Alpha-helix

Alpha-helix is a common motif in the secondary structure of proteins, characterized by a right-handed helix with a pitch of 5.4 Å and 3.6 amino acids per turn. The alpha-helix is stabilized by hydrogen bonds between the amide hydrogen and carbonyl oxygen of the backbone, four residues apart. This structure was first proposed by Linus Pauling and Robert Corey in 1951, marking a significant advancement in the understanding of protein structure.

Structure and Stability

The alpha-helix is a key structural element in proteins, providing a compact and rigid framework. Its stability is primarily due to hydrogen bonding between the backbone amide and carbonyl groups. Additionally, the side chains of the amino acids project outward from the helix, minimizing steric clashes and allowing for interactions with other elements of the protein or the environment.

Side chain interactions also play a crucial role in the stability and formation of alpha-helices. Amino acids like alanine, leucine, and methionine are considered helix-formers due to their propensity to adopt the helical conformation. In contrast, proline and glycine are often found at the ends of helices, as they are helix-breakers due to their structural constraints or flexibility, respectively.

Function

Alpha-helices are involved in a variety of biological functions, including the formation of transmembrane proteins, enzyme active sites, and the binding sites for DNA, RNA, and other proteins. The specific role of an alpha-helix within a protein is determined by its location, orientation, and the nature of its side chains.

Alpha-helix in Protein Folding

The formation of alpha-helices is a critical step in the folding of proteins. During folding, proteins often transition through intermediate states where alpha-helices can form and stabilize certain regions of the protein, guiding it towards its native conformation. Misfolding of proteins, including incorrect formation of alpha-helices, can lead to diseases such as Alzheimer's disease and Parkinson's disease.

Examples

Many proteins contain alpha-helices as part of their structure. For example, the hemoglobin molecule, responsible for oxygen transport in the blood, contains several alpha-helices. Another example is the transmembrane proteins, where alpha-helices span the cell membrane, facilitating the transport of molecules and ions.

See Also

• Protein structure
• Beta sheet
• Secondary structure
• Tertiary structure
• Quaternary structure