Question

Each polypeptide in a protein has amino acids linked with each other in a specific sequence. This sequence of amino acids is said to be (A) Primary structure of proteins (B) Secondary structure of proteins (C) Tertiary structure of proteins (D) Quaternary structure of proteins

Short answer

The specific sequence of amino acids in a polypeptide is represented by its primary structure. Thus, the answer is: (A) Primary structure of proteins

Step-by-step solution

Determine the Primary structure of proteins

The primary structure of proteins refers to the linear sequence of amino acids linked together by peptide bonds in a polypeptide chain. The order of amino acids in the sequence determines the properties and function of the protein. #step_2#

Determine the Secondary structure of proteins

The secondary structure of proteins refers to the localized arrangement of the polypeptide backbone into common structural patterns, such as alpha-helices and beta-sheets. These structures are stabilized by hydrogen bonding between the amino acids' backbone atoms. #step_3#

Determine the Tertiary structure of proteins

The tertiary structure of proteins refers to the overall three-dimensional shape resulting from the folding of a single polypeptide chain. This folding occurs due to various interactions between the side chains of amino acids, such as hydrogen bonding, ionic interactions, hydrophobic interactions, and disulfide bridges. #step_4#

Determine the Quaternary structure of proteins

The quaternary structure of proteins refers to the interaction and assembly of multiple polypeptide chains (also known as subunits) to form a functional protein complex. The individual polypeptide chains in a quaternary structure are called subunits and can be identical or different. #step_5#

Identify the correct option

Based on the information provided in steps 1-4, we can now determine the correct answer. A polypeptide's specific sequence of amino acids is represented by its primary structure. Therefore, the correct option is: (A) Primary structure of proteins

Key concepts

Primary Structure of Proteins

The primary structure of proteins is the most basic form of protein structure. It is essentially the sequence in which amino acids are linked together by peptide bonds within a polypeptide chain. This chain is like a string of beads, where each bead represents an amino acid connected sequentially. The specific order of these amino acids is critical, as it dictates the protein's properties and function.

Each unique sequence forms the foundation of a protein's identity, much like how a sequence of letters forms words and sentences. This sequence is determined by the genetic code within DNA, ensuring that each protein fulfills its specific role within the organism. A change in even a single amino acid can lead to significant changes in a protein's properties, which is why genetic mutations can have profound effects on protein function.

Understanding the primary structure is essential because it is the starting point for the higher levels of protein structure, influencing the folding and final shape of the protein.

Secondary Structure of Proteins

The secondary structure of proteins refers to the way in which the polypeptide chain folds or coils into regular structural patterns. These patterns are primarily driven by the backbone elements of the amino acids forming hydrogen bonds with each other.

The most common types of secondary structure elements are alpha-helices and beta-sheets.

• An alpha-helix resembles a right-handed coil, stabilized by hydrogen bonds that occur approximately every four amino acids. This structure is flexible and can stretch, which is why it appears in many structural proteins, such as those in hair and nails.
• Beta-sheets are formed when strands of polypeptides lie side by side, connected by hydrogen bonds. These sheets can be parallel or anti-parallel, and they form a "grid-like" structure, contributing to the strength and stability of a protein.

These secondary structures contribute to the overall shape and physical properties of the protein, playing a crucial role in its stability and function.

Tertiary Structure of Proteins

The tertiary structure relates to the complex 3D shape of a single polypeptide chain, formed when the secondary structures twist and fold into a unique and specific shape ready for function. This three-dimensional shape is crucial for the protein's functionality.

Various interactions between the side chains (R-groups) of amino acids contribute to maintaining the tertiary structure:

• Hydrogen bonds between polar side chains stabilize the structure.
• Ionic interactions occur between oppositely charged side chains.
• Hydrophobic interactions cluster nonpolar side chains away from water towards the inner part of the protein.
• Disulfide bridges, which are strong covalent bonds, form between cysteine residues, providing further stability.

The energy landscape for protein folding is like a funnel, driving the protein to fold into its most energetically favorable shape more than any other form. If this shape is perturbed, the protein might lose its function, indicating the critical nature of proper folding.

Quaternary Structure of Proteins

Moving beyond the individual polypeptide chain, the quaternary structure refers to the arrangement and interaction of multiple polypeptide chains, or subunits, into a single functional unit. Many proteins are not active until they achieve this multi-subunit complex form.

The quaternary structure can vary greatly among proteins. For example:

• Hemoglobin, the oxygen-carrying protein in blood, consists of four subunits—two identical alpha and two identical beta chains.
• Antibodies also have a quaternary structure, essential for their role in the immune system.

These subunits often interact through the same types of forces that stabilize tertiary structures, such as hydrogen bonds, hydrophobic interactions, and ionic bonds. The correct assembly of subunits is crucial, as it directly influences the biological activity and stability of the protein. The disruption of quaternary structures can lead to loss of function or diseases associated with protein misassembly, underscoring its importance.