中南大学 生物化学 考研课件 ch15 酶的特性和调控 enzyme specificity and regulation06.ppt

1、Biochemistry,For Speciality of Biotechnology or Bioengineering at CSU Mr. XIA Jinlan,Contents,Enzyme specificity (self-study) 酶的特异性(自学) The “lock and key” hypothesis 锁钥假设 The “induced fit” hypothesis 诱导契合假设 Control of enzyme action Control of enzyme amount 酶量的控制 Control of enzyme activity 酶活性的控制,Enz

2、yme specificity,Specificity is the result of molecular recognition The “lock and key” hypothesis The “induced fit” hypothesis,There are two models of binding of a substrate to an enzyme,Lock-and-key model 锁和钥匙模型 Founder: Emil Fisher, 1894 Principle Shape of the substrate and the active site of the e

3、nzyme are rigid and fixed, they fit together like a key into its lock Induced-fit model 诱导-契合模型 Founder: Daniel E., Koshland Jr., 1958 Principle Binding of substrate induces a conformational change in the active site of enzyme. Enzyme may distort the substrate, forcing it into a conformation similar

4、 to that of the transition state.,The induced fit model (ex. of hexokinase),Before glucose binding,After glucose binding,Substrate is highly specific to its enzyme,The specificity between enzyme and its substrate depends on the properties and spatial arrangement of the amino acid residues forming th

5、e active site of an enzyme Typical example Serine proteases Trypsin胰蛋白酶: cleaves C-terminal side of basic amino acid residues (Arg, Lys) Chymotrypsin 胰凝乳蛋白酶: cleaves C-terminal side of aromatic amino acid residues (Phe, Trp, Tyr) Elastase弹性蛋白酶(胰肽酶E)：cleaves C-terminal side of small uncharged amino a

6、cid residues (Gly, Ala),Question: Please compare the difference in mechanism of specificity among the serine proteases trypsin, chymotrypsin and elastase. 请比较丝氨酸胰蛋白酶、胰凝乳蛋白酶、弹性蛋白酶在特异性机制的区别,Carbamidine,Control of enzyme action,Control of enzyme amount Mainly by controlling gene transcription Control s

7、lowly Control of enzyme activity By controlling post-translation, i.e., enzyme (structural) modification Control fast,Feedback regulation 反馈调控 Reversible noncompetitive inhibition可逆非竞争抑制 Feedback regulation is usually accompanied by Allosteric action 反馈调控通常伴随着变构作用 Reversible covalent modification 可逆

8、共价修饰 The most common: Phosphorylation and dephosphorylation 最典型:磷酸化和去磷酸化 Irreversible modification of proenzymes 酶原的非可逆修饰 Example of allosteric control and covalent modification 变构调控和共价修饰的例子,Control of enzyme activity (酶活性控制),Feedback regulation,Control of enzyme activity,Example of allosteric regul

9、ation,A plot of V0 against S is sigmoid curve, rather than the hyperbolic plots predicted by the Michaelis-Menten equation. Substrate binding is cooperative 底物键合作用是协同性的 Two kinds of Factors binding to the regulatory subunits allosteric activator allosteric inhibitor,Characteristics of allosteric act

10、ion 变构作用的特征,Two types of allosteric effectors 两种变构效应剂,K-type,V-type,Two models of allosteric modification 两种变构修饰模型,Concerted or symmetry model (MWC Model),Sequential model,A dimeric protein can exist in either of two conformational states at equilibrium,Substrate binding shifts equilibrium in favor

11、of R,Basic points of MWC model, supposing that (1) In the absence of S, T0R0, (2) S binds only to R state, i.e., KRKT (3) In the presence of S, which binds to R cooperatively,Concerted or symmetry model 齐变模型 Monod-Wyman-Changeux (MWC) Model,Heterotropic effectors allosteric activator: binds to R sta

12、te allosteric inhibitor: binds to T state,Reversible covalent modification 可逆的共价修饰,Enzyme phosphorylation 酶的磷酸化 Catalyzed by protein kinase 蛋白激酶 Enzyme dephosphorylation 酶的去磷酸化 Catalyzed by protein phosphatase 蛋白磷酸（酯）酶 Two types of protein kinases 蛋白激酶的两种类型: Serine/threonine protein kinase (active s

13、ite: Ser or Thr residues OH group) Tyrosine kinase (active site: Tyr residues OH group),Phosphorylation and dephosphorylation,Reversible covalent modification 可逆的共价修饰,Irreversible modification of proenzymes or being zymogens 酶原的非可逆修饰,Enteropeptidase,Many enzymes are in forms of zymogens or proenzyme

14、s before activated,Zymogen is activated by proteolytic cleavage of its peptide bonds,R: Arg精氨酸 I: Ile异亮氨酸 L:Leu亮氨酸 Y:Tyr酪氨酸 A:ala丙氨酸 Ser: 丝氨酸 Thr: 苏氨酸 Asn:天冬酰胺酸,Example of allosteric control and covalent modification -Glycogen metabolism,Glycogen metabolism Glycogen degradation is catalyzed by glyco

15、gen phosphorylase, and glycogen synthesis by glycogen synthase The glycogen phosphorlyase and glycogen synthase are regulated allosterically and by way of covalent modification The covalent modification can be simultaneously regulated by protein kinase A Activation of PKA may be stimulated by hormon

16、al,Glycogen metabolism 糖原代谢 (1) Glycogen degradation糖原分解,Catalyzed by glycogen phosphorylase 磷酸水解酶,Glycogen synthesis catalyzed by glycogen synthase (p757),Glycogen metabolism 糖原代谢 (2) Glycogen synthesis 糖原合成,The mechanism of covalent modification and allosteric regulation of glycogen phosphorylase,

17、Glycogen phosphorlyase and glycogen synthase can be simultaneously regulated by protein kinase A,Covalent modification,Activation of Protein kinase A stimulated by hormone,Hydrolysis Of glycogen,Also see fig.15.19 p478,Activation of protein Kinase A stimulated by homone,Signal molecule,Activated Adenyl cyclase,receptor,Activated a subunit of G protein,Inactiv