that resembles the transition state, but has a lower energy. lead to multiple intermediate states that bypass the transition state. An intermediate state is a metastable state of a molecule.o In a non-catalyzed reaction the entropy may be highly negative due to the highly specific orientation required in order for a reaction to occur. Catalysts can lower the negative entropy by binding reacting molecules only in the proper mutual orientation, thus increasing their reactivityE + S ES EP E + PCollision TheoryMolecular velocity determines the binding of enzyme and substrate, thereby determine ES formation and Enzyme catalytic velocityTransient ES complex undergo INTRA-Molecular straining decreasing initial energy requirement for catalytic reaction to produce reaction productThe highest point of the energy profile is designated the transition state of the reactionIn all cases, the catalyst does not cause a shift in the equilibrium between reactant (s) and product (s) only increases the rate at which that equilibrium is attained.Various factors leading to the rate enhancements a. proximity and orientation effectsb. acid-base catalystc. covalent catalystd. strain or distortione. changes in environmentad.a proximity and orientation effectscommonly an enzyme could increase the rate of reaction involving more than one substrate by binding the substrates at adjacent sites and therefore cringing them into close proximity with each otherso reaction occur more readily Orientation with respect to each other influence the rate of reactionEnzymes make sure that the reactants are in the correct orientation as they approach each otherExample In this examples the restrictions placed on rotation of single bonds by bridge structure ensures that the preferred orientation of the reacting groups closely resembles that of the transition state of the reactionLess rotational entropy (degrees of freedom) occurs as the reaction proceed towards the transition stateThe smaller negative entropy of activation lead to an increase in the rate of reactionAdb. Acid-Base catalystMany reactions of the type catalyzed by enzymes are known to be catalyzed by acids and/or baseSince enzymes contain a number of amino acid side chains which are capable of acting as proton donors or acceptors acid-base catalysis importantAdc. Covalent catalysis (intermediate formation)Reactions can be speeded up by the formation of intermediates Many of the examples of covalent catalysis in enzyme-catalyzed reactions involve attack by a nucleophilic side chain at electron-deficient centre in the substrate nucleophilic catalysisAd.d Strain or distortionA substrate may be distorted on binding to the appropriate enzyme speed up the reaction if the distortion lowered the free energy of activation by making the geometry and electronic structure of the substrate more closely resemble the transition stateStrain also give a stabilization of the transition state of the reaction Enzymes make favorable contact with the transition state of the substrateAd5. changes in environmentThe rates of many organic reactions are highly sensitive to the nature of of the solvents in which they occurENZYME MODEL The induced fit model of enzyme action is a modification of the lock-and-key model originally proposed by Emil Fischer in 1894 The lock-and-key model proposes that an Enzyme/substrate pair is like a lock and key. Though it explains the specificity of enzyme /substrate pairs, it does little to explain catalysis, because a lock does not change a key the way an enzyme changes a substrate. In 1958, Daniel Koshland proposed the induced fit model to explain enzymatic catalysis proposed that distortion of the enzyme and the substrate is an important event in catalysis.Enzymes do more than simply bind and position substrates, however. Enzymes 1. Bind substrate(s); 2. Lower the energy of the transition state; and 3. Directly promote the catalytic event occur as a result of specific amino acid side chains that physically interact with the substrate and end up promoting the reaction.When the catalytic process is complete, the enzyme must be able to release the product or products and return to its original state for another round of catalysis.Triose Phosphate Isomerase CatalysisTriose phosphate isomerase catalyzes the following reaction: Glyceraldehyde-3-Phosphate (G3P) cis- enediol intermediate Dihydroxyacetone Phosphate (DHAP)The active enzyme is a dimer of two identical subunits. The active site (the place on the enzyme where catalysis occurs) can accommodate either G3P or DHAPAt the active site, a glutamate residue (Glu 165) and a histidine (His 95) are essential for function of the enzyme. The reaction steps : E + G3P E-G3P (Binding of G3P) E-G3P E-ed (Conversion to enediol) E-ed E-DHAP (Conversion to DHAP) E-DHAP E + DHAP (Release of DHAP)Like other enzymes, triose phosphate isomerase lowers the energy barriers of the transitionStatesBoth triose phosphate isomerase and serine proteases have a histidine and an acidic residuein their active site. Histidine is very common in active sites, because it readily accepts or donatesprotons at physiological pH.Multisubstrate Reactions Most biochemical reactions involve two or more substrates, often resulting in multiple products. An example is proteolysis, which involves two substrates (the polypeptide and water) and two products (the two fragments of the cleaved polypeptide chain). When an enzyme binds two o