Mechanism of formation
The spatial configuration of a protein depends on the possibility of free rotation around the bonds of the α-carbon atom (with nitrogen and with the carbon of the carbonyl group). However, the secondary structure arises due to the formation of hydrogen bonds directly between elements of the peptide backbone: the carbonyl oxygen ($-C=O$) and the hydrogen of the amide group ($-NH-$). Amino acid side chains are not involved in this process.
α-helix
In an α-helix, hydrogen bonds are formed between the oxygen atom of the carbonyl group of one amino acid and the hydrogen of the amide nitrogen of the fourth amino acid away from it.
Amino acid side chains are thus pushed to the periphery of the helix. The formation of such a structure can be hindered by:
- Equally charged or excessively bulky side chains (due to electrostatic repulsion and steric hindrance).
- Proline. In its structure, the nitrogen is incorporated into a rigid ring and lacks a hydrogen atom, so it cannot form a hydrogen bond. The fixed bond with the α-carbon causes the polypeptide chain to bend at the site of proline, disrupting the regularity of the helix.
β-structure (pleated sheet)
A β-structure is formed through the interaction of linear segments of the same or different polypeptide chains. Visually, it resembles a pleated sheet and is indicated in diagrams by broad arrows.
There are two types of β-structures depending on the direction of the chains:
- Parallel: The N- and C-termini of the interacting segments point in the same direction. They are usually connected by longer loops.
- Antiparallel: The chains run in opposite directions (N- and C-termini are opposite). They are often connected by a short β-turn.
Irregular regions and classification
The polypeptide chain does not always consist entirely of regular elements. It contains loops, turns, and bends that change the direction of the chain and help compact the molecule (e.g., connecting parallel β-chains).
Based on the predominant type of secondary structure, proteins can be divided into categories. For example, the third category includes proteins consisting solely of β-structures (immunoglobulins, superoxide dismutase). The fourth category unites proteins with a very low content of regular structures (small cysteine-rich proteins and metalloproteins). There are also proteins with mixed folding, combining both α-helices and β-sheets.
Briefly: answers to topical questions
What is the supersecondary structure (motifs) of a protein and what types exist?
Supersecondary structure refers to specific combinations and spatial arrangements of secondary structure elements (α-helices, β-structures) found in various proteins. This level of organization occupies an intermediate position between secondary and tertiary structures, representing a distinct folding of elements during the formation of a globule.
Typical types of motifs:
- "α-helix — turn — α-helix" — involved in DNA binding.
- "Leucine zipper" — a motif characteristic of transcription factors.
- "Zinc fingers" — an element of DNA-binding proteins.
Proline — the "helix breaker": due to its ring, it lacks the necessary hydrogen and disrupts the α-helix.
Class and exam questions
What bonds maintain the secondary structure of a protein?
Secondary structure is formed exclusively by hydrogen bonds between the carbonyl ($-C=O$) and amino groups ($-NH-$) of the peptide backbone.
Why does proline disrupt the α-helix?
The nitrogen in proline is part of a rigid ring and lacks a hydrogen atom to form a hydrogen bond. Additionally, the geometry of the ring creates a sharp bend in the polypeptide chain.
Do amino acid side chains participate in the formation of secondary structure?
No, hydrogen bonds are formed only between groups of the peptide backbone. Side chains are located on the periphery of helices and sheets, although excessively bulky or identically charged side chains can interfere with folding.
What else to study on this topic
- Features of peptide bond formation with proline
- Primary structure of proteins and its genetic determination
- Protein folding process and conformation
- Types of peptide bond hydrolysis (chemical and enzymatic)
- Relationship between genotype and spatial structure
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