食品專業(yè)英語 LESSON 4 Amino Acids And Proteins

1、食品專業(yè)英語 LESSON 4 Amino Acids And ProteinsProteins are molecules of great size, complexity, and diversity. They are the source of dietary amino acids, both essential and nonessential, that are used for growth, maintenance, and the general well-being of man. These macromolecules, characterized by their

2、 nitrogen contents, are involved in many vital processes intricately associated with all living matter. In mammals and many internal organs are largely composed of proteins. Mineral matter of bone is held together by collagenous protein. Skin, the protective covering of the body, often accounts for

3、about 10% of the total body protein.Some protein function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body. Fundamental life process, such as growth, digestion and metabolism, excretion, conversion of chemical energy into mechanical work, etc, are controlled by e

4、nzymes and hormones. Blood plasma proteins and hemoglobin regulate the osmotic pressure and PH of certain body fluids. Proteins are necessary for immunology reactions. Antibodies, modified plasma globulin proteins, defend against the invasion of foreign substances of microorganisms that can cause va

5、rious diseases, food allergies result when certain ingested proteins cause an apparent modification in the defense mechanism. This leads to a variety of painful, and occasionally drastic, conditions in certain individuals.Food shortages exist in many areas of the world, and they are likely to become

6、 more acute and widespread as the worlds population increases, providing第四課氨基酸和蛋白質(zhì)蛋白質(zhì)錯綜復(fù)雜、多種多樣的大分子物質(zhì)，是食物必須氨基酸和非必須氨基酸的來源。 人體利用這些氨基酸以滿足生長發(fā)育、修復(fù)組織和維持正常健康生活的要求。這些大分子以 含氮為其特征，參與了許多與各種有生命物質(zhì)有復(fù)雜聯(lián)系的生命過程。在包括人類在內(nèi)的哺 乳動物中，蛋白質(zhì)起著機體改造成分的作用，肌肉和許多體內(nèi)器官主要由蛋白質(zhì)構(gòu)成。骨骼 中的礦物質(zhì)靠膠原蛋白得以保持在一起。機體的保護層一皮膚中的蛋白質(zhì)通常占機體蛋白質(zhì) 總量的10%的左右。有些蛋白質(zhì)

7、有生物催化劑（酶和激素）的作用，以調(diào)節(jié)體內(nèi)的化學(xué)反響?；镜纳^ 程如生長、消化、代謝、排泄、化學(xué)能轉(zhuǎn)變成機械功等等都受酶和激素的控制。某些體液的 滲透壓和pH值受制于血漿蛋白和血紅蛋白。蛋白質(zhì)對免疫反響是必不可少的?？贵w（改性 的血漿球蛋白能引起疾病的外來雜質(zhì)和微生物的入侵。當(dāng)某些攝入的蛋白質(zhì)使防御機制產(chǎn)生 明顯的變化時，便發(fā)生人體的生物過敏。這就導(dǎo)致某些個體身上出現(xiàn)各種各樣的疾病，且有 時是急劇的病情。食物短缺現(xiàn)象在世界許多地區(qū)存在。隨著人口的增加，這個問題很可能愈來愈尖銳、愈 普遍。而蛋白質(zhì)供應(yīng)缺乏問題遠(yuǎn)比碳水化合物或脂肪供應(yīng)缺乏更為嚴(yán)重。蛋白質(zhì)不僅它的產(chǎn) 出費用要比碳水化合物或脂肪的

8、產(chǎn)出費用為高，而且每千克每天所需的蛋白質(zhì)量造整個成年期是恒定的，而每天所需的脂肪和碳水化合物量一般都隨著年齡的增長而逐漸減少。正如上面簡述的一樣，蛋白質(zhì)有多種不同的結(jié)構(gòu)、性質(zhì)和生理功能。許多蛋白質(zhì)容易受周圍 環(huán)境的一系列微妙變化的影響而發(fā)生變化。要想使現(xiàn)在和將來的食品加工能理想的滿足人類 的需要，就必須徹底了解原料特別是蛋白質(zhì)的組成結(jié)構(gòu)和化學(xué)性質(zhì)。目前，已經(jīng)有這方面的 大量資料可供利用，不過其中大局部是生物化學(xué)家利用某一特定食物成分作為模擬物系加以 提供的。氨基酸氨基酸是蛋白質(zhì)的“結(jié)構(gòu)單元”。因此，要了解蛋白質(zhì)的性質(zhì)，舊需要討論氨基酸的結(jié) 構(gòu)和性質(zhì)。氨基酸是既含氨基又含酸性竣基的化合物。蛋白質(zhì)

9、中的氨基酸在a-碳原子上同 時有氨基和竣基。a 氨基酸具有如下的一般結(jié)構(gòu)：在中性pH水溶液中，氨基和竣基都呈離子狀態(tài)?？⒒ヒ粋€質(zhì)子而帶負(fù)電荷，同時氨基 得到一個質(zhì)子而帶正電荷。結(jié)果氨基酸便具有偶極的特性。氨基酸的這種偶極形式（即兩性 形式）有如下的一般結(jié)構(gòu)：氨基酸有好幾種性質(zhì)都反映了這種結(jié)構(gòu)，這些性質(zhì)是：它們易溶于水而不易溶于極性很小的 溶劑：當(dāng)以晶體形式存在時，它們要在較高溫度（一般在200C以上）下熔化或分解；它們 在中性溶液種顯示出很大的偶極矩和介電常數(shù)。氨基酸的側(cè)鏈R基團對氨基酸和蛋白質(zhì)的化學(xué)性質(zhì)產(chǎn)生重大的影響。這些側(cè)鏈可以分為 四類。帶有極性非荷電的（親水的）R基團的氨基酸能與水

10、形成氫鍵，通常能溶于水溶液。絲 氨酸、蘇氨酸和酪氨酸的羥基，半胱氨酸的硫氫基（即硫醇）以及天冬酰胺和谷氨酰胺的酰 胺基時出現(xiàn)在這類氨基酸R基團中的功能局部，其中半胱氨酸的硫羥基和酪氨酸的羥基在 PH7時能輕度離子化，因而比這類中其它氨基酸更容易失去質(zhì)子。天冬酰胺和谷氨酰胺的酰 胺基容易被酸和堿水解，分別形成天冬氨酸和谷氨酸。帶有非極性（疏水的）R集團的氨基酸在水溶液中的溶解性比帶有極性非荷電的R基團 的氨基酸要小得多。帶有煌側(cè)鏈的五種氨基酸，其側(cè)鏈隨側(cè)鏈長度增加而降低。脯氨酸（以 及其煌基衍生物羥脯氨酸）的獨特結(jié)構(gòu)使這種氨基酸在蛋白質(zhì)結(jié)構(gòu)中有獨特的地位。PH6-7時帶正電荷（堿性的）R基團的氨

11、基酸有賴氨酸、精氨酸和組氨酸。賴氨酸帶 正電的原因主要在于氨基，而精氨酸那么具有帶正電荷的胭基。pH7時組氨酸分子中的咪嗖基 有10%質(zhì)子化，但在pH6時那么有50%以上帶正電荷。二羥基氨基酸（天冬氨酸和谷氨酸）在中性pH范圍內(nèi)帶凈負(fù)電荷，谷氨酸的一鈉鹽是 一種重要的膳食調(diào)味用的人造食品添加劑。當(dāng)一個氨基酸分子的氨基與另一個氨基酸分子的竣基起反響時，便形成一個肽鍵，同時 釋放出一分子水。這種C-N鍵把眾多的氨基酸連接在一起形成蛋白質(zhì)。這種肽鍵比其它簡單的C-N鍵略短，這說明由于肽鍵與好基氧的共振穩(wěn)定作用，肽鍵具有了 一定的雙鍵特性。這樣，緊鄰肽鍵的基團就不能自由轉(zhuǎn)動了。肽鍵的這種剛性使六個原子

12、保 持在一個平面上。H由于肽鍵的雙鍵性質(zhì)，亞氨基（-NH-）在pHO14之間均不能離子化。此外，由于立體位阻 現(xiàn)象，氨基酸殘基上的R基團迫使肽鍵上的氧原子和氫原子只能以反式構(gòu)型存在。因此，多 肽和蛋白質(zhì)的主鏈只可能在氨基酸之間的三個鍵中的兩個鍵上作自由轉(zhuǎn)動。如果少數(shù)幾個氨基酸以肽鍵連接起來，這樣的化合物就稱為“肽”大多數(shù)天然肽是由蛋 白質(zhì)局部水解形成的?？墒?，有少數(shù)肽那么是重要的代謝產(chǎn)物。鵝肌肽和肌肽是動物肌肉中組 氨酸的兩種衍生物，這些肽生物化學(xué)功能目前還不清楚。谷胱甘肽存在于哺乳動物血液、酵母之中，特別是快速分解的細(xì)胞質(zhì)組織中。一般認(rèn)為， 這種肽具有參與氧化代謝和解毒作用的功能。氧化過程中

13、，兩分子谷胱甘肽通過兩個半胱氨 酸殘基之間的二硫鍵-SS連接起來。在蛋白質(zhì)中未曾發(fā)現(xiàn)谷氨酸的Y-段基與半胱氨酸連成 的肽鍵。此外，還有具抗體和激素功能的肽。催產(chǎn)素和抗利尿素就是肽激素的例子。蛋白質(zhì)的結(jié)構(gòu)蛋白質(zhì)執(zhí)行的多種多樣的生物功能，而且由于它是數(shù)百個氨基酸組成的，故其結(jié)構(gòu)遠(yuǎn)比 肽為復(fù)雜。酶是生物中產(chǎn)生的球狀蛋白質(zhì)，目的是專門催化某些生物化學(xué)反響，不然的話，這些化 學(xué)反響在生理條件下是不會發(fā)生的。血紅蛋白和肌紅蛋白是輸送血液和肌肉中氧和二氧化碳 的含血紅素的蛋白質(zhì)。重要的肌肉蛋白一肌動蛋白和肌球蛋白把化學(xué)能轉(zhuǎn)變成機械能；而腱 中的蛋白質(zhì)（膠原蛋白和彈性蛋白）那么是將肌肉粘連在骨骼上。皮膚、毛發(fā)

14、、指（趾）甲是 蛋白質(zhì)類的保護物質(zhì)。食品科學(xué)家之所以關(guān)注食物蛋白質(zhì)，是因為掌握了蛋白質(zhì)結(jié)構(gòu)和功能 方面的知識，才能更好的加工和處理食品，造福于人類。用21種天然存在的氨基酸幾乎可以合成無數(shù)的蛋白質(zhì)?？墒?，據(jù)估計自然界只存在約 2000種不同的蛋白質(zhì)，如果考慮不同物種蛋白質(zhì)存在的微小差異，那么蛋白質(zhì)數(shù)目就超過此 數(shù)。蛋白質(zhì)分子內(nèi)氨基酸的直線排列次序被看作是蛋白質(zhì)的一級結(jié)構(gòu)。少數(shù)蛋白質(zhì)的一級結(jié) 構(gòu)已經(jīng)被確定。而且已在實驗室里合成了其中一種蛋白質(zhì)（核糖核酸）。正是這種氨基酸的 獨特排列順序賦予不同蛋白質(zhì)以許多基本特性,且在很大程度上決定了它們的二級和三級結(jié) 構(gòu)。如果蛋白質(zhì)中含有大量帶疏水基團的氨基酸

15、，那么它在水溶劑中的溶解性就可能比帶許多 親水基團的氨基酸的蛋白質(zhì)差。如果蛋白質(zhì)的一級結(jié)構(gòu)不是折疊的，那么蛋白質(zhì)分子就會很長很細(xì)。分子量為13,000 蛋白質(zhì)分子就應(yīng)有448納米長和3.7納米粗這種結(jié)構(gòu)將使它們可能與其它物質(zhì)發(fā)生過度的相 互反響，然而這樣的結(jié)構(gòu)在自然界還未發(fā)現(xiàn)。蛋白質(zhì)鏈中相互靠近的鏈節(jié)與鏈節(jié)之間的三維 排列方式即為蛋白質(zhì)的“二級結(jié)構(gòu)”。二級結(jié)構(gòu)的具體例子有羊毛蛋白的。螺旋結(jié)構(gòu)，蠶絲 蛋白的折疊片結(jié)構(gòu)和膠原蛋白的螺旋結(jié)構(gòu)。蛋白質(zhì)的自然結(jié)構(gòu)是含有最低可能自由能的結(jié)構(gòu)。因此，蛋白質(zhì)的結(jié)構(gòu)不是任意的，而 是有一定規(guī)那么的。當(dāng)球狀蛋白質(zhì)的聚氨基酸鏈上受到肽鍵約束時，右螺旋（即Q-螺旋）看

16、 來是最有規(guī)那么、最為穩(wěn)定的合理結(jié)構(gòu)之一。每一圈a-螺旋的蛋白質(zhì)主鏈上含有3.6個氨基酸殘基，而氨基酸的R基團那么從螺旋結(jié) 構(gòu)的軸線向外伸出，一個肽鍵的氮能夠與另一個沿蛋白質(zhì)主鏈相距四個氨基酸殘基處的氧形 成氫鍵。此氫鍵差不多與a-螺旋軸線平行，賦予螺旋結(jié)構(gòu)以強度。由于這樣的排列能使每 一個肽鍵都能形成氫鍵，因此大大加強了結(jié)構(gòu)的穩(wěn)定性。螺旋圈是非常緊密和堅固的，所以 即使象水那樣的有強烈參與形成氫鍵趨勢的物質(zhì)，也不能進入螺旋中央局部。出現(xiàn)在許多纖維狀蛋白質(zhì)中的二級結(jié)構(gòu)是8-折疊片結(jié)構(gòu)。在這構(gòu)型中，肽主鏈呈鋸齒 形，其氨基酸的R基團向肽鏈的上方和下方伸展。由于所有肽鍵都可供氫鍵形成，故這種構(gòu) 型

17、能夠使相鄰的多肽鏈之間充分形成交聯(lián)，從而具有良好的穩(wěn)定性。有兩種折疊片，即相鄰 多肽鏈走向相同的平行折疊片和走向相反的反平行折疊片，均有可能存在。如果多肽鏈中R 基團過大，或帶有同種電荷，那么R基團間的相互作用使B -折疊片不可能形成。蠶絲和昆蟲 纖維蛋白是8 折疊結(jié)構(gòu)的最好例子，而鳥類羽毛中所含的是這種構(gòu)型的復(fù)雜形式。纖維蛋白的另一種二極結(jié)構(gòu)是膠原螺旋。膠原螺旋是高等脊椎動物中最豐富的一種蛋白 質(zhì)，占動物體蛋白總量的1/3o膠原蛋白中含有13的甘氨酸和14脯氨酸或羥脯氨酸，能 抵抗拉伸，是腱的主要組分。由于R基團的剛性以及脯氨酸、羥脯氨酸參與的肽式鍵合不能 形成氫鍵的原因，所以Q-螺旋結(jié)構(gòu)無

18、法形成，迫使膠原多肽鏈變成一種零散結(jié)節(jié)式的螺旋 體，膠原多肽鏈中由甘氨酸構(gòu)成的肽鍵與另兩條多肽鏈形成了鍵間氫鍵，產(chǎn)生了一種穩(wěn)定的 三股螺旋。此三股螺旋結(jié)構(gòu)稱為“原膠原”，其分子量為30萬道爾頓。蛋白質(zhì)鏈中大鏈段的排列方式稱為蛋白質(zhì)的“三級結(jié)構(gòu)”。它包括二級結(jié)構(gòu)常規(guī)單元的 折疊以及無二級結(jié)構(gòu)肽鏈假設(shè)干區(qū)域的結(jié)構(gòu)化。例如，某些蛋白質(zhì)中包含了有a-螺旋結(jié)構(gòu)存 在的區(qū)域和另外不能形成這種結(jié)構(gòu)的區(qū)域。根據(jù)氨基酸順序的不同，此a-螺旋段的長度也 不同，并賦予獨特的三級結(jié)構(gòu)，這些折疊局部是靠R基團之間所形成的氫鍵，靠鹽鍵、疏水 相互作用以及共價二硫鍵（SS-）而結(jié)合在一起的。至此所討論的結(jié)構(gòu)僅涉及單個肽鏈的

19、結(jié)構(gòu)。各個（亞單位）多肽鏈相互作用變成天然蛋白質(zhì) 分子時所形成的結(jié)構(gòu)即為蛋白質(zhì)的“四級結(jié)構(gòu)”。使蛋白質(zhì)鍵結(jié)合在一起的鍵合機制通常與 三級結(jié)構(gòu)中所述的相同，可能的例外情況是雙硫鍵不參與蛋白質(zhì)四級結(jié)構(gòu)的保持。專業(yè)英語詞匯intricate a.復(fù)雜的，錯綜的,纏結(jié)的灘懂的 collagenous a.膠原的 globulin球蛋白 plasma血漿，原生質(zhì) immunological 免疫的hemoglobin血紅蛋白 basic amino堿性的氨基 acidic carboxyl酸性的較基 aqueous水的含水的水成的proton質(zhì)子，咒核 dipolar偶極的,兩極的 zwitterion

20、兩性離子 crystalline結(jié)晶的，晶狀 清澈的hydrophilic 親水的serine絲氨酸，羥基丙氨酸 threonine羥基丁氨酸,蘇氨酸 tyrosine酪氨酸,3-對羥苯基丙氨酸 sulfhydryl氫硫的 enzyme硫化氫解酶 group筑基cysteine半胱氨酸，筑基丙氨酸 cystine胱氨酸,雙疏丙氨酸amide酰胺氨化物 asparagine天門冬酰胺 glutamine谷氨酰胺 aspartic acid天門冬氨酸,丁氨二酸 glutamic acid 谷氨酸proline脯氨酸,氮戊環(huán)-2-基竣酸 lysine賴氨酸 arginine精氨酸 histidine

21、組氨酸,咪坐丙氨酸 quanidino 呱imidazol n.咪喋;1,3-二氮雜茂 adequate supplies of protein poses a much greater problem than providing adequate supplies of either carbohydrate or fat. Proteins not only are more costly to produce than fats or carbohydrates but the daily protein requirement per kilogram of bodyweight r

22、emains constant throughout adult life, whereas the requirements for fats and carbohydrates generally decrease with age.As briefly described above, proteins have diverse biological functions, structures, and properties. Many proteins are susceptible to alteration by a number of rather subtle changes

23、in the immediate environment. Maximum knowledge of the composition, structure, and chemical properties of the raw materials, especially proteins, is required if contemporary and future processing of foods is to best meet the needs of mankind. A considerable amount of information is already available

24、, although much of it has been collected by biochemists using a specific food component as a model system,Amino AcidsAmino acids are the “building blocks of proteins. Therefore, to understand the properties ofresonance n.回聲，反響 物共振,共鳴;諧振 醫(yī)叩響imino亞氨 steric a.空間的，位的 anserine鵝肌肽 carnosine 肌肽glutathione谷

25、胱甘肽 peptide肽,縮氨酸 oxytocin n.（垂體）后葉催產(chǎn)素 vasopressin n.后葉加（血）壓素,加壓素 carbonyl鍛基,碳酰 hemoglobin血紅蛋白 hemo表示“血”myoglobin肌紅蛋白 actin肌動蛋白 myosin肌球蛋白 tendon腱,筋根 collagen膠原,膠原蛋白 elastin彈性蛋白 ribonuclease核糖核酸酶 hydrophobic 疏水的restriction n.限制，限定，約束vertebrate n.脊椎動物a.有椎骨的，有脊椎的，脊椎動物的（作品等）結(jié)構(gòu)嚴(yán)密的 kink n.（繩索，頭發(fā)的）細(xì)結(jié)，絞纏奇想，

26、怪念頭，乖僻（奇特的）妙法（頸背等處的）肌肉痙攣，抽筋美（結(jié)構(gòu)或設(shè)計等的）缺陷 vt.使紐結(jié)，使絞纏vi紐結(jié)，打結(jié)glycine甘氨酸，氨基醋酸t(yī)ropocollagen 原膠原dalton道爾頓devoid a.缺乏/殳有（of）covalent共價 bond共價鍵quaternary a.四個一組的，四部組成的，第四的 四元的,四價的，季的 地第四紀(jì)的n.四，四個一組,第四組中的組成局部 數(shù)四進制 地第四紀(jì)zigzag之字形，Z字形，鋸齒形；之字形的線條（或道路、壕溝、裝飾等）；蜿蜒曲折，盤旋彎曲專業(yè)英語總結(jié)EnglishKnowledge point:1. be of +名詞，相當(dāng)于形容

27、詞。F： Proteins are molecules of great size, complexity, and diversity, proteins are molecules of great size, complexity, and diversity, roteins are molecules of great size, complexity, and diversity, proteins are molecules of great size, complexity, and diversity.2.at PH 7, at neutral PH （用介詞 at）SOME

28、 GOOD SENTENCE:Amino acids are the building blocks of proteins.Skin the protective covering of the body, often accounts for about 10% of the total body protein.Skin the protective covering of the body, often accounts for about 10% of the total body protein.Many proteins are susceptible to alteration

29、 by a number of rather subtle changes in the immediate environment.A considerable amount of information is already available, although much of it has been collected by biochemists using a specific food component as a model system.Several properties of amino acids provide evidence for this structure.

30、The amino is responsible for the positive charge of lysine .while arginine has a positively charged quanidino group.English Knowledge point:be of +名詞，相當(dāng)于形容詞。F： Proteins are molecules of great size, complexity, and diversity, proteins are molecules of great size, complexity, and diversity, roteins ar

31、e molecules of great size, complexity, and diversity, proteins are molecules of great size, complexity, and diversity.at PH 7, at neutral PH （用介詞 at）專業(yè)英語難點SOME GOOD SENTENCE:Amino acids are the “building blocks“ of proteins.Skin the protective covering of the body, often accounts for about 10% of th

32、e total body protein.2.Skin the protective covering of the body, often accounts for about 10% of the totalbody protein.Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment.A considerable amount of information is already available, although muc

33、h of it has been collected by biochemists using a specific food component as a model system.Several properties of amino acids provide evidence for this structure.The amino is responsible for the positive charge of lysine .while arginine has a positively charged quanidino teins, a discussion

34、 of the structures and properties o f amino acids is required. Amino acids are chemical compounds, which contain both basic amino groups and acidic carboxyl groups. Amino acids found in proteins have both the amino and carboxyl groups on the a-carbon atom; a-amino acids have the following general st

35、ructure:At neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized. The carboxyl group loses a proton and obtains a negative charge, while the amino group gains a proton and hence acquires a positive charge. As a consequence, amino acids possess dipolar characterist

36、ics. The dipolar; or zwitterions, form of amino acids has the following general structure:Several properties of amino acids provide evidence for this structure: they are more soluble in water than in less polar solvents; when present in crystalline form they melt or decompose at relatively high temp

37、eratures (generally above 200): and they exhibit large dipole moments and large dielectric constants in neural aqueous solutions.The R groups or side chains, of amino acids and proteins, these side chains may be classified in to four groups.Amino acids with polar-uncharged (hydrophilic) r groups can

38、 hydrogenbond with water and are generally soluble in aqueous solutions. The hydroxyls of serine, heroine, and tyrosine; the sulfhydryl of thinly of cysteine, and the amides of asparagines and glutamine are the functional moieties present in r groups of the class of amino acids. Two of these, the to

39、il of cysteine and the hydroxyl of tyrosine, are slightly ionized at PG 7 and can lose a proton much more readily than others in this class. The amides of asparagines and glutamine are readily hydrolyzed by acid or base to aspartic and glutamic acids, respectively.Amino acids with nonpolar (hydropho

40、bic) r groups are less soluble in aqueous solvents than amino acids with polar uncharged r groups. Five amino acids with hydrocarbon side chains decrease in polarity as the length of the side chain is increased. The unique structure of praline (and its hydoxylated derivative, hydroxyproline) causes

41、this amino acid to play a unique role in protein structure.The amino acids with positively charged (basic) r groups at ph 6-7 are lysine; argiine has a positively charged quanidino group. At ph 7.0 10% of the imidazole groups of histidine molecules are prorogated, but more than 50% carry positive at

42、 ph 6.0.The dicarboxylic amino acids, asparic glutamic, possess net negative charges n the neutral ph range. An important artificial meal-flavoring food additive is the monosodium salt of glutamic acid.PeptidesWhen the amino group of one amino acid reacts with the carboxyl group of another amino aci

43、d, a peptide bond is formed and a molecule of water is released. This can bond joins amino acids together to form proteinsThe peptide bond is slightly shorter than otter single c-n bonds. This indicates that the peptide bond has some characteristics of a double bond, because of resonance stabilizati

44、on with the carbonyl oxygen. Thus group adjacent to the peptide bond cannot rotate freely, this rigidity of the peptide bond holds the six atoms in a single plane. the amino (_NH_) group does not ionize between ph o and 14 due to the double-bond properties of the peptide bond. In addition, r groups

45、on amino acid residues, because of starch hindrance, force oxygen and hydrogen of the peptide bond to exist on a trans configuration. Therefore, the backbone of peptides and proteins has free rotation in two of the three bonds between amino acids.If a few amino acids are joined together by peptide b

46、onds the compound is called a most natural peptides are formed by the partial hydrolytic of proteins; however; a few peptides are important metabolites. Ansetime and carnosine are two derivatives of histamine that are found in muscles pf animals. The biochemical function of these peptides is not und

47、erstood.Glutathione occurs in mammalian blood, yeast, and especially in tissues of rapidly dividing cells.It is thought to function in oxidative metabolism and detoxification.Duirng oxidation, two moletcules of glutathiune join vin a disulfide bridge (-S-S) between two cysteine is not found in prote

48、ins.Other peptides functino as antibodies and hormones. Oxytocin and hormones. Oxytocin and vasopressin are examples of peptide hormones.Protein structureProteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids, their structures are much mere com

49、plex than those of peptides.Enzymes are globular proteins produced in living matter for the special purpose of catalyzing vital chemical reactions that otherwise do not occur under physiological conditions. Hemoglobin and myoglobin are hemo-containing proteins that transport oxygen and carbon dioxid

50、e in the blood and muscles. The major muscle proteins, actin and myosin, convert chemical energy to mechanical work, while proteins in tendons (collagen and elastim) bind muscles to bones, skin, hairy fingernails, and toenails are pertinacious protective substance. The food scientist is concerned ab

51、out proteins in foods since knowledge of protein structure and behavior allows him to more ably manipulate foods for the benefit mankind.Nearly an infinite number of proteins could be synthesized from the 21natural occurring amino acids. However, it has been estimated that only about 2000 different

52、proteins exist in nature.The number is greater than this if one considers the slight variations found in proteins from different species.The linear sequence of amino acids in protein is referred toast primary structure In a few proteins the primary structure has been determined and one protein (ribo

53、nuclease) has been synthesized in the laboratory. It is the unique sequence of amino acids that imparts many of the fundamental properties to different protein and tertiary structures. If the protein contains a considerable number of amino acids with hydrophobic groups, its solubility in aqueous sol

54、vents is probable less than that of proteins containing amino acids with many hydrophilic groups.If the primary structure of the protein were not folded, protein molecules would be excessively long and thin. A protein having a molecular weight of 13,000 would be 448 a thick. This structure allows ex

55、cessive interaction with other substances, and it is not found in nature The three-dimensional manner in which relatively close members of the protein chain are arranged is referred to as secondary structure/ examples or secondary structure are the a-helix of wool, the pleated-sheet configuration of

56、 silk, and the collagen helix.The native structure of a protein is that structure which possesses the lowest feasible free energy.Therefore, the structure of a protein is not random but somewhat ordered. when the restrictions of the peptide bond are superimposed on a polyamino acid chain of a globul

57、ar protein, a right handed coil, the -helix, appears to be one of the most ordered and stable structures feasible.the 0c-helix contains 3.6 amino acid residues per turn I of the protein backbone, with the r groups of the amino acids extending outward from the axis of the helical structure, hydrogen

58、bonding can occur between the nitrogen of one peptide bond and the oxygen of another peptide bond four residues along the protein chain, the hydrogen bonds are nearly parallel to the axis of the helix, lending strength to the helical structure, since this arrangement allows each peptide bond to form

59、 a hydrogen bond, the stability of the structure greatly enhanced. The coil of the helix is sufficiently compact and stables that even substances with strong tendencies to participate in hydrogen bonding, such as water, cannot enter the core.A secondary saturation found in many fibrous proteins is t

60、he p-pleated sheet configuration. In this configuration the peptide backbone forms a zigzag pattern, with the r groups of the amino acids extending alive and below the peptide chain. Since all peptide bonds are available for hydrogen bonding, this configuration allows maximum cross-linking between a