蛋白质作业,译文

1.Properties of the Peptide Bond肽键的特性In x-ray studies ofcrystalline peptides, Linus Pauling and Robert Corey foundthat 通过X—射线研究晶体肽，鲍霖和罗伯特发现the C-N bond in the peptide link is intermediate inlength (1.32 Å) between a肽键中的C—N键长(1.32A)处于typical C-N single bond (1.49 Å)and a C〓N double bond (1.27 Å).典型C—N单键键长（1.49A）和C—N双键键长（1.27）之间They alsofound that thepeptide bond is planar (all four atoms attached to the他们还发现肽键是平面的（即与C—N键相连的四个原子位于同一平面）C-Ngroup are located in the same plane) and that the twoα-carbon atoms因此与C—N键相连的α碳原子attached to the C-N are always trans to eachother (on opposite sides of thepeptide bond):通常处于反式（即在肽键的对立边）：(a)What does the length of the C-N bond in the peptidelinkage indicate about itsstrength and its bond order(i.e., whether it is single, double, or triple)?（a）根据肽链中的C—N键的键长可以推测出它怎样的键能和结构？（例如，是否是单键，双键或三键）(b) What do the observations of Pauling and Corey tellus about the ease of rotation about the C-N peptide bond?（b）鲍霖和科里关于C—N肽平面易旋转的发现告诉我们什么？2. Structural and Functional Relationships in FibrousProteins在纤维状蛋白中结构和功能的关系William Astbury discovered that the x-ray patternof wool shows a repeating威廉发现羊毛的X射线衍射图像展现了一个structural unit spaced about 5.2 Åalong the length of the wool fiber.重复的结构单元，在羊毛纤维上占据约5.2埃的长度When he steamed andstretched the wool, the x-ray pattern showed a new repeating 当他蒸煮并拉伸羊毛之后，X射线衍射图像呈现出一个新的结构单元structural unit at a spacing of 7.0 Å. Steaming and stretchingthe wool and then letting 大约占据7.0埃的长度。

蒸煮绷紧的羊毛再让它收缩之后，it shrink gave an x-ray pattern consistentwith the original spacing of about 5.2 Å.X射线衍射图像与原始的图像一致，又回复到5.2埃。

Although theseobservations provided important clues to the molecular structureof尽管这些发现为羊毛分子的结构的破解提供了很重要的线索wool, Astbury was unable to interpret them at the time.但是在当时他并没有能力解释它们。

(a)Given our current understanding of the structure ofwool, interpret Astbury’sobservations.（a）根据我们现在对羊毛结构的认识，试着解释阿斯特伯里的发现。

(b) When wool sweaters or socks are washed in hot wateror heated in a dryer, they shrink. Silk, on the other hand,does not shrink under the same conditions. Explain. （b）羊毛衫或者羊毛袜用热水洗或者在烘干机里受热之后，它们会缩小。

然而，丝绸在同样的条件下却不会缩小，为什么？3. Disulfide Bonds Determine the Properties of ManyProteins二硫键决定许多蛋白质的特性A number of natural proteins are very rich indisulfide bonds, and their mechanical许多天然蛋白质都富含二硫键，他们的机械性质properties (tensilestrength, viscosity, hardness, etc.) are correlated with the degreeof（如张力、粘性、硬度等）取决于二硫键的键合程度disulfide bonding. For example, glutenin, a wheat proteinrich in disulfide bonds, is例如，麦谷蛋白，一种富含二硫键的小麦蛋白，responsible for the cohesiveand elastic character of dough made from wheat flour.就是小麦面粉和的生面团既有粘性又有弹性的原因Similarly,the hard, tough nature of tortoiseshell is due to theextensive disulfide同样的，龟甲粗糙坚硬的特性也是因为它的α角蛋白中含有大量的二硫键bonding in its α-keratin.(a)What is the molecular basis for the correlation betweendisulfide-bond content andmechanical properties ofthe protein?（a）二硫键含量与蛋白质机械性质的相关性的分子基础是什么？(b) Most globular proteins are denatured and lose theiractivity when briefly heated to65 o C. However, globular proteinsthat contain multiple disulfide bonds often must beheated longer at higher temperatures to denature them. Onesuch protein is bovine pancreatic trypsin inhibitor (BPTI),which has 58 amino acid residues in a single chain and containsthree disulfide bonds. On cooling a solution of denaturedBPTI, the activity of the protein is restored. What isthe molecular basis for this property?（b）大多数球蛋白被瞬间加热到65摄氏度时，会变性失活。

然而，含有很多二硫键的球蛋白，如果想让它变性，需要在更高的温度下加热更长的时间。

一种蛋白质叫牛胰岛素抑制剂，它的一条单链中含有58个氨基酸残基，其中包括3个二硫键。

在冷却牛胰岛素抑制剂溶液之后，蛋白质的活性又恢复了。

这一特性的分子基础是什么？4. Amino Acid Sequence and ProteinStructure氨基酸序列和蛋白质结构Ourgrowing understanding of how proteins fold allows researchersto make随着我们对蛋白质折叠方式的了解逐步深入，研究者可以根据predictions about protein structure basedon primary amino acid sequence data.一级氨基酸序列的数据对蛋白质结构进行预测。

(a)In the amino acid sequence above, where would youpredict that bends or βturnswould occur?（a）在上述氨基酸序列中，请推测在哪会发生弯曲或者是β转角？(b) Where might intrachain disulfide cross-linkages beformed?（b）在哪会形成链内的二硫键交联？(c) Assuming that this sequence is part of a larger globularprotein, indicate theprobable location (the external surfaceor interior of the protein) of the following amino acidresidues: Asp, Ile, Thr, Ala, Gln, Lys. Explain your reasoning.（c）假定这段序列是一个很大的球蛋白的一部分，请推测以下氨基酸残基的可能位置（位于外表面或是蛋白质的内部），并说明原因。

Asp, Ile, Thr, Ala, Gln, Lys.。