Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

Which functional group participates in disulphide bond formation in proteins? (a) thioester (b) thioether (c) thiol (d) thioacetone

Step-by-Step Solution

Answer

(c) thiol.

Step 1: Identify Potential Functional Groups

We need to identify which of the provided functional groups are involved in forming disulphide bonds. The options given are thioester, thioether, thiol, and thioacetone. Each has a distinct sulfur-containing functional group.

Step 2: Understand Disulfide Bond Formation

Disulfide bonds are covalent bonds derived from the oxidation of two thiol groups (-SH groups). The process involves the removal of hydrogen atoms from the thiol groups to form a sulfur-sulfur bond (-S-S-).

Step 3: Match Functional Group with Reaction

Review the functional groups: - Thioester (RCO-SR') is involved in ester linkages with sulfur. - Thioether (R-S-R') is a simple sulfur linkage between carbon atoms. - Thiol (R-SH) contains a sulfur-hydrogen bond. - Thioacetone (acetonylsulfur derivative) contains complex sulfur compounds. Thiol groups (R-SH) are the ones involved in disulfide bond formation upon oxidation.

Step 4: Confirm Correct Answer

Since thiol groups can oxidize to form disulfide bonds, and considering the descriptions of the other groups, thiol is confirmed as the correct functional group.

Key Concepts

Thiol Functional Group

The thiol functional group, often denoted as R-SH, is a key player in biochemistry, particularly when it comes to the formation of disulfide bonds in proteins. This group contains a sulfur atom bonded to a hydrogen atom, and it can behave in unique ways because sulfur is similar to oxygen but displays different reactivity.
When you see a thiol group, imagine the following characteristics:

• A sulfur atom ( S ) bonded to a hydrogen atom ( H ).
• This bond is polar, meaning there is an uneven distribution of charge across the bond.
• Thiol groups are analogous to hydroxyl groups (alcohols) but are more reactive due to the presence of sulfur.

The high reactivity of thiol groups underlies their important role in biological systems. They can undergo oxidation, leading to the formation of disulfide bonds, which can significantly affect protein structure and function.

Protein Structure

Proteins are complex molecules that perform a myriad of functions in biological systems. They are composed of chains of amino acids, and their specific three-dimensional structures dictate their specific functions. One key aspect of protein structure is the formation of disulfide bonds between cysteine residues, each containing a thiol group.

• Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.
• Disulfide bonds mainly affect the tertiary structure by creating covalent links between different parts of a protein or between separate protein chains.
• This form of bonding adds stability and can influence protein folding, which is crucial for biological activity.
• Correct protein folding is vital for function; any misfolding can lead to diseases such as cystic fibrosis or Alzheimer’s disease.

Hence, the bonding between thiol groups into disulfide bridges is fundamental to maintaining the structure and stability needed for proteins to carry out their functions effectively.

Oxidation of Thiols

The oxidation of thiols is a chemical process where a thiol group ( SH ) transforms into a disulfide bond ( S-S ), a covalent bond formed between two sulfur atoms. This transformation is a crucial reaction that influences protein stability and functionality in biological systems.
Here’s how it generally works:

• Two thiol groups approach each other under oxidative conditions.
• Each thiol group loses a hydrogen atom (as a hydrogen ion and an electron).
• The sulfur atoms from the two thiols then form a covalent bond, resulting in a disulfide bond.
• This reaction can occur under biological conditions and is often facilitated by enzymes that oxidize thiols.

Through the oxidation of thiols, proteins can achieve more stable structures, affecting their dynamics and functionality. These disulfide bonds are also key in modulating protein interactions and pathways in cellular processes.