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Protein secondary structure is the local spatial conformation of the polypeptide backbone excluding the side chains. [1] The two most common secondary structural elements are alpha helices and beta sheets , though beta turns and omega loops occur as well.
Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet. A β-strand is a stretch of polypeptide chain typically 3 to 10 amino acids long with backbone in an extended conformation .
For example, in a sequence of 5 residues, both residues 1 to 4 and residues 2 to 5 form a turn; in such a case, one speaks of an (i, i + 1) double turn. Multiple turns (up to sevenfold) occur commonly in proteins. [5] Beta bend ribbons are a different type of multiple turn.
In proteins in general all four beta turn types occur frequently but I is most common, followed by II, I' and II' in that order. Beta turns are especially common at the loop ends of beta hairpins ; they have a different distribution of types from the others; type I' is the most common, followed by types II', I and II.
Polypeptide direction, NH 2 and COOH termini Small arrows on one or both of the termini, or letters. For β-strands, the direction of the arrow is sufficient. Today, the direction of the polypeptide chain is often indicated by a colour ramp. Disulfide bonds Interlocked SS symbol or a zigzag, like a stylized lightning stroke.
β-alanine, an example of a β-amino acid. The amino group attaches not to the α carbon but to the β-carbon, which in this case is a methylene group.. Beta-peptides (β-peptides) are peptides derived from β-amino acids, in which the amino group is attached to the β-carbon (i.e. the carbon two atoms away from the carboxylate group).
A beta bend ribbon can be regarded as an aberrant 310 helix (3/10-helix) that has lost some of its hydrogen bonds. [9] Two websites are available to facilitate finding and examining these features in proteins: Motivated Proteins; [10] and PDBeMotif. [11] A beta bend ribbon with 12 alanine residues. Colors of atoms: carbon, green; oxygen red ...
Protein structures range in size from tens to several thousand amino acids. [2] By physical size, proteins are classified as nanoparticles, between 1–100 nm. Very large protein complexes can be formed from protein subunits. For example, many thousands of actin molecules assemble into a microfilament.