南京農(nóng)業(yè)大學(xué)生物化學(xué)最新課件

1、Enzyme An Introduction to Enzymes 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

2、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.” 一、 酶的概念 An Introduction to Enzymes He postulated that these ferments were inseparable from the structure of li

3、ving yeast cells; this view, called vitalism(活力論活力論), 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. Khne called the

4、se 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. An Introduction to Enzymes The isolation and crystallization of urease by J a m e s S u m n e r i n 1 9 2 6 p r o v i d e d

5、 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 Sumners conclusion widely accepted, after

6、 John Northrop and Moses Kunitz crystallized pepsin, trypsin, and other digestive enzymes and found them also to be proteins. 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, Hal

7、dane 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. 一、 酶的概念 With the exception of a small group of catalytic RNA molecules, all

8、 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

9、destroyed. Thus the primary, secondary, tertiary, and quaternary structures of protein enzymes are essential to their catalytic activity. Most Enzymes Are Proteins 一、 酶的概念 生物體內(nèi)的反應(yīng)是在很生物體內(nèi)的反應(yīng)是在很溫和溫和的條件（如溫和的條件（如溫和的溫度、接近中性的的溫度、接近中性的pHpH）下進行的，而同樣的反）下進行的，而同樣的反應(yīng)若在非生物條件下進行，則需要高溫、高壓、應(yīng)若在非生物條件下進行，則需要高溫、高壓、強酸、強

10、堿等劇烈的條件強酸、強堿等劇烈的條件。 酶的概念酶的概念酶是由生物細胞產(chǎn)生的以蛋白質(zhì)酶是由生物細胞產(chǎn)生的以蛋白質(zhì)為主要成分的生物催化劑。為主要成分的生物催化劑。 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

11、 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

12、? question?- Keeping the Sweet Taste of Corn 一、 酶的概念酶是酶是但酶發(fā)揮其催化作用并不局限于活細胞內(nèi)，在許但酶發(fā)揮其催化作用并不局限于活細胞內(nèi)，在許多情況下，細胞內(nèi)產(chǎn)生的酶需分泌到細胞外或轉(zhuǎn)移到多情況下，細胞內(nèi)產(chǎn)生的酶需分泌到細胞外或轉(zhuǎn)移到其它組織器官中發(fā)揮作用，如胰蛋白酶、脂酶、淀粉其它組織器官中發(fā)揮作用，如胰蛋白酶、脂酶、淀粉酶等水解酶。酶等水解酶。把由細胞內(nèi)產(chǎn)生并在細胞內(nèi)發(fā)揮作用的把由細胞內(nèi)產(chǎn)生并在細胞內(nèi)發(fā)揮作用的酶稱為胞內(nèi)酶，酶稱為胞內(nèi)酶，而將細胞內(nèi)產(chǎn)生后分泌細胞外起作用而將細胞內(nèi)產(chǎn)生后分泌細胞外起作用的酶叫胞外酶。的酶叫胞外酶。 一、

13、酶的概念 在本章節(jié)中把酶所催化的反應(yīng)稱作酶促反應(yīng)在本章節(jié)中把酶所催化的反應(yīng)稱作酶促反應(yīng)，發(fā)生化學(xué)反應(yīng)前的物質(zhì)稱底物，發(fā)生化學(xué)反應(yīng)前的物質(zhì)稱底物（substratesubstrate），而反應(yīng)后生成的物質(zhì)稱產(chǎn)物（而反應(yīng)后生成的物質(zhì)稱產(chǎn)物（productproduct）。）。 二、 酶的特性 1. 酶具有共同于一般催化劑的特征v用量少用量少; ;v只能催化熱力學(xué)上允許的反應(yīng)只能催化熱力學(xué)上允許的反應(yīng); ;v不改變反應(yīng)的平衡點，而只能縮短時不改變反應(yīng)的平衡點，而只能縮短時間。催化機理都是降低反應(yīng)所需的活間。催化機理都是降低反應(yīng)所需的活化能?；?。 二、 酶的特性 2. 酶不共同于一般催化劑的特征v

14、催化效率極高催化效率極高 二、 酶的特性 1000109 二、 酶的特性 二、 酶的特性 2. 酶不共同于一般催化劑的特征 鉑：催化許多反應(yīng)，包括有機反應(yīng)鉑：催化許多反應(yīng)，包括有機反應(yīng)H H+ +：淀粉、脂肪、蛋白質(zhì)、蔗糖等：淀粉、脂肪、蛋白質(zhì)、蔗糖等酶：只作用于結(jié)構(gòu)近似的分子，甚至酶：只作用于結(jié)構(gòu)近似的分子，甚至 只催化一種化合物。只催化一種化合物。v 專一性很強專一性很強 二、 酶的特性 2. 酶不共同于一般催化劑的特征v 酶對環(huán)境條件極為敏感酶對環(huán)境條件極為敏感v 酶活性可以調(diào)控酶活性可以調(diào)控 酶催化的專一性酶催化的專一性(specificity)(specificity)是指酶對它所催

15、是指酶對它所催化的反應(yīng)及其底物具有的嚴格的選擇性。通常一種酶化的反應(yīng)及其底物具有的嚴格的選擇性。通常一種酶只能催化一種或一類化學(xué)反應(yīng)。只能催化一種或一類化學(xué)反應(yīng)。由酶對底物選擇的嚴格程度，可將酶的專一性分為多由酶對底物選擇的嚴格程度，可將酶的專一性分為多種類型：種類型： 三、酶的專一性的類型 三、酶的專一性及其類型 H2NNH2CO 2NH3 + CO2脲酶H2OA A 基團專一性基團專一性(group specificity)(group specificity)在催化在催化A-BA-B化合化合物中，酶對其中的一個基團具有高度甚至是絕對專一物中，酶對其中的一個基團具有高度甚至是絕對專一性，而

16、對另一個基團則具有相對專一性的特性。性，而對另一個基團則具有相對專一性的特性。OCH2OHOHOHOHOR 葡萄糖苷 三、酶的專一性及其類型 B B 鍵專一性鍵專一性(bond specificity)(bond specificity)在催化在催化A-BA-B化合物化合物中，酶對中，酶對A,BA,B基團的結(jié)構(gòu)要求不嚴，而要求有一定的基團的結(jié)構(gòu)要求不嚴，而要求有一定的化學(xué)鍵便能進行催化反應(yīng)特性。化學(xué)鍵便能進行催化反應(yīng)特性。 三、酶的專一性及其類型 C C 立體專一性立體專一性(stereo specificity)(stereo specificity)一種酶只能對一種酶只能對一種立體異構(gòu)體起

17、催化作用，對其對映體則全無作用一種立體異構(gòu)體起催化作用，對其對映體則全無作用的特性。的特性。a a. .立體異構(gòu)專一性立體異構(gòu)專一性: :L 氨基酸 酮酸 + NH3 + H2O2L 氨基酸氧化酶H2O + O2 三、酶的專一性及其類型 C C 立體專一性立體專一性(stereo specificity)(stereo specificity)一種酶只能對一種酶只能對一種立體異構(gòu)體起催化作用，對其對映體則全無作用一種立體異構(gòu)體起催化作用，對其對映體則全無作用的特性。的特性。B.B.幾何異構(gòu)專一性幾何異構(gòu)專一性 : :HOOCCHHCCOOH 延胡索酸 延胡索酸酶CH2COOHCHCOOHHO 蘋果酸 三、酶的專一性及其類型 Summry絕對專一性絕