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Proteins 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 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 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 enzymes 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 various 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 more acute and widespread as the world’s population increases. providing
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 remains 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 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, although much of it has been collected by biochemists using a specific food component as a model system,
Amino Acids
Amino acids are the “building blocks” of proteins. Therefore, to understand the properties of proteins, a discussion 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 structure:
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 characteristics. 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 temperatures (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 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 toil 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 (hydrophobic) 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 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 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.
Peptides
When the amino group of one amino acid reacts with the carboxyl group of another amino acid, a peptide bond is formed and a molecule of water is released. This can bond joins amino acids together to form proteins
The 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 stabilization with the carbony1 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 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 bonds 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 understood.
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 proteins.
Other peptides functino as antibodies and hormones. Oxytocin and hormones. Oxytocin and vasopressin are examples of peptide hormones.
Protein structure
Proteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids, their structures are much mere complex 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 dioxide 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 about 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 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 (ribonuclease) 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 solvents 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 excessive 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 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 globular protein, a right handed coil, the ∝-helix, appears to be one of the most ordered and stable structures feasible.
the ∝-helix contains 3.6 amino acid residues per turn lof the protein backbone, with the r groups of the amino acids extending outward from the axis of the helical structure, hydrogen 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 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 the β-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 adjacent polypeptide chains and thus good stability. Both parallel-pleated sheet, where the polypeptide chains run in opposite directions, are possible. Where groups are bulky or have little charges, the interactions of the r groups do not allow the pleated-sheet configuration to exist. silk and insect fibers are the best examples of theβ-sheet, although feathers of birds contain a complicated form of these configuration.
Another type of secondary structure of fibrous proteins is the collagen helix. collagen is the most abundant protein in higher vertebrates, accounting for one-third of the total body protein, collagen resists stretching, is the major component of tendons, and contains one-third glycine and one-fourth proline or hydroxyprolinethe rigid r groups, and the lack of hydrogen bonding by peptide linkages involving proline and hydroxyproline, prevents formation of an ∝-helical structure and forces the collagen polypeptide chain into an odd kinked-type helix. Peptide bonds composed of glycine form interchain hydrogen bonds with two other collagen polypeptide chains, and this results in a stable triple helix. This triple-helical structure is called “tropocollagen” and it has a molecular weight of 3000,000 Daltons.
The manner in, which large portions of it protein chain are arranged is referred to as tertiary structure. This involves folding of regular unts of the secondary structure as well as the structuring of areas of the peptide chain that are devoid of secondary structure. for example, some proteins contain areas where ∝-helical structure exists and other areas where this structure cannot form. depending on the amino acid sequence, the length of the ∝-helical portions are held together by hydrogen bonds formed between r groups, by salt linkages, by hydrophobic interactions, and by covalent disulfide(-s-s-0 linkages.
The structures discussed so far have involved only a single peptide chain. The structure formed when individual (subunit) polypeptide chains interact to form a native protein molecule is referred to as “quaternary structure”. The bonding mechanisms that hold protein chains together are generally the same as those involved in tertiary structure, with the possible exception that disulfide bonds do not assist in maitaining the quaternary structures of proteins
第四课 氨基酸和蛋白质
蛋白质错综复杂、多种多样的大分子物质,是食物必须氨基酸和非必须氨基酸的来源。人体利用这些氨基酸以满足生长发育、修复组织和维持正常健康生活的要求。这些大分子以含氮为其特征，参与了许多与各种有生命物质有复杂联系的生命过程。在包括人类在内的哺乳动物中，蛋白质起着机体改造成分的作用，肌肉和许多体内器官主要由蛋白质构成。骨骼中的矿物质靠胶原蛋白得以保持在一起�；宓谋；げ恪し糁械牡鞍字释ǔＵ蓟宓鞍字首芰康�10%的左右。
有些蛋白质有生物催化剂（酶和激素）的作用，以调节体内的化学反应�；镜纳倘缟�、消化、代谢、排泄、化学能转变成机械功等等都受酶和激素的控制。某些体液的渗透压和pH值受制于血浆蛋白和血红蛋白。蛋白质对免疫反应是必不可少的�？固澹ǜ男缘难虻鞍啄芤鸺膊〉耐饫丛又屎臀⑸锏娜肭�。当某些摄入的蛋白质使防御机制产生明显的变化时，便发生人体的生物过敏。这就导致某些个体身上出现各种各样的疾病，且有时是急剧的病情。
食物短缺现象在世界许多地区存在。随着人口的增加，这个问题很可能愈来愈尖锐、愈普遍。而蛋白质供应不足问题远比碳水化合物或脂肪供应不足更为严重。蛋白质不仅它的产出费用要比碳水化合物或脂肪的产出费用为高，而且每千克每天所需的蛋白质量造整个成年期是恒定的，而每天所需的脂肪和碳水化合物量一般都随着年龄的增长而逐渐减少。
正如上面简述的一样，蛋白质有多种不同的结构、性质和生理功能。许多蛋白质容易受周围环境的一系列微妙变化的影响而发生变化。要想使现在和将来的食品加工能理想的满足人类的需要，就必须彻底了解原料特别是蛋白质的组成结构和化学性质。目前，已经有这方面的大量资料可供利用，不过其中大部分是生物化学家利用某一特定食物成分作为模拟物系加以收集的。
氨基酸
氨基酸是蛋白质的“结构单元”。因此，要了解蛋白质的性质，旧需要讨论氨基酸的结构和性质。氨基酸是既含氨基又含酸性羧基的化合物。蛋白质中的氨基酸在α-碳原子上同时有氨基和羧基。α-氨基酸具有如下的一般结构：
在中性pH水溶液中，氨基和羧基都呈离子状态。羧基失去一个质子而带负电荷，同时氨基得到一个质子而带正电荷。结果氨基酸便具有偶极的特性。氨基酸的这种偶极形式（即两性形式）有如下的一般结构：
氨基酸有好几种性质都反映了这种结构，这些性质是：它们易溶于水而不易溶于极性很小的溶剂：当以晶体形式存在时，它们要在较高温度（一般在200℃以上）下熔化或分解；它们在中性溶液种显示出很大的偶极矩和介电常数。
氨基酸的侧链R基团对氨基酸和蛋白质的化学性质产生重大的影响。这些侧链可以分为四类。
带有极性非荷电的（亲水的）R基团的氨基酸能与水形成氢键，通常能溶于水溶液。丝氨酸、苏氨酸和酪氨酸的羟基，半胱氨酸的硫氢基（即硫醇）以及天冬酰胺和谷氨酰胺的酰胺基时出现在这类氨基酸R基团中的功能部分，其中半胱氨酸的硫羟基和酪氨酸的羟基在pH7时能轻度离子化，因而比这类中其它氨基酸更容易失去质子。天冬酰胺和谷氨酰胺的酰胺基容易被酸和碱水解，分别形成天冬氨酸和谷氨酸。
带有非极性（疏水的）R集团的氨基酸在水溶液中的溶解性比带有极性非荷电的R基团的氨基酸要小得多。带有烃侧链的五种氨基酸，其侧链随侧链长度增加而降低。脯氨酸（以及其烃基衍生物羟脯氨酸）的独特结构使这种氨基酸在蛋白质结构中有独特的地位。
pH6～7时带正电荷（碱性的）R基团的氨基酸有赖氨酸、精氨酸和组氨酸。赖氨酸带正电的原因主要在于氨基，而精氨酸则具有带正电荷的胍基。pH7时组氨酸分子中的咪唑基有10%质子化，但在pH6时则有50%以上带正电荷。
二羟基氨基酸（天冬氨酸和谷氨酸）在中性pH范围内带净负电荷，谷氨酸的一钠盐是一种重要的膳食调味用的人造食品添加剂。
肽
当一个氨基酸分子的氨基与另一个氨基酸分子的羧基起反应时,便形成一个肽键，同时释放出一分子水。这种C-N键把众多的氨基酸连接在一起形成蛋白质。
这种肽键比其它简单的C-N键略短,这说明由于肽键与羰基氧的共振稳定作用，肽键具有了一定的双键特性。这样，紧邻肽键的基团就不能自由转动了。肽键的这种刚性使六个原子保持在一个平面上。 H
由于肽键的双键性质，亚氨基（-NH-)在pH0～14之间均不能离子化。此外，由于立体位阻现象，氨基酸残基上的R基团迫使肽键上的氧原子和氢原子只能以反式构型存在。因此，多肽和蛋白质的主链只可能在氨基酸之间的三个键中的两个键上作自由转动。
如果少数几个氨基酸以肽键连接起来，这样的化合物就称为“肽”大多数天然肽是由蛋白质部分水解形成的。可是，有少数肽则是重要的代谢产物。鹅肌肽和肌肽是动物肌肉中组氨酸的两种衍生物，这些肽生物化学功能目前还不清楚。
谷胱甘肽存在于哺乳动物血液、酵母之中，特别是快速分解的细胞质组织中。一般认为，这种肽具有参与氧化代谢和解毒作用的功能。氧化过程中，两分子谷胱甘肽通过两个半胱氨酸残基之间的二硫键-S-S-连接起来。在蛋白质中未曾发现谷氨酸的γ-羰基与半胱氨酸连成的肽键。
此外，还有具抗体和激素功能的肽。催产素和抗利尿素就是肽激素的例子。
蛋白质的结构
蛋白质执行的多种多样的生物功能，而且由于它是数百个氨基酸组成的，故其结构远比肽为复杂。
酶是生物中产生的球状蛋白质，目的是专门催化某些生物化学反应，不然的话，这些化学反应在生理条件下是不会发生的。血红蛋白和肌红蛋白是输送血液和肌肉中氧和二氧化碳的含血红素的蛋白质。重要的肌肉蛋白—肌动蛋白和肌球蛋白把化学能转变成机械能；而腱中的蛋白质（胶原蛋白和弹性蛋白）则是将肌肉粘连在骨骼上。皮肤、毛发、指（趾）甲是蛋白质类的�；の镏�。食品科学家之所以关注食物蛋白质，是因为掌握了蛋白质结构和功能方面的知识，才能更好的加工和处理食品，造福于人类。
用21种天然存在的氨基酸几乎可以合成无数的蛋白质�？墒�，据估计自然界只存在约2000种不同的蛋白质，如果考虑不同物种蛋白质存在的微小差异，则蛋白质数目就超过此数。
蛋白质分子内氨基酸的直线排列次序被看作是蛋白质的一级结构。少数蛋白质的一级结构已经被确定。而且已在实验室里合成了其中一种蛋白质（核糖核酸）。正是这种氨基酸的独特排列顺序赋予不同蛋白质以许多基本特性，且在很大程度上决定了它们的二级和三级结构。如果蛋白质中含有大量带疏水基团的氨基酸，则它在水溶剂中的溶解性就可能比带许多亲水基团的氨基酸的蛋白质差。
如果蛋白质的一级结构不是折叠的，那么蛋白质分子就会很长很细。分子量为13,000蛋白质分子就应有448纳米长和3.7纳米粗这种结构将使它们可能与其它物质发生过度的相互反应，然而这样的结构在自然界还未发现。蛋白质链中相互靠近的链节与链节之间的三维排列方式即为蛋白质的“二级结构”。二级结构的具体例子有羊毛蛋白的α-螺旋结构，蚕丝蛋白的折叠片结构和胶原蛋白的螺旋结构。
蛋白质的自然结构是含有最低可能自由能的结构。因此，蛋白质的结构不是任意的，而是有一定规则的。当球状蛋白质的聚氨基酸链上受到肽键约束时，右螺旋（即α-螺旋）看来是最有规则、最为稳定的合理结构之一。
每一圈α-螺旋的蛋白质主链上含有3.6个氨基酸残基，而氨基酸的R基团则从螺旋结构的轴线向外伸出，一个肽键的氮能够与另一个沿蛋白质主链相距四个氨基酸残基处的氧形成氢键。此氢键差不多与α-螺旋轴线平行，赋予螺旋结构以强度。由于这样的排列能使每一个肽键都能形成氢键，因此大大加强了结构的稳定性。螺旋圈是非常紧密和坚固的，所以即使象水那样的有强烈参与形成氢键趋势的物质，也不能进入螺旋中央部分。
出现在许多纤维状蛋白质中的二级结构是β-折叠片结构。在这构型中，肽主链呈锯齿形，其氨基酸的R基团向肽链的上方和下方伸展。由于所有肽键都可供氢键形成，故这种构型能够使相邻的多肽链之间充分形成交联，从而具有良好的稳定性。有两种折叠片，即相邻多肽链走向相同的平行折叠片和走向相反的反平行折叠片，均有可能存在。如果多肽链中R基团过大，或带有同种电荷，则R基团间的相互作用使β-折叠片不可能形成。蚕丝和昆虫纤维蛋白是β-折叠结构的最好例子，而鸟类羽毛中所含的是这种构型的复杂形式。
纤维蛋白的另一种二极结构是胶原螺旋。胶原螺旋是高等脊椎动物中最丰富的一种蛋白质，占动物体蛋白总量的1/3。胶原蛋白中含有1\3的甘氨酸和1\4脯氨酸或羟脯氨酸，能抵抗拉伸,是腱的主要组分。由于R基团的刚性以及脯氨酸、羟脯氨酸参与的肽式键合不能形成氢键的原因，所以α-螺旋结构无法形成，迫使胶原多肽链变成一种零散结节式的螺旋体，胶原多肽链中由甘氨酸构成的肽键与另两条多肽链形成了键间氢键，产生了一种稳定的三股螺旋。此三股螺旋结构称为“原胶原”，其分子量为30万道尔顿。
蛋白质链中大链段的排列方式称为蛋白质的“三级结构”。它包括二级结构常规单元的折叠以及无二级结构肽链若干区域的结构化。例如，某些蛋白质中包含了有α-螺旋结构存在的区域和另外不能形成这种结构的区域。根据氨基酸顺序的不同，此α-螺旋段的长度也不同，并赋予独特的三级结构，这些折叠部分是靠R基团之间所形成的氢键，靠盐键、疏水相互作用以及共价二硫键（-S-S-）而结合在一起的。
至此所讨论的结构仅涉及单个肽链的结构。各个（亚单位）多肽链相互作用变成天然蛋白质分子时所形成的结构即为蛋白质的“四级结构”。使蛋白质键结合在一起的键合机制通常与三级结构中所述的相同，可能的例外情况是双硫键不参与蛋白质四级结构的保持。
专业英语词汇
intricate a.复杂的,错综的,缠结的,难懂的
collagenous a.胶原的
globulin 球蛋白
plasma 血浆，原生质
immunological 免疫的
hemoglobin 血红蛋白
basic amino 碱性的氨基
acidic carboxyl 酸性的羧基
aqueous ①水的 ②含水的 ③水成的
proton 质子,氕核
dipolar 偶极的,两极的
zwitterion 两性离子
crystalline ①结晶的,晶状 ②清澈的
hydrophilic 亲水的
serine 丝氨酸,羟基丙氨酸
threonine 羟基丁氨酸,苏氨酸
tyrosine 酪氨酸, 3-对羟苯基丙氨酸
sulfhydryl 氢硫的 ～enzyme 硫化氢解酶 ～ group 巯基
cysteine 半胱氨酸,巯基丙氨酸
cystine 胱氨酸,双巯丙氨酸
amide ①酰胺 ②氨化物
asparagine 天门冬酰胺
glutamine 谷氨酰胺
aspartic acid 天门冬氨酸,丁氨二酸
glutamic acid 谷氨酸
proline 脯氨酸,氮戊环-[2]-基羧酸
lysine 赖氨酸
arginine 精氨酸
histidine 组氨酸,咪唑丙氨酸
quanidino 胍
imidazol n. 咪唑;1,3-二氮杂茂
resonance n. ①回声,反响 ②[物]共振,共鸣;谐振 ③[医]叩响
imino 亚氨
steric a.空间的,位的
anserine 鹅肌肽
carnosine 肌肽
glutathione 谷胱甘肽
peptide肽,缩氨酸
oxytocin n.(垂体)后叶催产素
vasopressin n.后叶加(血)压素,加压素
carbonyl 羰基,碳酰
hemoglobin 血红蛋白 hemo-表示“血”
myoglobin 肌红蛋白
actin 肌动蛋白
myosin 肌球蛋白
tendon 腱,筋根
collagen 胶原,胶原蛋白
elastin 弹性蛋白
ribonuclease 核糖核酸酶
hydrophobic 疏水的
restriction n. 限制,限定,约束
vertebrate n.脊椎动物 a.①有椎骨的,有脊椎的,脊椎动物的 ②(作品等)结构严密的
kink n.①(绳索,头发的)细结,绞缠 ②奇想,怪念头,乖僻 ③(奇特的)妙法 ④(颈背等处的)
肌肉痉挛,抽筋 ⑤[美](结构或设计等的)缺陷 vt.使纽结,使绞缠 vi纽结,打结
glycine 甘氨酸,氨基醋酸
tropocollagen 原胶原
dalton 道尔顿
devoid a. 缺乏,没有(of)
covalent 共价 ～ bond共价键
quaternary a.①四个一组的,四部组成的,第四的 ②四元的,四价的,季的 ③[地]第四纪的
n.①四,四个一组,第四组中的组成部分 ②[数]四进制 ③[地]第四纪
zigzag ①之字形，Z字形，锯齿形；②之字形的线条（或道路、壕沟、装饰等）；③蜿蜒曲折，盘旋弯曲
专业英语总结
English Knowledge point:
1. be of +名词，相当于形容词。
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 (用介词at)
SOME GOOD SENTENCE:
1. 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.
2. Skin the protective covering of the body, often accounts for about 10% of the total body protein.
3. Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment.
4. 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.
5. Several properties of amino acids provide evidence for this structure.
6. The amino is responsible for the positive charge of lysine .while arginine has a positively charged quanidino group.
English Knowledge point:
1. be of +名词，相当于形容词。
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 (用介词at)
专业英语难点
SOME GOOD SENTENCE:
1. 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.
2. Skin the protective covering of the body, often accounts for about 10% of the total body protein.
3. Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment.
4. 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.
5. Several properties of amino acids provide evidence for this structure.
6. The amino is responsible for the positive charge of lysine .while arginine has a positively charged quanidino group.
