2.5 Protein Structure I - Primary structure and the peptide bond

The course centers on understanding protein structure and architecture, which is a core topic essential for subsequent modules in biochemistry and related fields.
Grasping the intricate details of protein structure is crucial for students as it interlinks with various biological processes and functions, laying foundational knowledge applicable in advanced topics.

The course prepares students for practical studies of protein samples using techniques such as x-ray crystallography, allowing for the examination of protein structures at an atomic level. This preparation is essential for understanding how proteins perform their specific functions in biological systems.

To provide a comprehensive foundational understanding of protein structure, emphasizing its complexity and relevance in biological contexts.
To highlight the significance of protein structure/function relationships, aiding students in understanding the biological mechanisms and implications of protein functionality within life sciences.
To explore the pivotal relationship between amino acid sequence and protein function, demonstrating how slight variations can lead to profound functional changes in proteins.

Definition: Interactions between permanent dipoles involving hydrogen atoms. These bonds are integral to protein stability and folding.
Important Components:
- Donor: An atom that is covalently bonded to hydrogen (e.g., N-H, O-H).
- Acceptor: An electronegative atom that can attract the hydrogen atom from the donor.
Key Interactions in Proteins: Includes O-H and N-H bonds, which are critical for maintaining protein conformation.
Optimal Distance: For effective hydrogen bonding to occur, atoms must be within 2.5 - 3.0 Å of each other.

Definition: Attractive molecular interactions that arise from dipole moments and charge distributions. These forces, though weak individually, contribute significantly to the overall stability and shape of proteins when combined.

Concept: The behavior of molecules in aqueous solutions where polar molecules (like water) can form hydrogen bonds, while non-polar molecules cannot, leading to the clustering of hydrophobic regions in a protein's interior.
Hydrophilic vs. Hydrophobic:
- Hydrophilic (water-loving): Molecules that can interact with water and join the hydrogen-bond network.
- Hydrophobic (water-fearing): Molecules that are excluded from interacting with water, leading to their aggregation in aqueous environments.
- Amphipathic molecules: Have both hydrophilic and hydrophobic regions, crucial for the structural integrity of membrane proteins and lipids and their interactions with varying environments.

There are 20 distinct variations of amino acids that serve as the fundamental components of proteins.
Proteins can have a vast range in length, from 50 to over 1000 amino acids, which influences their shape and functionality.
The order of amino acids, dictated by the sequence of nucleotides in DNA, plays a critical role in determining the protein's ultimate structure and function.

The central carbon atom has four tetrahedral single bonds, allowing for a diverse array of structures and configurations in proteins.

Definition: The linear sequence of amino acids in a protein chain. It serves as the most fundamental aspect of protein architecture and is vital for determining the protein's function and overall structure.
Importance: Understanding the primary structure is essential for:
- Functional analysis of proteins.
- Identifying genetic mutations related to diseases.
- Relating to homologous proteins across different species.
  - Example: Myoglobin's primary structure shows variations in sequences that correlate with its function in diverse organisms.
Average Lengths:
- Prokaryotes: Approximately 212 residues, MW: 24,000 Da.
- Eukaryotes: Approximately 280 residues, MW: 31,700 Da.

Various mutations can lead to significant changes in protein function, which can result in diseases:
- Sickle Cell Anaemia: A specific mutation in the hemoglobin gene (glu6 to val6) leading to less soluble hemoglobin, causing distorted red blood cells and reduced oxygen transport.
- Cystic Fibrosis: Caused by a 3-base deletion resulting in a missing phenylalanine in a protein, significantly affecting lung function.
Understanding these genetic defects is crucial for developing targeted treatments and interventions for diseases.

Amino acids are linked by peptide bonds through a condensation reaction, which involves the removal of water to form a bond between the carboxyl group of one amino acid and the amino group of another.
Characteristics of the Peptide Bond:
- Exhibits double bond character due to resonance, which restricts rotation around the bond, thus affecting the structure of proteins.

The peptide bond usually adopts a trans configuration, ensuring that the carbonyl oxygen (C=O) and the nitrogen (N-H) are positioned on opposite sides of the peptide bond.
Rotation about the C-N bond is not allowed due to the rigidity of the peptide bond; however, rotation is permitted at the α-carbon bonds. This leads to two angles (ϕ and ψ) that describe the folding of proteins, which are restricted due to atomic size and steric hindrance.

These fundamental concepts of protein structure and interactions form the groundwork for advanced understanding and exploration of biological macromolecules, their structures, and their myriad functions within living systems. An appreciation of these principles is essential for any aspiring biochemist or molecular biologist to excel in the field.