Stages of conformational formation
The spatial organization of a protein molecule arises in stages:
- Secondary structure — local ordering of the chain into an α-helix or β-pleated sheet due to hydrogen bonds between peptide groups ($CO$ and $NH$).
- Supersecondary structure (motif) — specific combinations of secondary structure elements, such as the zinc finger or leucine zipper.
- Tertiary structure — the final formation of the three-dimensional globule due to the apposition of parts of the chain that are distant from each other.
Chemical bonds in maintaining tertiary structure
The compact shape of the globule is held together by the following interactions:
- Hydrophobic interactions: nonpolar amino acid side chains (Ala, Val, Leu, Ile, Phe, Trp, Met, Pro) are hidden inside the protein away from water.
- Ionic bonds (salt bridges): electrostatic attraction between oppositely charged amino acid groups (Asp, Glu, Lys, Arg, His).
- Hydrogen bonds: occur between polar uncharged side chains (Ser, Thr, Tyr, Asn, Gln, Cys).
- Disulfide bonds: strong covalent bridges formed between Cys residues with the formation of cystine.
Conformational labiality and its significance
Conformational labiality refers to the ability of a protein to reversibly alter its spatial architecture under the influence of external factors or upon ligand binding. This property is due to the flexibility of the polypeptide chain in non-rigid zones and the weakness of the stabilizing bonds.
Labiality is critically important for:
- Enzymatic catalysis via the induced-fit mechanism;
- Allosteric regulation of activity;
- Transport of substances with a change in ligand affinity (e.g., in hemoglobin);
- Signal transduction via receptor structures.
Domain organization of proteins
Many large proteins consist of domains — discrete regions of the polypeptide chain that fold independently and often perform separate tasks.
A striking example is steroid hormone receptors, which contain:
- A ligand-binding domain for hormone recognition;
- A DNA-binding domain for contact with the genome;
- A transactivation domain to initiate transcription.
This modular structure endows proteins with polyfunctionality.
To memorize the amino acid groups for hydrophobic interactions, keep this rule in mind: Ala, Val, and Leu with Ile, Phenyl, Trp, Met, and Pro hide water deep within (accounting for standard three-letter abbreviations).
Class and exam questions
What bonds stabilize the secondary structure of a protein?
The secondary structure is stabilized by hydrogen bonds formed between the peptide groups ($CO$ and $NH$) of the polypeptide chain.
Which amino acids are responsible for the formation of disulfide bonds?
Disulfide covalent bonds are formed with the participation of the amino acid cysteine (Cys), which converts into cystine.
What is conformational protein labiality?
This is the ability of a protein to reversibly alter its spatial structure upon interaction with ligands or under the influence of environmental factors, which ensures enzymatic activity, transport, and signal transduction.
What is a protein domain?
A domain is a discrete structural and functional region of the polypeptide chain that folds independently and can perform a separate specific function.
What else to study on this topic
- Algorithm for analyzing the relationship between primary structure and biological function of peptides
- Denaturation and aggregation of globular proteins under external influences
- The role of proline and glycine in forming bends of the polypeptide chain
- Mechanisms of allosteric enzyme regulation
- Structural and functional features of steroid hormone receptors
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