Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

An example of a sulphur containing amino acid is (a) lysine (b) serine (c) cysteine (d) tyrosine

Step-by-Step Solution

Answer

Cysteine is a sulphur-containing amino acid.

Step 1: Understand the Exercise

The task is to identify a sulphur-containing amino acid from the given options. Amino acids are building blocks of proteins, and some contain sulphur in their side chains.

Step 2: Recall Sulphur-Containing Amino Acids

There are a few amino acids known to contain sulphur: cysteine and methionine. Both have sulphur atoms in their structures.

Step 3: Analyze the Given Options

Review the list of given amino acids: (a) lysine, (b) serine, (c) cysteine, (d) tyrosine. Compare these with the known sulphur-containing amino acids.

Step 4: Identify the Correct Answer

From the options, cysteine is the only amino acid that is known to contain sulphur in its structure. Lysine, serine, and tyrosine do not have sulphur.

Key Concepts

Sulphur-Containing Amino Acids

Amino acids are the fundamental building blocks of proteins, vital for various biological functions. Among the 20 standard amino acids, a few contain the element sulphur in their molecular makeup. These are primarily cysteine and methionine.

Sulphur is a unique element in these amino acids, playing a crucial role in the chemical properties and biological functions of the proteins they compose. In cysteine, sulphur is present in the form of a thiol group, while in methionine, it is part of a thioether linkage. The presence of sulphur enables these amino acids to form strong disulfide bonds, which are important for the tertiary and quaternary structures of proteins.

• Disulfide bonds: These are covalent bonds formed between two sulphur atoms, often stabilizing the protein’s structure.
• Unique roles: Sulphur-containing amino acids are involved in catalytic and regulatory functions within cells.
• Reactive nature: The sulphur atom allows cysteine and methionine to participate in a variety of chemical reactions.

Cysteine

Cysteine is a naturally occurring sulphur-containing amino acid instrumental in forming proteins. Its standout feature is the thiol side chain (-SH), which helps in creating disulfide bonds. These bonds are covalent and are vital for maintaining the 3D structure of proteins by linking different parts of the protein molecule or even different protein molecules together.

Cysteine is often considered a semi-essential amino acid, meaning that under normal physiological conditions, it can be synthesized by the human body from methionine. However, in specific situations or dietary insufficiencies, external supplementation might be necessary.

• Synthesis: Cysteine can be synthesized from methionine if enough of the latter is available.
• Stability: The thiol group in cysteine can form a disulfide bridge, stabilizing protein structures.
• Antioxidant role: Cysteine is a precursor to glutathione, an antioxidant that helps protect cells from oxidative damage.

Protein Building Blocks

Proteins are essential biomolecules composed of amino acids, acting like building blocks analogous to how bricks form the structure of a building. Each protein has a specific sequence of amino acids, which determines its unique structure and function.

There are 20 standard amino acids, and they combine in various sequences to form a vast array of proteins, each tailored to perform a distinct role in biological organisms. These roles include catalyzing biochemical reactions, providing structural support, maintaining cell shape, and facilitating communication between cells.

• Versatile functions: Proteins can act as enzymes, hormones, carriers, and antibodies.
• Structural role: Proteins like collagen offer structural integrity to tissues.
• Dynamic composition: Proteins can change shape or form complexes with other biomolecules to perform a specific function.