Question

Which of these amino acids has a side chain that can become ionized in cells? (A) Histidine (B) Leucine (C) Proline (D) Threonine

Short answer

A: Histidine

Step-by-step solution

- Identify ionizable side chains

Determine which amino acids have side chains that can gain or lose a proton (ionize) at physiological pH. Most common ionizable side chains are those of amino acids like Aspartic acid, Glutamic acid, Histidine, Lysine, Arginine, Cysteine, and Tyrosine.

- Analyze each option

Examine the side chains of each given amino acid to see if they can ionize: (1) Histidine: contains an imidazole side chain that can become ionized.(2) Leucine: contains a nonpolar side chain which cannot ionize.(3) Proline: has a unique cyclic structure but its side chain does not ionize.(4) Threonine: has a hydroxyl group but it generally doesn't ionize in cells.

- Determine the correct answer

Based on the analysis, Histidine is the only amino acid among the given options with a side chain that can become ionized in cells.

Key concepts

Physiological pH

In biological systems, the term physiological pH usually refers to the pH of human blood, which is around 7.35 to 7.45. This pH range is crucial for maintaining various bodily functions and enzyme activity.
Understanding physiological pH is important because many biochemical processes are pH-dependent. Proteins, particularly enzymes, require a specific pH range for optimal function, and any deviation from this range can result in loss of activity or denaturation.
Amino acids in a solution are influenced by the pH of the environment. The ionization state of amino acids changes depending on whether the environment is acidic, neutral, or basic.
At physiological pH, some amino acid side chains can gain or lose a proton, affecting their charge and overall behavior in the cell.

Histidine

Histidine is a unique amino acid because its side chain contains an imidazole group that can become ionized. This means it can gain or lose a proton depending on the pH of the environment.
The imidazole side chain of histidine has a pKa value around 6.0, which is close to the physiological pH of 7.4. This proximity allows histidine to act effectively in enzyme active sites and proteins by participating in acid-base catalysis.
Histidine's ability to ionize makes it essential in buffering systems within cells. It helps maintain pH stability by either accepting or donating a proton. Therefore, histidine plays a crucial role in various metabolic and cellular processes.
Membrane proteins and many enzymes often have histidine residues that are strategically located to improve their functionality and stability.

Ionizable Side Chains

Ionizable side chains are specific parts of amino acids capable of gaining or losing a proton, depending on the pH. These side chains can influence the overall charge and solubility of proteins.
Common amino acids with ionizable side chains include:

• Aspartic Acid and Glutamic Acid: both have carboxyl groups that can lose a proton.
• Histidine: has an imidazole side chain that can gain or lose a proton.
• Lysine and Arginine: contain amino groups that can gain a proton.
• Cysteine: has a thiol group, and Tyrosine: contains a phenol group, both of which can ionize.

The ionization state of these side chains depends on the surrounding pH, influencing the protein's 3D structure, bonding capabilities, and interaction with other molecules.
This property is crucial in the function of enzymes and signal transduction pathways.

Amino Acid Properties

Amino acids are the building blocks of proteins and have varied properties due to their side chains. The properties primarily include polarity, ionization state, and reactivity.
These properties influence how amino acids interact with each other and with other molecules, affecting the overall structure and function of proteins. Amino acids can be categorized based on their side chains as:

• Nonpolar: like Leucine, which have hydrophobic properties.
• Polar but uncharged: such as Threonine, which can form hydrogen bonds but generally do not ionize.
• Charged: including Histidine, which can be positively or negatively charged depending on the pH and are critical in biological processes.

Amino acids with unique side chains, like Proline with its cyclic structure, introduce kinks in protein structures, whereas flexible ones like Glycine provide structural versatility.
Each amino acid's distinct properties contribute to the diversity and functionality of proteins, making them essential for different biochemical roles within cells.