第七章 酶

1. 第七章 酶 第一节 酶的概念及特点 Enzyme

2. An Introduction to Enzymes 一、 酶的概念 Much of the history of biochemistry is the history of enzyme research. Biological catalysis was first recognized and described in the late 1700s, in studies on the digestion of meat by secretions of the stomach, and research continued in the 1800s with examinations of the conversion of starch to sugar by saliva and various plant extracts. In the 1850s, Louis Pasteur concluded that fermentation of sugar into alcohol by yeast is catalyzed by “ferments.”

3. An Introduction to Enzymes He postulated that these ferments were inseparable from the structure of living yeast cells; this view, called vitalism(活力论), prevailed for decades. Then in 1897 Eduard Buchner discovered that yeast extracts could ferment sugar to alcohol, proving that fermentation was promoted by molecules that continued to function when removed from cells. Frederick W. Kühne called these molecules enzymes. As vitalistic notions of life were disproved, the isolation of new enzymes and the investigation of their properties advanced the science of biochemistry.

4. An Introduction to Enzymes The isolation and crystallization of urease by James Sumner in 1926 provided a breakthrough in early enzyme studies. Sumner found that urease crystals consisted entirely of protein, and he postulated that all enzymes are proteins. In the absence of other examples, this idea remained controversial for some time. Only in the 1930s was Sumner’s conclusion widely accepted, after John Northrop and Moses Kunitz crystallized pepsin, trypsin, and other digestive enzymes and found them also to be proteins.

5. An Introduction to Enzymes During this period, J. B. S. Haldane wrote a treatise entitled Enzymes. Although the molecular nature of enzymes was not yet fully appreciated, Haldane made the remarkable suggestion that weak bonding interactions between an enzyme and its substrate might be used to catalyze a reaction. This insight lies at the heart of our current understanding of enzymatic catalysis.

6. 一、 酶的概念 Most Enzymes Are Proteins With the exception of a small group of catalytic RNA molecules, all enzymes are proteins. Their catalytic activity depends on the integrity of their native protein conformation. If an enzyme is denatured or dissociated into its subunits, catalytic activity is usually lost. If an enzyme is broken down into its component amino acids, its catalytic activity is always destroyed. Thus the primary, secondary, tertiary, and quaternary structures of protein enzymes are essential to their catalytic activity.

7. 一、 酶的概念 酶的概念—酶是由生物细胞产生的以蛋白质为主要成分的生物催化剂。 生物体内的反应是在很温和的条件（如温和的温度、接近中性的pH）下进行的，而同样的反应若在非生物条件下进行，则需要高温、高压、强酸、强碱等剧烈的条件。

8. question?-- Keeping the Sweet Taste of Corn The sweet taste of freshly picked corn (maize) is due to the high level of sugar in the kernels. Store-bought corn (several days after picking) is not as sweet, because about 50% of the free sugar is converted to starch within one day of picking. To preserve the sweetness of fresh corn, the husked ears can be immersed in boiling water for a few minutes (“blanched”) then cooled in cold water. Corn processed in this way and stored in a freezer maintains its sweetness. What is the biochemical basis for this procedure?

9. 一、 酶的概念 酶是但酶发挥其催化作用并不局限于活细胞内，在许多情况下，细胞内产生的酶需分泌到细胞外或转移到其它组织器官中发挥作用，如胰蛋白酶、脂酶、淀粉酶等水解酶。把由细胞内产生并在细胞内发挥作用的酶称为胞内酶，而将细胞内产生后分泌细胞外起作用的酶叫胞外酶。

10. 一、 酶的概念 在本章节中把酶所催化的反应称作酶促反应，发生化学反应前的物质称底物（substrate），而反应后生成的物质称产物（product）。

11. 二、 酶的特性 1. 酶具有共同于一般催化剂的特征 • 用量少; • 只能催化热力学上允许的反应; • 不改变反应的平衡点，而只能缩短时间。催化机理都是降低反应所需的活化能。

14. 二、 酶的特性 2. 酶不共同于一般催化剂的特征 • 催化效率极高 酶促反应的速度比非酶促反应通常要快 105~1017 倍 如此高的催化效率使生物体内含量甚微的酶能催化大量的物质转化。

15. 二、 酶的特性 2 H2O2 2H2O + O2 铁屑 肝糜 肝糜(煮) 用简单的实验证明酶的催化效率： 1000 109

16. 二、 酶的特性

17. 二、 酶的特性 2. 酶不共同于一般催化剂的特征 • 专一性很强 铂：催化许多反应，包括有机反应 H+：淀粉、脂肪、蛋白质、蔗糖等 酶：只作用于结构近似的分子，甚至 只催化一种化合物。

18. 二、 酶的特性 2. 酶不共同于一般催化剂的特征 • 酶对环境条件极为敏感 • 酶活性可以调控

19. 三、酶的专一性的类型 酶催化的专一性(specificity)是指酶对它所催化的反应及其底物具有的严格的选择性。通常一种酶只能催化一种或一类化学反应。 由酶对底物选择的严格程度，可将酶的专一性分为多种类型： 1. 绝对专业性—除一种底物外，酶都不能催化其它反应的特性

20. 三、酶的专一性及其类型

21. 三、酶的专一性及其类型 2 相对专一性—酶能催化在结构上类似的一系列化合物反应特怔。 A 基团专一性(group specificity)—在催化A-B化合物中，酶对其中的一个基团具有高度甚至是绝对专一性，而对另一个基团则具有相对专一性的特性。

22. 三、酶的专一性及其类型 2 相对专一性—酶能催化在结构上类似的一系列化合物反应特怔。 B 键专一性(bond specificity)—在催化A-B化合物中，酶对A,B基团的结构要求不严，而要求有一定的化学键便能进行催化反应特性。

23. 三、酶的专一性及其类型 C 立体专一性(stereo specificity)—一种酶只能对一种立体异构体起催化作用，对其对映体则全无作用的特性。a.立体异构专一性:

24. 三、酶的专一性及其类型 C 立体专一性(stereo specificity)—一种酶只能对一种立体异构体起催化作用，对其对映体则全无作用的特性。B.几何异构专一性:

25. 三、酶的专一性及其类型 Summry