2.5 Protein Structure I - Primary structure and the peptide bond(1)

Module: Biochemistry
Date: 16/11/2024
The primary structure of a protein refers to the unique sequence of amino acids that are linked together by peptide bonds.
Peptide bonds are formed through a dehydration synthesis reaction between the carboxyl group of one amino acid and the amino group of another, resulting in the release of a water molecule.
This sequence is critical as it determines the protein's overall structure and function, influencing how it will fold into secondary and tertiary structures.

Objective: To provide a basic understanding of protein structure and architecture.
Importance: Core material for the LSC-20015 (Molecular, Cellular, and Structural Immunology) and LSC-20016 (Metabolism in Health and Disease) at level 5, and LSC-30016 (Structural Biology & Macromolecular Function) at Level 6.
Introduction to protein sample preparation for X-ray crystallography included.

Involve interactions between permanent dipoles, where one includes a hydrogen atom (H) and the other has a lone pair of electrons.
Mechanism: The H atom is partly shared among a H donor atom and an acceptor atom.
Common Examples in Proteins:
Donor (e.g., OH) and Acceptor (e.g., N): Δ-δ+ δ- interactions occur within proximity (2.5-3.0 Å).

Definition: Molecular interactions arising from dipole and charge interactions.
These forces operate as a net attractive force between all molecules, enhancing stability, especially in structurally fit molecules.

Polar vs Non-Polar: Polar molecules (hydrophilic) participate in hydrogen bonding while non-polar (hydrophobic) molecules are excluded.
Importance: Fundamental for protein stability and organization; some molecules, like lipids, exhibit both properties (amphipathic).
Functional Groups: -OH, -NHn, -CO act as polar (hydrophilic) groups; hydrocarbon chains (CH groups) are non-polar (hydrophobic).

Basic Unit: Each protein consists of amino acids, the primary building blocks.
Variation: 20 different amino acids combine in sequences that range from 50 to 1000+ residues.
Significance of Order: The sequence of amino acids, determined by DNA, dictates the protein's character and functional roles.
Structural Feature: Central carbon (α carbon) is tetrahedral, linking to four distinct atoms.

Defined as the sequence of amino acids in a linear form.
Role of Primary Structure:
1. Assists in understanding the protein function.
2. Identifies mutations potentially linked to genetic diseases.
3. Provides relationships to other proteins.
Limitations: Knowledge of the sequence alone offers minimal insight into three-dimensional structure without prior characterized protein families or homologous structures.

Variability: Length can range from 200-500 residues typically, with some proteins reaching up to 25,000 residues.
Frequency Distribution: Analysis shows a non-equal representation of amino acids within average sequences, e.g., Leu 9%, Ala 8.3%.
Link to Disease: Mutations can affect structure/function—example includes sickle cell anemia arising from a single amino acid alteration.

Structure: Proteins are built through peptide bonds linking amino acids. Peptide bonds exhibit characteristics of both single and double bonds and do not allow for free rotation.
Formation Process: A condensation reaction results in linkage while producing water (H2O).

Planarity and Configuration: The peptide bond ensures that amino acids are aligned in a rigid plane, with notable trans configurations favored over cis to avoid steric clashes.
Rotational Flexibility: While the peptide bond itself is fixed, rotation is possible around Cα-N and Cα-C bonds, creating flexibility in polypeptide formation influenced by sterics.

Structural organization in proteins progresses from primary (linear sequence), secondary (alpha helices and beta strands), tertiary (complete 3D shape), to quaternary (aggregate units).
Importance of Analysis: Understanding the complete conformation through adjacent peptide relationships dictated by allowed rotational angles (ϕ, ψ) is crucial for structural biology.