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.

第六课  酶的定义和命名

    酶是复杂的球状蛋白质催化剂，它在37℃左右的温度下，能以10¹²-10²⁰倍于非催化反应的速率加速化学反应。相比之下，工业催化剂(无机物质)的效率在相应条件下要比酶的效率低若干个数量级。例如，在37℃下，由过氧化氢酶催化的过氧化氢还原反应比由胶态铂催化的该反应快1千万倍。

    酶的催化效率非常高，一个酶分子每分钟可转化多达10,000-1,000,000个底物分子。正是酶的这种在较低温度下的催化效果，使得酶成了食品科学家手里非常重要的法宝。这就是说食品可以在适中的温度(譬如25-50℃)下利用酶进行加工或改性，而在同样的温度下，要不然就不会以明显的速率发生变化。然而，这也意味着，内源的酶在同样的条件下也有活性，这可能有益，也可能有害。

    同时，酶因其巨大的催化本领和较低的活化能而在冰点以下仍有活性，所以它可能是冷藏食品或冷冻食品降解反应的主要刺激物。

    当然，食品热处理的理论根据之一就是使食品中的酶变性和钝化，从而使食品不再继续受到酶的作用。食品科学家必须对食品的酶变性现象有所了解，以便适当地对食品进行加工。

    除了催化本领以外，酶活性的另一重要方面是酶促反应的专一性。工业催化剂缺乏这种反应专一性，因此不能用于食品体系中一些特定组分的改性。例如，氢离子催化剂的催化范围特性很宽，反之，许多酶执行的只是单一的功能，譬如一种单键或一种键型的水解。正是酶的这种专一性使得食品科学家能够有选择地改变食品的个别组分而不影响其他组分。

    酶的敏感性和专一性也使酶成为对食品科学家来说很重要的分析工具。许多情况下，食品成分的分析可以利用酶技术加以简化，这方面伯格梅厄和圭鲍尔特有详细的论述。        酶的命名多年来，分离和鉴定出来的酶的数目一直以惊人的速度在不断增加。起先，习惯上是由分离和鉴定购的人给酶命名的。而这在许多情况下造成了给同一种酶取了不同的名称，或者给不同的酶取了相同的名称。因此，酶的命名变得相当混乱，于是国际生物化学联合会成立了酶命名分类委员会，制订了一种现已作为标准并在酶著作中必须予以采用的命名系统。该系统给每种酶以一个四位数的代码，每个数字由句点分开，并依照下列规则排列。第一个数字表示该酶所属的大类，即：(1)氧化还原酶类，(2)转移酶类，(3)水解酶类，(4)裂合酶类，(5)异构酶类，(6)连接酶类。第二个数字表示酶所属的亚类，它用更具体的条款确认该酶。第三个数字确切说明酶活性的类型。第四个数字是该酶在亚—亚类中的系列号。这样，前三个数字就清楚地指出了酶的性质。例如，  1．2．3．4表示一种以醛为电子给体、以02为电子受体的氧化还原酶，  而且它在具体系列中的编号为第四。除了代号以外，还给予每种酶一个系统名称，许多情况下这个名字太麻烦，不使用于常规文献。因而，有人建议在普通场合下使用俗名。俗名作一般用时相当简短，但不一定很确切，很系统；它是早已在大量例子中普遍使用的名字。国际生物化学联合会1972年推荐列入《酶的命名和分类》目录上的酶有l700多种，其中氧化还原酶、转移酶和水解酶各有400多种。

除了与动物宰后，植物采后生理变化有关的酶以外，载入目录的其它酶几乎没有一种是食品科学家直接感兴趣的。食品加工中用到最多的一类酶是水解酶。氧化还原酶和 异构酶用得很少，而转移酶、裂解酶或连接酶则几乎没有用到。

名词解释

    在酶学文献中经常遇到下列各词：

    1．全酶：酶的蛋白质部分以及酶催化活力必要时的辅酶。

    2．酶蛋白：酶中决定酶特异性的不耐热的蛋白质部分。

    3．辅酶、辅助因子、辅基：这些词经常不加选择地用来描述与酶蛋白同时作用以催化某—一反应的辅催化剂。然而，本哈德提出了辅助因子与辅酶的区别。辅基通常是与    蛋白质结合得非常紧的辅催化剂。

    4．同功酶；同一物种来源的某种酶的多种形式。它们催化同一反应，并且多种形式来源于遗传决定的一级结构上的差异。“酶的多种形式”一词应作为广义的词使用，包    括了所有具有相同催化活性并天然存在于单一物种中的所有蛋白质。

                              Lesson 6    

globular 球状的

magnitude n. ①巨大，广大 ②重大，重要性  ③大小，积，量，数量，音量

factor n. ①因素，要素  ② 因子，因素，系数

catalyst 催化剂，触酶

catalase  过氧化氢酶，接触酶

hydrogen peroxide 过氧化氢

colloidal 胶体的，胶质的，胶态的  colloid  ①胶体，胶质，胶态  ②胶体的，胶质的

platium 铂

activation  活化             

inactivate 钝化，失活

denature 变性

refrigerate 冷藏

degradative 降解

specificity  特性,专一性

hydrolysis 水解效应

sensitivity  敏感性

analysis 分析

isolate 使隔离

classification 分类

lyase 裂合酶                    

hydrolase 水解酶

oxidoreductases 氧化还原酶        

transferase  转移酶

isomerase  异构酶                

ligase  连接酶

cumbersome  a. ①拖累的，麻烦的  ②笨重的，不方便的

catalogue  n. 目录，目录册  vt.为···编目录，把···编入目录，按目录分类

enzymology  n. 酶学

period  n. ①句号，句点，结束，终止 ②学时，课时 ③ 整句

subclass 亚类

postmortem 死后的

physiology  生理学

holoenzyme 全酶             coenzyme 辅酶              apoenzyme 酶蛋白

thermolabile  不耐热的

prosthetic  group  酶活动基，辅基

cofactor  辅助因子

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℃.（用介词at）

  2．  By contrast, industrial catalysts (inorganic substances) are orders of magnitude less effective(若干个数量级) 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较结构)

  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 .(其意思为“要不然”，“否则”，此处其位置叫为特别)

  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.（意为承受得起，后接动名词）

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

7．  By far the largest group of enzymes used in food processing is the hydrolases.(意为“远远超过”或“显然”)

句中，the second 指酶系统命名中四位数代码的第二位数，在此作主语，which 引导的从句是定语从句修饰 the subclass.

    本表达式中，the protein portion of …….for catalytic activity 是名词短语， 作holoenzyme 的同位语。另外，也将此名词短语理解为it is the protein portion of …for catalytic activity 的句子。这时由if it (指coenzyme)is needed for catativity 省略而成的条件状语从句if needed…activity 作状语，修饰主句谓语is the protein portion of the enzyme and the conezyme.这里the protein portion 和the coenzyme 是并列的。

    句中，they 指上句中的mutiple forms, catalyze和arise是并列谓语.from genetically determinated differences in primary structure 是介词短语，作状语，修饰不及物动词arise.介词短语in primary structure 作定语，修饰名词differences.determined 是过去分词，作定语，也修饰differences. 副词genetically 修饰determined.