Enzymes are complex globular protein catalysts that accelerate chemical reaction rates by factors of 1012-1020 over that of uncatalyzed reactions at temperatures around 37℃.By contrast, industrial catalysts (inorganic substances) are orders of magnitude less effective than enzymes under comparable conditions. For example the reduction of hydrogen peroxide catalyzed by cataloes ,occurs 10 million times faster than it does when catalyzed by colloidal platinum at 37℃.

The catalytic efficiency of enzymes is very high, whereby one molecule of enzymes can transform as many as 10,000-1,000,0000 molecules of molecule of substrate per minute. it is this catalytic efficient of enzymes at low temperature which makes them important to the food scientism. This means that foods can be processed or modified by enzymes at moderate temperature ,say 25-50℃,where food products would not otherwise undergo changes at a significant rate .It also means, however, that endogenous enzymes are active under these conditions as well, and this can be beneficial or deleterious.

Furthermore, enzymes because of their tremendous catalytic power and low activation energies are active at subfreezing temperatures and therefore can be important stimulants of degradative reactions in refrigerated or frozen foods.

Of course, one basis for heat processing is to denature and inactivate enzymes so that the food is not subjected to continuing enzymes activity. The food scientist must have an understanding of the denaturtion phenomenon in order to properly process foods.

Another important aspect of enzymes activity in addition to catalytic power is the specificity of enzymes reactions. Industrial catalysts lack this specificity of reaction, and so cannot be used for modifying specific components of a food system. The specificity of hydrogen ion catalysts, for example, is very broad, whereas many enzymes perform only a single function, such as hydrolysis of a single bond or bond type. It is this enzymes specificity, which allows the food scientist to selectively modify individual food components and no affect others.

The sensitivity and specificity of enzymes also make them important to the food scientist as analytic tools. Analysis for food constituents in many instances can be simplified using enzymes techniques, which are detailed by berg Meyer, and jailbait. 

Enzymes nomenclature

Over the years, the number of enzymes isolated and characterized has continued to increase at an enormous rate. Previously it was custom for individual who isolated and characterized the enzyme to also name it. However, in many instances the same name. Consequently, the nomenclature for enzymes because so chaotic that the international union of biochemistry instituted a commission on nomenclature and classification of enzymes to prepare a system of nomenclature that has become standard and should be used in enzyme work. Each enzyme is assigned a code number of four numerals, each separated by periods and arranged according to the following principles .

The first numeral is the main division to which the enzyme belongs, i.e. (1) oxidoreductases, (2) transferases, (3) hydrolase, (4) lyases, (5) isomerases, and (6) ligases; the second is the subclass which identifies the enzyme in more specific terms; the third precisely defines the type of enzyme activity; and the fourth numerals clearly number of the enzyme in its sub-subclass.

Thus the first three numerals clearly designate the nature of the enzyme. For example, 1.2.3.4 denotes an oxidoreductase with an aldehyde as a donor and O2 as an acceptor, and it is the fourth numbered enzyme in particular series. In addition to the code number each enzyme is assigned a systematic name, which in many instances is too cumbersome to be used in the literature on a routine basis. Consequently, a trivial name has been recommended of common usage. The trivial name is sufficiently short for general use but is not necessarily very exact or systematic; in a great of the international union of biochemistry o nomenclature and classification of enzymes catalogued over 1700 enzymes each.

Aside from enzymes involved in postmortem and post harvest physiogy, few of the catalogued enzymes are of direct interest to the food scientist. By far the largest group of enzymes used in food processing is the hydrolases. A few oxidoreductases and isomerases are used, but hardly any transferees, assessor lipases. 

Definitions

The following terms are encountered in the enzymology literature.

  1. Holoenzyme: The protein portion of the enzyme and the coenzyme, it needed for catalytic activity.      

  2. Apoenzyme: The thermolabile protein component of the enzyme theat determines specificity.

3. Coenzyme, cofactor, prosthetic group: These terms are often used interchangeably to describe cocatalsts which act in conjunction with the apoenzyme to catallyze a reaction. However, Bernhard draws a distinction between cofactors and coenzymes. Prosthetic groups are usually those cocatalysts that are very tightly bound to the protein.

  4. Isoenzymes or isozymes: Multiple forms of an enzyme occurring in the same species. They catalyze the same reaction and arise from genetically determined differences in primary structure.The term "multiple forms of the enzyme " should be used as a broad term covering all proteins possessing the same enzymic activity and occurring naturally in a single species.

第六課  酶的定義和命名

    酶是復(fù)雜的球狀蛋白質(zhì)催化劑，它在37℃左右的溫度下，能以10¹²-10²⁰倍于非催化反應(yīng)的速率加速化學(xué)反應(yīng)。相比之下，工業(yè)催化劑(無機(jī)物質(zhì))的效率在相應(yīng)條件下要比酶的效率低若干個數(shù)量級。例如，在37℃下，由過氧化氫酶催化的過氧化氫還原反應(yīng)比由膠態(tài)鉑催化的該反應(yīng)快1千萬倍。

    酶的催化效率非常高，一個酶分子每分鐘可轉(zhuǎn)化多達(dá)10,000-1,000,000個底物分子。正是酶的這種在較低溫度下的催化效果，使得酶成了食品科學(xué)家手里非常重要的法寶。這就是說食品可以在適中的溫度(譬如25-50℃)下利用酶進(jìn)行加工或改性，而在同樣的溫度下，要不然就不會以明顯的速率發(fā)生變化。然而，這也意味著，內(nèi)源的酶在同樣的條件下也有活性，這可能有益，也可能有害。

    同時，酶因其巨大的催化本領(lǐng)和較低的活化能而在冰點以下仍有活性，所以它可能是冷藏食品或冷凍食品降解反應(yīng)的主要刺激物。

    當(dāng)然，食品熱處理的理論根據(jù)之一就是使食品中的酶變性和鈍化，從而使食品不再繼續(xù)受到酶的作用。食品科學(xué)家必須對食品的酶變性現(xiàn)象有所了解，以便適當(dāng)?shù)貙κ称愤M(jìn)行加工。

    除了催化本領(lǐng)以外，酶活性的另一重要方面是酶促反應(yīng)的專一性。工業(yè)催化劑缺乏這種反應(yīng)專一性，因此不能用于食品體系中一些特定組分的改性。例如，氫離子催化劑的催化范圍特性很寬，反之，許多酶執(zhí)行的只是單一的功能，譬如一種單鍵或一種鍵型的水解。正是酶的這種專一性使得食品科學(xué)家能夠有選擇地改變食品的個別組分而不影響其他組分。

    酶的敏感性和專一性也使酶成為對食品科學(xué)家來說很重要的分析工具。許多情況下，食品成分的分析可以利用酶技術(shù)加以簡化，這方面伯格梅厄和圭鮑爾特有詳細(xì)的論述。        酶的命名多年來，分離和鑒定出來的酶的數(shù)目一直以驚人的速度在不斷增加。起先，習(xí)慣上是由分離和鑒定購的人給酶命名的。而這在許多情況下造成了給同一種酶取了不同的名稱，或者給不同的酶取了相同的名稱。因此，酶的命名變得相當(dāng)混亂，于是國際生物化學(xué)聯(lián)合會成立了酶命名分類委員會，制訂了一種現(xiàn)已作為標(biāo)準(zhǔn)并在酶著作中必須予以采用的命名系統(tǒng)。該系統(tǒng)給每種酶以一個四位數(shù)的代碼，每個數(shù)字由句點分開，并依照下列規(guī)則排列。第一個數(shù)字表示該酶所屬的大類，即：(1)氧化還原酶類，(2)轉(zhuǎn)移酶類，(3)水解酶類，(4)裂合酶類，(5)異構(gòu)酶類，(6)連接酶類。第二個數(shù)字表示酶所屬的亞類，它用更具體的條款確認(rèn)該酶。第三個數(shù)字確切說明酶活性的類型。第四個數(shù)字是該酶在亞—亞類中的系列號。這樣，前三個數(shù)字就清楚地指出了酶的性質(zhì)。例如，  1．2．3．4表示一種以醛為電子給體、以02為電子受體的氧化還原酶，  而且它在具體系列中的編號為第四。除了代號以外，還給予每種酶一個系統(tǒng)名稱，許多情況下這個名字太麻煩，不使用于常規(guī)文獻(xiàn)。因而，有人建議在普通場合下使用俗名。俗名作一般用時相當(dāng)簡短，但不一定很確切，很系統(tǒng)；它是早已在大量例子中普遍使用的名字。國際生物化學(xué)聯(lián)合會1972年推薦列入《酶的命名和分類》目錄上的酶有l(wèi)700多種，其中氧化還原酶、轉(zhuǎn)移酶和水解酶各有400多種。

除了與動物宰后，植物采后生理變化有關(guān)的酶以外，載入目錄的其它酶幾乎沒有一種是食品科學(xué)家直接感興趣的。食品加工中用到最多的一類酶是水解酶。氧化還原酶和 異構(gòu)酶用得很少，而轉(zhuǎn)移酶、裂解酶或連接酶則幾乎沒有用到。

名詞解釋

    在酶學(xué)文獻(xiàn)中經(jīng)常遇到下列各詞：

    1．全酶：酶的蛋白質(zhì)部分以及酶催化活力必要時的輔酶。

    2．酶蛋白：酶中決定酶特異性的不耐熱的蛋白質(zhì)部分。

    3．輔酶、輔助因子、輔基：這些詞經(jīng)常不加選擇地用來描述與酶蛋白同時作用以催化某—一反應(yīng)的輔催化劑。然而，本哈德提出了輔助因子與輔酶的區(qū)別。輔基通常是與    蛋白質(zhì)結(jié)合得非常緊的輔催化劑。

    4．同功酶；同一物種來源的某種酶的多種形式。它們催化同一反應(yīng)，并且多種形式來源于遺傳決定的一級結(jié)構(gòu)上的差異。“酶的多種形式”一詞應(yīng)作為廣義的詞使用，包    括了所有具有相同催化活性并天然存在于單一物種中的所有蛋白質(zhì)。

                              Lesson 6    

globular 球狀的

magnitude n. ①巨大，廣大 ②重大，重要性  ③大小，積，量，數(shù)量，音量

factor n. ①因素，要素  ② 因子，因素，系數(shù)

catalyst 催化劑，觸酶

catalase  過氧化氫酶，接觸酶

hydrogen peroxide 過氧化氫

colloidal 膠體的，膠質(zhì)的，膠態(tài)的  colloid  ①膠體，膠質(zhì)，膠態(tài)  ②膠體的，膠質(zhì)的

platium 鉑

activation  活化             

inactivate 鈍化，失活

denature 變性

refrigerate 冷藏

degradative 降解

specificity  特性,專一性

hydrolysis 水解效應(yīng)

sensitivity  敏感性

analysis 分析

isolate 使隔離

classification 分類

lyase 裂合酶                    

hydrolase 水解酶

oxidoreductases 氧化還原酶        

transferase  轉(zhuǎn)移酶

isomerase  異構(gòu)酶                

ligase  連接酶

cumbersome  a. ①拖累的，麻煩的  ②笨重的，不方便的

catalogue  n. 目錄，目錄冊  vt.為···編目錄，把···編入目錄，按目錄分類

enzymology  n. 酶學(xué)

period  n. ①句號，句點，結(jié)束，終止 ②學(xué)時，課時 ③ 整句

subclass 亞類

postmortem 死后的

physiology  生理學(xué)

holoenzyme 全酶             coenzyme 輔酶              apoenzyme 酶蛋白

thermolabile  不耐熱的

prosthetic  group  酶活動基，輔基

cofactor  輔助因子

isoenzymes 同功酶

English-bug

  1．  Enzymes are complex globular protein catalysts that accelerate chemical reaction rates by factors of 1012-1020 over that of uncatalyzed reactions at temperatures around 37℃.（用介詞at）

  2．  By contrast, industrial catalysts (inorganic substances) are orders of magnitude less effective(若干個數(shù)量級) than enzymes under comparable（比得上的） conditions.

  3．  For example the reduction of hydrogen peroxide catalyzed by cataloes ,occurs 10 million times faster than it does when catalyzed by colloidal platinum at 37℃.(注意表示多少倍的比較結(jié)構(gòu))

  4．  This means that foods can be processed or modified by enzymes at moderate temperature ,say 25-50℃,where food products would not otherwise undergo changes at a significant rate .(其意思為“要不然”，“否則”，此處其位置叫為特別)

  5．  Of course, one basis for heat processing is to denature and inactivate enzymes so that the food is not subjected to continuing enzymes activity.（意為承受得起，后接動名詞）

  6．  Another important aspect of enzymes activity in addition to catalytic power is the specificity of enzymes reactions.（意為除…以外）

7．  By far the largest group of enzymes used in food processing is the hydrolases.(意為“遠(yuǎn)遠(yuǎn)超過”或“顯然”)

句中，the second 指酶系統(tǒng)命名中四位數(shù)代碼的第二位數(shù)，在此作主語，which 引導(dǎo)的從句是定語從句修飾 the subclass.

    本表達(dá)式中，the protein portion of …….for catalytic activity 是名詞短語， 作holoenzyme 的同位語。另外，也將此名詞短語理解為it is the protein portion of …for catalytic activity 的句子。這時由if it (指coenzyme)is needed for catativity 省略而成的條件狀語從句if needed…activity 作狀語，修飾主句謂語is the protein portion of the enzyme and the conezyme.這里the protein portion 和the coenzyme 是并列的。

    句中，they 指上句中的mutiple forms, catalyze和arise是并列謂語.from genetically determinated differences in primary structure 是介詞短語，作狀語，修飾不及物動詞arise.介詞短語in primary structure 作定語，修飾名詞differences.determined 是過去分詞，作定語，也修飾differences. 副詞genetically 修飾determined.