生物化学ppt---生物酶.ppt

单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,单击此处编辑母版标题样式,Chapter 3 Enzymes,Molecular structure and function of enzyme,Characteristic and Mechanism of Enzyme-Catalyzed Reaction,Kinetics of Enzyme-Catalyzed Reaction,Regulation of Enzyme,Naming and Classification of Enzyme,What are enzymes?,Enzymes are biologic polymers that catalyze the chemical reactions.,Biological catalysts,The vast majority of enzymes are,proteins,.,RNA:,ribozyme,DNA:,DNAzyme,Enzyme,E,Substrate,S,Product,P,1,Molecular structure and function of enzyme,Molecular component,Active site,Simple enzymes:,enzymes require no other chemical groups other than their amino acid residues for activity.,trypsin,chymotrypsin,ribonuclease,A,Conjugated enzymes:,enzymes,contain chemical groups other than AA,the non-amino acid parts are usually called,prosthetic groups(coenzymes&cofactors),and the protein part alone called,apoprotein,.,1.,Molecular component,enzyme,Prosthetic groups could be inorganic metal ions(e.g.,Fe,2+,Mg,2+,Zn,2+,)or complex organic molecules called coenzymes.,A complete catalytically active enzyme(including its prosthetic group)is called a,holoenzyme,.,holoenzyme,=,apoprotein,+prosthetic group,Coenzymes often function as transient carriers of specific(functional)groups during catalysis.,Many vitamins,organic nutrients required in small amounts in the diet,are precursors of coenzymes.,Coenzymes,VI,Substrates are bound to,a specific region,of an enzyme called the active site.,The active site is,a three-dimensional conformation,formed by groups that come from,different parts,of the linear amino acid sequence.,Active site contains,catalytic groups,(participate in the catalytic processes)and,binding groups,(specificity of binding).,2.Active site,Active site,Catalytic groups,Binding groups,Substrate,Other essential groups,Ser:OH,Cys,:SH,His:,imidazole,Active site is,cleft,or,pocket,with a generally,nonpolar,character.,In many enzymes,the active site has shape complementary to those of their substrates only after the substrates are bound(,the induced fit,).,For conjugated enzymes,the active site always contains,coenzymes,.,3.,Isozymes,*,Distinct forms that catalyze the same reaction,differ in molecular components and physical&chemical properties.,Two or more polypeptide chains,Different polypeptide chains are products of different genes,differ in AA sequence.,Are tissue specific,LDH,Heart,Muscle,Lactate,Dehydrogenase,LDH is a tetramer of two non-identical subunits,Cellular/Plasma specific activities of various tissues,Tissue,Heart,Kidney,RBC,Brain,WBC,Skeletal,Muscle,Liver,Diagnostic Enzyme Analysis,Normal serum,Myocardial infarction,Acute hepatitis,2,The C,haracteristic,and Mechanism of E,nzyme-,C,atalyzed,R,eaction,Characteristics of enzymes,Mechanism of E,nzyme-,C,atalyzed,R,eaction,1.Characteristics of enzymes,The two most striking characteristics of enzymes,High efficiency,High,specificity,Be regulated,High efficiency,Urease,is a good example:,Catalyzed rate:3x10,4,/sec,Uncatalyzed,rate:3x10,-10,/sec,Ratio is,1x10,14,!,Most reactions in biological systems do not occur at perceptible rates in the absence of enzymes.,Enzymes accelerate reactions,by factors of at least a million.,The rate enhancements(rate with enzyme catalysis divided by rate without enzyme catalysis)brought about by enzymes are often in the range of,10,7,to 10,14,What Enzymes Do.,Enzymes accelerate reactions by,lowering activation energy,Enzymes do this by binding the,transition state,of the reaction better than the substrate,Enzymes,like all other catalysts,do not affect reaction,equilibria,only accelerate reactions.,G,F,The rate constant of a reaction(k),is related to the free energy difference between the transition state and the ground state of the substrate(,G,F,),Activation energy,Transition state,is a fleeting molecular moment(not a chemical species with any significant stability)that has the highest free energy during a reaction.,G,F,An enzyme increases the rate constant of a reaction(k)by lowering its,G,F,.,G,F-,uncat,G,F-cat,The combination of a substrate and an enzyme creates a,new reaction pathway,whose,transition state,energy is lower than that of the reaction in the absence of enzyme.,G,F-,uncat,G,F-cat,High specificity,Enzymes are highly specific both in the,reaction,catalyzed and in their choice of,substrates,.,An enzyme usually catalyzes a single chemical reaction or a set of closely related reactions.,Low specificity,:some peptidases,phosphatases,etc.,Intermediate specificity,:trypsin,chymotrypsin,etc,Absolute or,stereochemical,specificity,:,urease,succinate,dehydrogenase,etc,stereochemical,specificity:lactate,dehydrogenase,Enzymes exhibit various degrees of specificity,Weak interactions,between substrates and enzymes may generate,conformational changes,on the enzyme.,Active site,Induced fit,2.Mechanism of E,nzyme-,C,atalyzed,R,eaction,Amino acids in general acid-base catalysis,S,E,T,pH,Inhibitor,Activator,3,Ki,netics of Enzyme-Catalyzed Reaction,V,i,was measured at the beginning of the enzyme-catalyzed reaction,when substrate concentration can be considered constant(S will decrease as the reaction progresses),Just change one factor,Initial velocity(V,i,),1.S,At early times in the reaction,the concentration of the product(P)is negligible and the overall reaction can be written as(,no reverse reaction,),k,1,k,2,E+S,ES E+P,k,-1,V,i,rectangular hyperbola,The,Michaelis-Menton,equation,i,Michaelis,18751949,Menten,18791960,The,Michaelis-Menten,Equation,It assumes the formation of an,enzyme-substrate complex,It assumes that the ES complex is in,rapid equilibrium,with free enzyme,Breakdown of ES to form products is assumed to be,slower,than 1)formation of ES and 2)breakdown of ES to re-form E and S,K,m,K,m,equals to the,substrate concentration,at which the reaction rate is half its maximal value(,V,i,=V,max,/2,).,K,m,is a,constant,K,m,=(k,-1,+k,2,)/k,1,k,2,E,，,V,max,=k,3,E,3.T,Optimal T,Optimal pH or pH range,4.pH,Hydrogen ion concentration,5.Inhibitor,Reversible versus Irreversible,inhibitors,Reversible inhibition,:inhibitors interact with an enzyme via,noncovalent,associations,Irreversible inhibition,:inhibitors interact with an enzyme via,covalent,associations,Reversible inhibition,Competitive inhibition,-inhibitor(I)binds only to E,not to ES,Noncompetitive inhibition,-inhibitor(I)binds either to E and/or to ES,Uncompetitive inhibition,-inhibitor(I)binds to ES,Competitive inhibitors are often compounds that,resemble,the substrates,the inhibitor,competes with the substrate for the active site,(binding of one prevents binding of the other,forming ES or EI complexes),Competitive inhibition,v,i,1,1,V,max,is not affected.,K,m,is increased.,Double reciprocal plot,Succinate,dehydrogenase,succinate,malonate,The inhibitor binds to,a site distinct from,the active site which binds the substrate.,Inhibitor binding,does not affect,substrate binding and vice versa.,Noncompetitive inhibition,V,i,1,V,max,is lowered.,K,m,is not affected.,Double reciprocal plot,The inhibitor binds at a site distinct from the substrate active site and,but binds only to the ES complex.,Uncompetitive inhibition,V,max,is lowered.,K,m,is,decreased,.,Double reciprocal plot,Many irreversible inhibitors modify essential groups covalently,thus inactivating the enzymes.,Irreversible inhibition,Inactive enzyme,Aicd,Active enzyme,Phosphide,5.Activator,Essential activator,Non-essential activator,4,Regulation of enzyme,The enzymatic activity of some enzymes are precisely and tightly regulated in living organisms to meet physiological requirements.,Regulation of enzyme activity,Regulation of enzyme quantity,Isoenzyme,1.Regulation of enzyme activity,Allosteric,regulation,Covalent modification,Zymogens and activation of zymogens,(1),Allosteric,regulation,*,Allosteric,enzymes are regulated by,reversible,noncovalent,binding of,modulators,(often being metabolites).The modulators usually bind,not to the active site,but to another specific,regulatory site,.,Modulators:,allosteric,effectors,The enzyme,activity,varies when the concentration of the,allosteric,effectors,vary.,Enzymes situated at,key steps,in metabolic pathways are,modulated,by,allosteric,effectors.,These effectors are usually produced elsewhere in the pathway.,Effectors may be,positive,or,negative,.,V,i,Kinetics of,allosteric,enzymes are sigmoid,Feedback inhibition,i.e.,building up of a pathways,end product,ultimately slows the entire pathway,is often realized through,allosteric,enzymes,.,The enzyme catalyzing the,first step,of a synthetic pathway is often an,allosteric,enzyme.,Feedback inhibition,The activity of many enzymes are regulated by,reversible covalent,modifications.,Phosphorylation/dephosphorylation,the most common reversible covalent modification,is a highly effective means of switching the activity of target enzymes.,(2)Covalent modification,Protein,phosphatases,catalyze the hydrolysis of,phosphoryl,groups attached to proteins,thus reversing the effects of,kinases,.,Proteinkinases,catalyze the transfer of a phosphate group from an ATP molecule to the side chains of Ser,Thr,or,Tyr,residues in proteins.,Many enzymes are synthesized as,inactive precursors,called,zymogens,.,These enzymes are,activated,by,specific,proteolytic,cleavage,of one or several specific,peptide bonds,.,(3)Zymogens and activation of zymogens,Primary structure,changs,and then,3-dimensional conformation,changs,that form the,active site,.,Protect the tissue of origin from,autodigestion,.,Facilitate rapid mobilization of activity.,Both the synthesis and degradation of the certain important enzymes are tightly controlled.,2.Regulation of enzyme quantity,Synthesis and degradation,5,The Naming and classification of enzyme,Enzymes are classified by the substrates or the reactions they catalyze.,Common name,International classification,Many enzymes have been named by adding the suffix,“-,ase,”to the name of their substrate or to a word or phrase describing their activity(type of reaction).,Common name,Lactate,dehydrogenase,succinate,dehydrogenase,urease,ribonuclease,A,phosphatase,etc.,Enzymes are classified into,six major classes,by international union of biochemists(IUB).,Each enzyme is assigned a,four-digit number,with the first digit denoting the class it belongs,the other three further clarifications on the reaction catalyzed.,Hexokinase,:E.C.2.7.1.1,Class 2,transferase,Each enzyme is given a systematic name which identifies the reaction catalyzed.,Hexokinase,-ATP:D-hexose-6-phosphotrasferase,6,The Relation of Enzyme and Medicine,Occurrence of disease,Diagnosis of disease,Therapeutics of disease,summary,Enzymes,produced by living cells are proteins which have,high specificity and efficiency,in catalytic reactions.,Simple enzymes,are composed solely by amino acids,while,prosthetic enzymes,(,holoenzymes,),additionally require,nonprotein,components,called,prosthetic groups,.,Active site of enzyme,.,Enzyme kinetics,studies on the factors which affect the velocity of enzyme reaction.,They includes,T,pH,S,and the inhibitors.,The effect of S on enzyme velocity can be described by the,Michaelis-Menten,equation.K,m,the constant of,Michaelis-Menten,equation,K,m,equals to the substrate concentration at V,i,=V,max,/2.,In,competitive inhibition,reactions,the apparent K,m,is increased,while,V,max,is not affected.,In,noncompetitive inhibition,reaction,K,m,is not affected,while,V,max,is lowered.,Isozyme,;,Allosteric,regulation;Covalent modification;,Zymogen,