21.8
肽键
肽键通过脱水反应，同价连接氨基酸。 一个氨基酸的羧基组和另一个氨基酸的氨基组结合在一起，释放了一个水分子。 产生的键是肽键。 这种联系形成的生成物是肽。 随着更多的氨基酸加入这一生长链，形成的链是多肽。 每一种多肽在一端都有一个游离氨基组。 这一端有 N 端子或氨基端子，另一端有一个游离羧基组— C…
氨基酸通过肽键 彼此共价结合。肽键将一个氨基酸的羧基 连接到另一个氨基酸的氨基上。肽是由 2 到 50 个单体组成的 氨基酸小链。肽键通过释放水分子的 缩合反应形成。当氨基和羧基连接在一起时，就会形成酰胺基团。肽键具有刚性平面结构，并表现出一些双键特征。这是因为羰基上的双键 可以与碳氮肽键 形成共振结构。这种共振导致单键长度减少，双键长度增加，并抑制肽键周围基团的 旋转。肽键存在顺式和反式两种构象。在顺式构象中，阿尔法碳 位于肽键的同一侧，在反式构象中，它们位于键的相对侧。除脯氨酸的氨基参与成键外，肽键大多以反式构象 出现。通过肽键连接的氨基酸 有一个氮 端和一个碳 端。氮端是氨基不参与 肽键形成的端，碳端是羧基不参与 肽键形成的端。肽或蛋白质中的氨基酸序列 总是从氮端开始，到碳端结束。当两个氨基酸通过肽键连接在一起时，称为二肽。当三个和四个氨基酸 通过肽键连接在一起时，分别称为三肽和四肽。当多个氨基酸通过肽键连接在一起时，称为多肽。肽键可通过使用化学催化剂水解 而迅速分解，例如酸或 称为蛋白酶的酶。肽键的断裂需要加入 一个水分子。或者，这些键可以在水存在的情况下，通过缓慢的过程自发断裂。
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Q1: How does a peptide bond form between amino acids?
A peptide bond forms through a condensation reaction where the carboxyl group of one amino acid links to the amino group of another, releasing a water molecule. This covalent bond creates an amide group, joining the two amino acids together. The reaction is reversible through hydrolysis, which adds water back to break the bond.
Q2: What is the structural difference between cis and trans peptide bonds?
In cis conformation, the alpha carbons of both amino acids are on the same side of the peptide bond, while in trans conformation they are on opposite sides. Peptide bonds predominantly occur in the trans conformation, except when proline contributes its amino group to bond formation, where cis bonds are more common.
Q3: Why does a peptide bond have a rigid planar structure?
The peptide bond exhibits rigidity due to resonance between the carbonyl group's double bond and the carbon-nitrogen single bond. This electron sharing decreases the single bond length and increases the double bond length, preventing rotation around the peptide bond and creating its characteristic planar structure.
Q4: What are N-terminal and C-terminal ends in a peptide chain?
The N-terminal is the end of a peptide chain with a free amino group not involved in peptide bond formation, while the C-terminal has a free carboxyl group. Amino acid sequences always begin at the N-terminal and end at the C-terminal, establishing the directional orientation of the polypeptide chain.
Q5: How are peptides classified based on the number of amino acids?
Peptides are classified by chain length: dipeptides contain two amino acids, tripeptides contain three, and tetrapeptides contain four. When several amino acids link through peptide bonds, they form polypeptides. Polypeptides that fold into specific three-dimensional structures with distinct functions are called proteins.
Q6: How can peptide bonds be broken down?
Peptide bonds break through hydrolysis, which adds a water molecule to cleave the bond. This can occur rapidly using chemical catalysts like acids or enzymes called proteases, or slowly through spontaneous hydrolysis in the presence of water alone. Both mechanisms reverse the condensation reaction.
Q7: What role does resonance play in peptide bond chemistry?
Resonance in peptide bonds involves electron sharing between the carbonyl group's double bond and the carbon-nitrogen single bond. This resonance stabilizes the bond structure, reduces rotation around the peptide linkage, and contributes to the rigid planar geometry essential for protein folding and function.