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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. providingadequate 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:
NH2
│
R ─ C ─ COOH
│
H
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:
NH3++
│
R ─ C ─ COO-
│
H
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
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。机体的�；げ�—皮肤中的蛋白质通常占机体蛋白质总量的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。抗体（改性的血浆球蛋白能引起疾病的外来杂质和微生物的入侵。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：
NH2
│
R ─ C ─ COOH
│
H
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：
NH3+
│
R ─ C ─ COO-
│
H
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。
