Beta Strand: Videos & Practice Problems

The beta strand is a crucial type of secondary structure in proteins, characterized by an extended zigzag conformation of the protein backbone. This structure is periodic, repeating every two amino acid residues, and is visually represented by the alternating positions of the R groups, which appear to zigzag. The rise, defined as the distance covered per amino acid residue, is approximately 3.5 angstroms for beta strands, significantly greater than the 1.5 angstroms observed in alpha helices. This indicates that beta strands are more extended compared to the coiled structure of alpha helices.

The pitch of a beta strand, which refers to the length of one complete turn of the backbone, measures about 7 angstroms, again longer than the 5.4 angstroms of an alpha helix. To illustrate the differences between these two structures, consider a comparison of five amino acid residues in each conformation. The beta strand, being more extended, has a total length calculated by multiplying the number of amino acids (5) by the rise (3.5 angstroms), resulting in a length of 17.5 angstroms. In contrast, the alpha helix, with the same number of amino acids, yields a length of only 7.5 angstroms when calculated using its rise of 1.5 angstroms.

This comparison highlights the fundamental differences in structure and spatial arrangement between beta strands and alpha helices, emphasizing the extended nature of beta strands versus the coiled configuration of alpha helices. Understanding these distinctions is essential for grasping protein structure and function.

The beta strand is characterized by an extended zigzag conformation of the protein backbone, commonly represented as broad arrows in structural depictions. These arrows indicate the orientation of the strands, always pointing towards the C-terminal end of the protein. This directional representation aligns with the convention of reading protein sequences from the N-terminal to the C-terminal end.

In contrast to alpha helices, where hydrogen bonds run parallel to the helical axis, the hydrogen bonds in beta strands are perpendicular to the direction of the strands. This means that when the hydrogen bonds intersect with the strands, they form right angles (90 degrees). For instance, in a single beta strand, the arrow pointing towards the C-terminal end signifies that the opposite end is the N-terminal end. When multiple beta strands are present, each strand is depicted with its own arrow, and the arrows are connected, indicating that they are part of a single polypeptide chain.

As beta strands come together, they are stabilized by hydrogen bonds formed between the peptide backbones. It is crucial to remember that these hydrogen bonds run perpendicular to the strands, which is a key distinction from the bonding in alpha helices. Understanding these structural features is essential for grasping the overall architecture of proteins and their functional implications.