Leucine Zipper
This type of supersecondary structure is primarily designed for dimerization—the stable joining of two separate protein molecules. Such a motif is extremely characteristic of DNA-binding proteins that function not alone, but as part of large oligomeric complexes.
Architecture and Geometric Features The basis of the motif consists of $\alpha$-helical regions located on the surface of interacting proteins. The key structural feature is the presence of at least four leucine residues embedded in the polypeptide chain with strict periodicity. Leucine occurs exactly every six amino acids, meaning it occupies every seventh position.
Because one complete turn of a classical $\alpha$-helix contains 3.6 amino acid residues, this periodicity creates an interesting spatial effect. Leucine residues end up on the surface of every second turn and align in a straight line strictly along one side of the helix.
Binding Mechanism When two such helices approach each other, the exposed leucine radicals of one molecule interact tightly with the analogous radicals of the other. Powerful hydrophobic interactions arise between them. This interaction resembles the interlocking of zipper teeth, which gave the structure its name. As a result, a very strong contact is formed between the subunits.
Biological Significance Illustrated by Histones An excellent example of the leucine zipper in action is histones—nuclear proteins required for DNA packaging. They contain a huge number of positively charged amino acids (up to 80% of their composition is arginine and lysine). To form a nucleosome, histone molecules must assemble into an octamer (a complex of 8 monomers). However, identical positive charges cause strong electrostatic repulsion. It is precisely the hydrophobic contacts of the leucine zippers that allow this repulsion to be overcome and hold the histones together securely.
Zinc Finger
This variant of supersecondary structure is critically important for specific DNA-binding proteins. Visually, it represents a small elongated fragment on the surface of the protein globule, resembling a finger in shape.
Structure and Metal Ion Coordination On average, the length of a single "finger" is about 20 amino acid residues. For this elongated loop to maintain its shape and not collapse, a reliable stabilizing factor is required. This role is played by a zinc atom ($Zn^{2+}$).
The central zinc ion is coordinated by the radicals of four amino acids. There are two main variants of this coordination:
- Classical type: zinc binds to two cysteine (Cys) residues and two histidine (His) residues.
- Alternative type: zinc interacts with four cysteine residues.
In the primary structure of the protein, these coordinating amino acids are arranged in a specific manner: a pair of closely spaced cysteine residues is separated from the second pair of coordinating amino acids (histidines or cysteines) by a chain consisting of approximately 12 amino acids.
Function and Specificity of DNA Interaction
The formed zinc finger structure includes an $\alpha$-helical region. It is this $\alpha$-helix that is responsible for direct contact with the nucleic acid. It precisely inserts into the major groove of DNA and binds to its regulatory regions.
An essential property of this structural motif is ensuring individual specificity. The exact nucleotide sequence to which the protein attaches depends entirely on the amino acid composition within the $\alpha$-helix region of the finger (these amino acids are often denoted by single-letter codes, for example: E, V, K, F, S, A, L, R).
Such motifs are typical for regulatory proteins, such as steroid hormone receptors. Their main task is participating in the fine regulation of transcription, the process of copying genetic information from a DNA template to an RNA molecule.