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Ch. 24 - Amino Acids, Peptides, and Proteins
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Wade 9th EditionCh. 24 - Amino Acids, Peptides, and Proteins문제 41
24장, 문제 41

Histidine is an important catalytic residue found at the active sites of many enzymes. In many cases, histidine appears to remove protons or to transfer protons from one location to another.
(a) Show which nitrogen atom of the histidine heterocycle is basic and which is not.
(b) Use resonance forms to show why the protonated form of histidine is a particularly stable cation.
(c) Show the structure that results when histidine accepts a proton on the basic nitrogen of the heterocycle and then is deprotonated on the other heterocyclic nitrogen. Explain how histidine might function as a pipeline to transfer protons between sites within an enzyme and its substrate.

검증된 단계별 안내
1
Step 1: Analyze the structure of histidine's heterocyclic ring (imidazole). The imidazole ring contains two nitrogen atoms: one is part of a double bond (sp2 hybridized) and the other is bonded to a hydrogen atom (sp2 hybridized but capable of donating a lone pair). The nitrogen bonded to hydrogen is basic because it can donate its lone pair to accept a proton.
Step 2: To determine why the protonated form of histidine is stable, draw resonance structures for the protonated imidazole ring. Show how the positive charge can delocalize across the ring, stabilizing the cation through resonance.
Step 3: When histidine accepts a proton on the basic nitrogen, draw the resulting structure. Then, show the deprotonation of the other nitrogen atom in the heterocycle. This results in a neutral imidazole ring with a proton transferred from one nitrogen to the other.
Step 4: Explain how histidine functions as a proton pipeline. The ability of histidine to accept and donate protons through its imidazole ring allows it to transfer protons between the enzyme active site and the substrate. This is facilitated by the resonance stabilization of the intermediate forms.
Step 5: Relate the structure of urocanic acid (shown in the image) to histidine. Urocanic acid is a derivative of histidine where the amino group is replaced by a double bond to a carbon chain. This structure highlights the importance of the imidazole ring in proton transfer and catalytic activity.

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주요 개념

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Basicity of Nitrogen Atoms

In histidine, the basicity of nitrogen atoms is determined by their ability to accept protons. The nitrogen in the imidazole ring can be protonated, making it a basic site, while the other nitrogen is part of a double bond and is less likely to accept a proton due to resonance stabilization. Understanding which nitrogen is basic is crucial for analyzing histidine's role in enzymatic reactions.
추천 영상:
가이드 코스
1:45
Nitrogenous Bases Concept 3

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, distributing electron density across the molecule. In the case of protonated histidine, resonance forms show that the positive charge can be delocalized over the nitrogen atoms and the adjacent carbon atoms, enhancing the stability of the cation. This concept is essential for understanding the stability of charged species in biochemical reactions.
추천 영상:
03:43
The radical stability trend.

Proton Transfer Mechanism

Histidine can act as a proton shuttle in enzymatic reactions, facilitating the transfer of protons between different sites. When histidine accepts a proton on its basic nitrogen, it can later donate it from the other nitrogen, effectively transferring protons within the enzyme's active site. This mechanism is vital for many enzymatic processes, as it allows for the regulation of pH and the stabilization of transition states.
추천 영상:
가이드 코스
3:05
Polyurethane Mechanism Example 1
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(a) Explain why an NHS ester is much more reactive than a simple alkyl ester.

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