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  • Enzymes

  • contentsIntroduction of enzymesCatalytic Mechanisms of enzymesEnzymatic reactionkinetics

  • Enzymes are biocatalysts with high specificity and highly catalytic efficency produced by living cells. Besides proteins, ribozymes and deoxyribozymes are also enzymes. What is enzyme? Introduction of enzymes

  • Structure and Function of Enzymes

  • Structure of enzymes1.Composition of Enzyme MoleculesHoloenzyme = Apoenzyme + Cofactors

  • 2. Active site of an enzymeIt is an area of enzymeIt is consist of a few amino acids residuesThe residues are closed to each other in spaceIt combines substrate and catalyzes a reaction.

  • substrate Essential groups outside of active site Binding groups catalytic group active siteActive site of enzymes

  • 3. Structure of enzyme

  • Properties and Catalytic Mechanisms of Enzymes

  • 2. Highly Specificity of Enzymes 3. Activities of enzymes can be regulated 1. Highly Catalytic Activity of Enzymes Properties of Enzymes

  • Catalytic Mechanisms of enzymesEnzymes accelerate reactions by decreasing the activation energy.

  • For a biochemical reaction to proceed, the energy barrier needed to transform the substrate molecules into the transition state has to overcome. The transition state has the highest free energy in the energy pathway. Formation of Enzyme-substrate Complex and Induced-fit Hypothesis The combination of substrate and enzyme forces the substrate to become the transition state.

  • Energy diagram for catalyzed and uncatalyzed reactions

  • Enzyme-substrate complex

  • Induced fit model

  • Induced fit model

    Active sites in the uninduced enzymeBinding of the first substrate (gold) induces a conformational shift that facilitates binding of the second substrate (blue), with far lower energy than otherwise required.When catalysis is complete, the product is released, and the enzyme returns to its uninduced state.

  • Hexokinase-D-glucose complexHexokinaseInduced fit

  • (1) Proximity and orientation effects Catalytic mechanisms of enzymes

  • FIGURE 3-7 Proximity effects and orientation arrange

  • (2) Electrostatic effects The active sites of enzymes are often hydrophobic. Water is largedly excluded from the active site because of the low dielectric constant. This increases the electrostatic interactions between enzymes and substrates and accelerate velocity of the reaction.

  • (3) Acid-base catalysis Side chain groups in enzyme active sites, act as proton donors or acceptors are termed general acids or general bases. The transition state is stabilized and the rate of reaction is increased by adding or removing proton from reactants. These chemical groups are imidazole group, amino group, carboxyl group, and so on.

  • (4) Covalent catalysis The attack of nucleophilic or electrophilic group in the enzyme active site upon the substrate results in covalent binding of substrate to the enzyme as an intermediate in the cause of catalysis.

  • Enzymatic reaction kinetics

  • Effect of concentration of substrates on enzyme reaction speed

  • 2. sense and application of Km() sense 1Kmthat is the concentration of substrate when reaction speed reach to V max. 2Km can present the affinity between enzyme and substrate.3Km is the mark constant.

  • Application:1To be used to identify enzyme; 2To estimate the best substrate of enzyme; 3To calculate the concentration of substrate at the fixed speed;4To infer whether the enzyme is regulated by concentration of substrate; 5To estimate the trend of reaction.

  • 3. Calculation metheds of Km1.

  • V[S]KmVmax[S]Vmax+=1-Km

  • Effect of pH on enzyme reaction speed 1. affect the combination of enzyme and substrate

    2. affect the conformation of enzyme molecule

  • pH

  • Effect of temperature on enzyme reaction speed

  • Effect of concentration of enzyme on reation speed

  • Effect of activator on enzyme reation speed activator: the substance that could increase the activity of enzyme and elevate the enzyme reaction speed.

  • Effect of inhibitor on enzyme reation speed

    Inhibitor: the substance could decrease the activity of enzyme.

  • Types of inhibiting1. irreversible inhibition specifity inhibition non-specifity inhibition2. reversible inhibition competitive inhibition noncompetitive inhibition uncompetitive inhibition

  • non-specifity inhibition1. irreversible inhibition

  • specifity inhibition

  • 2. reversible inhibition the inhibitor was attached to enzyme with non covalent bond to decrease or lose the activity of enzyme, but the inhibitor could be remove by dialysis.

  • competitive inhibition

  • Characteristic of competitive inhibition (KmVm

  • noncompetitive inhibition

  • Characteristic of noncompetitive inhibition Km()Vm

  • uncompetitive inhibition

  • Characteristic of uncompetitive inhibition1 2()KmVmax3[S] [ I ]

  • comparison of reversible inhibition

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