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The α-helix is a common secondary structure that occurs in proteins. It is a right-handed coiled or spiral conformation of the polypeptide chain, held together by hydrogen bonds. In this structure, the backbone carbonyl (C=O) group of one amino acid residue forms a hydrogen bond with the backbone amide hydrogen (N-H) of another amino acid residue that is four positions further along the sequence.

In an α-helix, 3.6 amino acid residues are present per turn of the helix, and there is a rise of about 1.5 Å (angstroms) along the helix axis per residue or approximately 5.4 Å per helical turn. The side chains of the amino acids in the helix protrude outward from the helix backbone, allowing them to potentially participate in interactions and reactions with other molecules or regions of the protein.

There are many different types of proteins that exhibit α-helix secondary structure. Some examples include: 1. Globins: Hemoglobin and myoglobin, which are oxygen-binding proteins found in muscles and red blood cells. 2. Enzymes: Some enzymes like helicase, which is involved in DNA replication and repair, adopt α-helix structures as they unwind the DNA double helix. 3. Transmembrane proteins: Certain proteins that span the cell membrane, such as the ion channels and receptor proteins, may contain α-helix regions within their structure. 4. Transcription factors: Proteins that are responsible for binding to DNA and regulating gene expression, such as homeodomain-containing proteins, may also possess α-helix motifs. 5. Coiled-coil proteins: Some proteins, like the fibrous structural proteins keratin and myosin, contain α-helices that twist around each other to form coiled-coils. It is important to note that many proteins exhibit a mix of secondary structures, including α-helices, β-sheets, and random coils. The overall three-dimensional structure of a protein depends on the combination and arrangement of these secondary structures.