华南师范大学生命科学学院05级技术(2)班 刘俏敏 enzyme 华南师范大学生命科学学院05级技术(2)班 刘俏敏

Enzyme classification Active center Mechanism Kinetics Enzyme’s name Enzyme classification Active center Mechanism Kinetics Some important enzyme

Enzyme’s name recommended name Systematic name substrate + reaction nature + enzyme (底物＋反应性质＋酶) 谷丙转氨酶(recommended name) 丙氨酸:а酮戊二酸基转移酶(systematic name)

Enzyme classification Enzymes are grouped into six classes according to the type of reactions catalyzed Oxidoreductase (氧化-还原酶) Transferase (转移酶) Hydrolase (水解酶) Lyase (裂解酶) Isomerase (异构酶) Ligase (连接酶)

The four-digit number The four-digit number indicates the following Class (1-6) Sub-classes based on type of substrates Sub-sub classes based on cofactor type A unique number for each individual enzyme within a sub-sub-class EC+酶大类号．亚类号底物．亚亚类号．序号 eg: D-葡萄糖-6-磷酸磷酸水解酶 EC 3.1.3.9

Active center (active site) The active site of an enzyme is the place where all of the action occurs. It contains the functional groups (amino acid side chains) that bind the substrate (s) and catalyze It’s conversion to product (s) active site Binding site(结合部位) Catalytic site(催化部位)

What does the active site do? The active site binds the substrates and positions them in the proper orientation for the reaction to occur The active site contains chemical groups that stabilize the transition state of the reaction The active site determines the specificity of the enzyme (i.e. it determines whether a particular substrate is bound and whether a particular product is made)

酶分子中各种残基的作用

Characteristics of active sites THE active site takes up a small part of the total volume of the enzyme The active site is 3-dimensional and is generally found in a crevice or cleft on the enzyme The active site displays highly specific substrate binding The active site is responsible for whether there is ordered or random binding of substrates and release of products

Kinetics The velocity is affected by some factors 1) enzyme concentration 2) substrate concentration 3) temperature 4) pH 5) activator 6) inhibitor

Affected by enzyme concentration ∝ when [S]>>[E] V [E] 反应浓度 酶浓度

Affected by substrate concentration 单底物酶促反应动力学 V V=Vmax [S]/(Km +[S]) [S]

Michaelis-constant Km is a constant derived from rate constant Km=(K-1 + K2 )/K1 Km is under true Michaelis-Menten conditions, an estimate of the dissociation constant of E from S ,because K1 [E][S]=K-1 [ES] at equilibrium, Reversible reaction E+S ES, dissociation constant is Kd= = K1 [E][S] K-1 [ES] K-1 K1

Michaelis-constant Km equals to the substrate concentration at which V=1/2Vmax Small Km means tight substrate binding; high Km means weak substrate binding.

Lineweaver-Burk作图法(双倒数作图法) 1/ v=(Km / Vmax ).(1/[s]) + 1/Vmax

Affected by substrate concentration 双底物酶促反应动力学 有序反应 (order reaction) 序列反应 (sequential reaction) 双底物双产的反应 随机反应 (random reactions) 乒乓反应 (ping pong reaction)

Affected by temperature 1、 在最适温度以前反应速度随温度的升高而升高 2 、在最适温度以后反应速度随温度的升高而降低 （酶蛋白变性） Optimum temperature V

Affected by pH Reason 影响酶构象 影响酶与底物的解离 Optimum pH

Optimum pH

Affected by activator Affected:凡是能提高酶活性的物质。 无机离子 激活剂 小分子有机物 大分子有机物 激活剂 小分子有机物 大分子有机物 金属离子 阴离子 氢离子 某些还原剂 EDTA

affected by inhibitor Irreversible inhibition Reversible inhibition 1)competitive inhibition 2)noncompetitive inhibition 3)uncompetitive inhibition

Competitive inhibition 交于 y 轴

Noncompetitive inhibition 交于x轴

Uncompetitive inhibition 一组平行直线

Vmax Km Competitive inhibition 不变 增加 Noncompetitive inhibition 减小 不变 Uncompetitive inhibition 减小 减小

调节酶 allosteric enzyme enzyme activity is regulated by four different mechanisms 1) allosteric control 2)covalent modification 3) proteolytic activation 4) stimulation or inhibition by control proteins

别构酶调节酶活性的机理 symmetry or concerted model Sequential model

Symmetry model

Sequential model

Isozymes(同工酶) Isozymes are physically distinct forms of the same enzyme. Isozymes may differ from each other by differences in their amino acid sequences or by the presence of different posttranslational modifications in each isozyme. The relative abundance of different isozymes varies for different tissues. The ability to control which isozymes are expressed in a particular cell allows each cell to adjust the enzyme activity based on the specific conditions that exist in the cell.

1 In this reaction , the enzyme belongs A Oxidoreductase B Transferase C Hydrolase D Lyase E Isomerase F Ligase