Problem 175 - A Complete Resource Book in Chemistry for JEE Main

Question

Thiol Group is present in? (a) Cystine (b) Cysteine (c) Cytosine (d) Methionine

Step-by-Step Solution

Answer

The thiol group is present in Cysteine (option b).

Step 1: Understand the Question

We need to identify which compound among the given options contains a thiol group. A thiol group is denoted by the functional group \(-SH\).

Step 2: Analyze Each Option

Look at each option one by one to determine which contains the \(-SH\) group. - **Cystine**: Formed by two cysteine molecules linked by a disulfide bond (\(-S-S-\)), thus lacks a free thiol group.- **Cysteine**: Contains the \(-SH\) group as part of its primary structure.- **Cytosine**: A nitrogenous base which does not contain sulfur or thiol group.- **Methionine**: Contains sulfur but in a thioether group, not a free thiol \(-SH\).

Step 3: Identify the Correct Answer

After analyzing, we find that **Cysteine** is the only amino acid that has a free thiol group \(-SH\).

Key Concepts

Cysteine

Cysteine is a unique amino acid found in many proteins, playing a critical role in a variety of biological functions. One of its distinguishing characteristics is the presence of a thiol group, which is denoted as \(-SH\).
This thiol group makes cysteine a polar molecule and contributes to its reactivity, especially in forming bonds and reactions within the body.
In its natural form, cysteine can be found in many high-protein foods such as poultry, eggs, and legumes.

• It is considered a semi-essential amino acid because, under certain conditions, the body can synthesize it from other amino acids.


• Apart from its presence in diverse proteins, cysteine contributes to metabolic processes and the synthesis of other important compounds like glutathione.

Cysteine's ability to form disulfide bonds is essential in maintaining the tertiary and quaternary structure of proteins.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.
These groups behave consistently across different molecules and are crucial in determining the properties and functions of organic compounds.

• The thiol group \(-SH\), found in cysteine, is a type of functional group. It is known for its ability to engage in important chemical reactions, such as oxidation and reduction.


• Another example of a functional group within amino acids is the carboxyl group \(-COOH\), which lends acidic characteristics to these compounds.

Understanding functional groups helps in predicting how a molecule will react and interact within biological systems, making them foundational in fields like chemistry and biochemistry.

Amino Acids

Amino acids are the building blocks of proteins, playing a pivotal role in biochemistry by forming peptide bonds to create long polypeptide chains.
There are 20 standard amino acids that make up proteins in the human body, each with a unique side chain that influences its properties and function.

• Cysteine, for instance, is one of these amino acids with a distinct side chain that includes a thiol group \(-SH\), allowing it to form disulfide bonds.


• Amino acids are classified based on the characteristics of their side chains, such as polar, non-polar, acidic, or basic.

They contribute to countless bodily functions, including enzyme catalysis, immune response, and tissue repair, proving indispensable to life. Furthermore, amino acids are involved in the synthesis of hormones and neurotransmitters that regulate physiological processes.

Disulfide Bonds

Disulfide bonds, also known as disulfide bridges, are covalent bonds that form when the thiol groups of two cysteine molecules oxidize, linking them together.
These bonds play a critical role in stabilizing the three-dimensional structure of proteins, ensuring proper folding and resilience.

• Disulfide bonds are fundamental in protein tertiary and quaternary structure, helping to maintain the protein's functional shape.


• They can be found in many structural proteins, like those in hair (keratin), and also in enzymes and antibodies.

For the structural integrity of many proteins, the formation of disulfide bonds is essential. Additionally, these bonds can influence the activity and lifespan of proteins, making them instrumental in cellular processes and signaling pathways.