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1、第二章 蛋白质化学Protein Chemistry,2.1 蛋白质分子的组成一、蛋白质定义 蛋白质是一切生物体中普遍存在的，由天然氨基酸(amino acid)通过肽键(peptide bond)连接而成的生物大分子（macromolecule）。,the most widely distributed biomoleculesthe most abundant biomolecules(45%of human body)the most complex biomolecules the most diversified biological functions蛋白质是一种高分子含氮化合物，
2、分子量一般在一万到一百万，甚至高达数百万。,How are about proteins?,二、蛋白质的化学组成 1、元素组成major elementsC(5055%),H(7%),O(1920%),N(1319%),S(4%)trace elementsP,Fe,Cu,Zn,I,蛋白质元素组成的一个特征是无论样品来源如何，其含N含量一般约为16%，即1克的N相当于6.25克的蛋白质。粗蛋白%=N%6.25,2、蛋白质的分子组成蛋白质=多肽链+非蛋白质物质(辅基)polypeptide nonprotein（cofactor）,2.1.1 蛋白质的基本结构-氨基酸,一、蛋白质的水解 蛋白质可
3、以被酸、碱和蛋白酶水解，在水解过程中逐渐降解成分子量越来越小的肽段，直到最后成为氨基酸的混合物。蛋白质 多肽 寡肽 二肽 氨基酸 根据蛋白质的水解程度可分为完全水解 部分水解,三种蛋白质水解方法的优缺点1、酸水解6mol/L盐酸或 4mol/L硫酸水解20小时左右，完全水解。优点不引起消旋作用，得到的是L-氨基酸。缺点色氨酸完全被沸酸破坏，羟基氨基酸 和酰胺被部分水解。2、碱水解5mol/LNaOH共煮10-20小时，即可 完全水解。在水解过程中，多数氨基酸有不同程度的破坏，产生消旋作用。特别是引起精氨酸脱氨。,3、酶水解 不产生消旋作用，也不破坏氨基酸。但一种酶往往水解不彻底，需要几种酶的协
4、同作用才能使蛋白质完全水解，并且酶水解所需时间较长。常用的蛋白酶有动物蛋白酶 植物蛋白酶 微生物蛋白酶,二、氨基酸的结构与分类1、氨基酸的结构通式 COOH H2N-CH RAA的结构特点都是a-AA；除Gly外都具有光学活性(有D-型和L-型两种光学异构体)。,L-Amino acid,2、常见氨基酸的分类及结构 根据R基团的结构和性质，分为(1)、根据R基团的化学性质分为脂肪族氨基酸(Aliphatic AA)芳香族氨基酸(Aromatic AA)杂环族氨基酸（Heterocyclic AA）(2)、根据R基团的酸碱性分为酸性氨基酸(Acidic AA)碱性氨基酸(Basic AA)(3)
5、、根据R基团的电性质分为R基团带电荷：R基团不带电荷：极性氨基酸 Polar 非极性氨基酸 Nonpolar,R groups are non-polar,hydrophobic aliphatic or aromatic groups.R groups are uncharged.AAs are insoluble in H2O.,Non-polar and hydrophobic AAs,Polar and uncharged AAs,R groups are polar:-OH,-SH,and-NH2.R groups are highly reactive.AAs are solubl
6、e in H2O,that is,hydrophilic.,R groups have one-NH2.R groups are positively charged at neutral pH(=7.0).AAs are highly hydrophilic.,Basic AAs,Acidic AAs,R groups have COOH.R groups are negatively charged at physiological pH(=7.4).AAs are soluble in H2O.,Aspartic acid glutamic acid(Asp or D)(Glu or E
7、),Special amino acids-Gly,optically inactive,Special amino acids-Pro,Having a ring structure and imino亚胺 group,active thiol 巯基 groups to form disulfide bond,蛋白质分子中的稀有氨基酸和非蛋白氨基酸,羧基谷氨酸,羟基脯氨酸,锁链（赖氨）素,鸟氨酸,瓜氨酸,尿素循环的中间体,必需氨基酸：8种,Val Ile Leu Phe Met Trp Thr Lys 缬 异 亮 苯 蛋 色 苏 赖,携一两本带色书来,2.1.1.2 氨基酸的理化性质,rot
8、ation,4 吸光性所有的氨基酸在可见光范围内均无吸收，只有下列氨基酸在近紫外有吸收。Tyr:max=275nm Trp:max=280nm Phe:max=259nm,5、氨基酸的两性解离及等电点AA的两性解离,(两性化合物）(两性离子),纯晶体状态,COO-,H3N+,水溶后等电荷状态,总电荷 0,加 酸,总电荷“+”,COO-,H3N+,水溶后等电荷状态,总电荷 0,加 碱,总电荷“-”,H2N,H2O,pH=pI,pHpI,pHpI,amphoteric,cation,anion,氨基酸的滴定曲线的用途(1)反映在不同的pH条件下，AA的带电状态、两性解离及等电点。(2)给出氨基和羧
9、基的解离常数 pH=pK+lg质子受体/质子供体,溶液在某一pH时，AA的氨基和羧基的解离度完全相同，此时，AA分子所带的正、负电荷相等，即AA分子的净电荷为零，它在电场中既不向阳极移动，也不向阴极移动，此时，AA溶液所处的pH就称为该AA的等电点(用pI表示)。,等电点,pI of most protein is 5.0,and negatively charges in body fluid(pH7.4)pI 7.4:basic proteins:protamine,histonepI 7.4:acidic proteins:pepsin,R-CH-COOH k1 R-CH-COO-+H+
10、NH+3 NH3+（H 2A+）（HA+-）k2 R-CH-COO-+H+NH2（A-）K1=(HAH+)/H2A+K2=(A-H+)/HApH=pK1+logHA/H2A+pH=pK2+logA-/HA2pH=pK1+pK2+logA-/H2A+,当A-H2A+时，即为等电点，故pH=pI=(pK1+pK2)/2AA在溶液中的带电状态与溶液pH的关系当pH=pI时，净电荷为0；当pHpI时，AA带负电。当溶液pH偏离pI越远，AA所带的净电荷数目越多。,Table of a-Amino Acids Found in Proteins,对于一氨基一羧基的AA:pI=(pK1+pK2)/2,对于
11、一氨基二羧基的AA:pI=(pK1+pKR)/2,对于二氨基一羧基的AA:pI=(pKR+pK2)/2,2、氨基酸的重要化学反应 A、与茚三酮的反应,水合茚三酮,水合茚三酮,紫色物质,B、桑格尔反应（Sanger reaction）该反应用于鉴定多肽和蛋白质的氨基末端。,2，4二硝基氟苯（DNFB）,丹磺酰氯,C、埃德曼反应（Edman reaction）,Labeling the N-terminal residue with a fluorophore.Cleaving the labeled residue without breaking the peptide bonds of th
12、e rest part of the peptide.Determining the N-terminal residue with chromatography.Repeating the same procedure to the rest peptide until the whole sequence is determined.,First cycle of Edman degradation,Edman degradation,该反应用于测定多肽链N端氨基酸的排列顺序，是“蛋白质顺序自动分析仪”的工作原理。,2.1.2 肽（Peptide),A peptide bond is a
13、covalent bond formed between the carboxyl group of one AA and the amino group of its next AA with the elimination of one H2O molecule.,Peptides can be extended by adding multiple AAs through multiple peptide bonds in a sequential order.dipeptide,tripeptide,oligopeptide,polypeptide,AAs in peptides ar
14、e called as residues.,Biologically active peptides,Glutathione(GSH)Glutamic acidcystein glycine,GSH peroxidase,GSH reductase,NADPH+H+,NADP+,As a reductant to protect nucleic acids and proteins from toxin by discharging free radical or H2O2,Peptide hormones secreted from peptidergic neurons肽能神经元 or S
15、omatostatin生长抑素,Noacosapeptide,Octapeptide,Thyrotropin-release hormone促甲状腺素释放激素,Antidiuretic hormone抗利尿激素 Neuropeptides神经肽 responsible for signal transductionEnkephalin脑啡肽,Endorphin内啡肽,Dynorphin强啡肽,Substance P P物质,Neuropeptide Y神经肽Y,Peptides,thyrotropin-release hormone,Pyroglutamic acid histidine pr
16、olinamide,Neuropeptide,2.1.3 蛋白质的分类(1)根据蛋白质的化学组成分类简单蛋白质-只含有一条多肽链。如溶菌酶复合蛋白质-由简单的蛋白质和非蛋白质物质 结合而成。如血红蛋白(血红素)复合蛋白质又可分为 色蛋白、糖蛋白、磷蛋白、核蛋白、脂蛋白,(2)根据蛋白质的分子形状分类球状蛋白质-易溶解、能成结晶。纤维状蛋白质-分子形状不对称，类似棒 状或纤维状。(3)根据蛋白质的功能分类活性蛋白质如酶、激素、抗体等。非活性蛋白质是一类结构蛋白质，多起保护和支撑作用。如胶原蛋白和角蛋白。,The light produced by fireflies is the result
17、of a light-producing reaction involving luciferin and ATP that is catalyzed by the enzyme luciferase荧光素酶,Erythrocytes红细胞 contain large amounts of the oxygen-transporting protein hemoglobin,Transport Proteins,Transport proteins in blood plasma血浆 bind and carry specific molecules or ions from one orga
18、n to another.Hemoglobin of erythrocytes binds oxygen as the blood passes through the lungs,carries it to the peripheral tissues,and there releases it to participate in the energy-yielding oxidation of nutrients.The blood plasma contains,which carry lipids from the liver to other lipoproteins organs.
19、,Other kinds of transport proteins are present in the plasma membranes and intracellular membranes of all organisms;these are adapted to bind glucose,amino acids,or other substances and transport them across the membrane,Nutrient and Storage Proteins,The seeds of many plants store nutrient proteins
20、required for the growth of the germinating seedling.Particularly well-studied examples are the seed proteins of wheat,corn,and rice.Ovalbumin卵清蛋白,the major protein of egg white,and casein酪蛋白,the major protein of milk,are other examples of nutrient proteins.The ferritin found in some bacteria and in
21、plant and animal tissues stores iron.,Contractile or Motile Proteins,Some proteins endow cells and organisms with the ability to contract,to change shape,or to move about.Actin肌动蛋白and myosin肌球蛋白 function in the contractile system of skeletal muscle and also in many nonmuscle cells.Tubulin微管蛋白 is the
22、 protein from which microtubules are built.Microtubules act in concert with the protein dynein in flagella鞭毛and cilia纤毛 to propel cells.,Structural Proteins,Many proteins serve as supporting filaments,cables,or sheets,to give biological structures strength or protection.The major component of tendon
23、s and cartilage肌腱 is the fibrous protein collagen纤维软骨胶原蛋白,which has very high tensile strength.Leather is almost pure collagen.Ligaments韧带contain elastin弹性蛋白,a structural protein capable of stretching in two dimensions.,Hair,fingernails,and feathers consist largely of the tough,insoluble protein ker
24、atin.The major component of silk fibers and spider webs is fibroin.The wing hinges of some insects are made of resilin弹性蛋白,which has nearly perfect elastic properties.,Defense Proteins,Many proteins defend organisms against invasion by other species or protect them from injury.The immunoglobulins or
25、 antibodies,specialized proteins made by the lymphocytes淋巴细胞 of vertebrates,can recognize and precipitate or neutralize invading bacteria,viruses,or foreign proteins from another species.Fibrinogen and thrombin凝血酶 are blood-clotting proteins that prevent loss of blood when the vascular血管 system is i
26、njured.,Snake venoms,bacterial toxins,and toxic plant proteins,such as ricin蓖麻毒素,also appear to have defensive functions.Some of these,including fibrinogen纤维蛋白原,thrombin凝血酶,and some venoms,are also enzymes.,such as ricin,also appear to have defensive functions,The spider webs is fibroin,Regulatory P
27、roteins,Some proteins help regulate cellular or physiological activity.Among them are many hormones.Examples include insulin,which regulates sugar metabolism,and the growth hormone of the pituitary垂体.The cellular response to many hormonal signals is often mediated by a class of GTP-binding proteins
28、called G proteins(GTP is closely related to ATP,with guanine鸟嘌呤 replacing the adenine portion of the molecule;,Other regulatory proteins bind to DNA and regulate the biosynthesis of enzymes and RNA molecules involved in cell division in both prokaryotes and eukaryotes,Other Proteins,There are numero
29、us other proteins whose functions are rather exotic and not easily classified.Monellin,a protein of an African plant,has an intensely sweet taste.It is being studied as a nonfattening,nontoxic food sweetener for human use.The blood plasma of some Antarctic fish contains antifreeze proteins,which pro
30、tect their blood from freezing,蛋白质的功能与分布单细胞生物,胞外酶鞭毛-蛋白质（运动器官）细胞壁-糖、脂、肽细胞膜-载运蛋白、酶蛋白细胞质-酶蛋白（催化物质代谢）细胞核-酶蛋白（催化遗传）,多细胞动物,个体层次,毛发、皮肤-角蛋白,骨骼、牙齿-钙+胶原蛋白,食道（口、胃、肠）-消化酶（细胞外）血管中-免疫蛋白（Antibody）激素蛋白,器官层次,巨噬细胞,细胞水平,提问：为什么人体细胞会有不同的形状呢？细胞内有支撑体,细胞内丝网状骨架蛋白（决定细胞形状及运动）；,地衣细胞,带有绿色荧光的骨架蛋白质,食物酵母细胞,细胞粘连物质-胶原蛋白,结构蛋白,均为纤维状；,功能 蛋 白,大多为 球 状；,细胞膜载体蛋白（物质、信号）、酶,细胞质(包括细胞器)酶（物质代谢）、信号传递,细胞核酶（遗传）、结构蛋白（染色体骨架）,
