A toroid repeat is a

protein fold A protein superfamily is the largest grouping (clade) of proteins for which common ancestry can be inferred (see homology). Usually this common ancestry is inferred from structural alignment and mechanistic similarity, even if no sequence similarit ...

composed of repeating subunits, arranged in circular fashion to form a closed structure.

Structure

In the case when the N- and C-terminal repeats lie in close physical contact in a

tandem repeat Tandem repeats occur in DNA when a pattern of one or more nucleotides is repeated and the repetitions are directly adjacent to each other. Several protein domains also form tandem repeats within their amino acid primary structure, such as armadil ...

domain, the result is a topologically compact, closed structure. Such domains typically display a high rotational symmetry (unlike open repeats that only have translational symmetries), and assume a wheel-like shape. Because of the limitations of this structure, the number of individual repeats is not arbitrary. In the case of

WD40 repeat The WD40 repeat (also known as the WD or beta-transducin repeat) is a short structural motif of approximately 40 amino acids, often terminating in a tryptophan-aspartic acid (W-D) dipeptide. Tandem copies of these repeats typically fold togethe ...

s (perhaps the largest family of closed solenoids) the number of repeats can range from 4 to 10 (more usually between 5 and 7). Kelch repeats, beta-barrels and beta-trefoil repeats are further examples for this architecture.

Function

Closed solenoids frequently function as protein-protein interaction modules: it is possible that all repeats must be present to form the ligand-binding site if it is located at the centre or axis of the domain "wheel". The

is a prime example of this function.

Classification

The following major sub-classes of toroid repeat proteins can be found: #

TIM barrel The TIM barrel (triose-phosphate isomerase), also known as an alpha/beta barrel, is a conserved protein fold consisting of eight alpha helices (α-helices) and eight parallel beta strands (β-strands) that alternate along the peptide backbone. ...

structures composed of eight units with alternating beta strands and alpha helices #

Beta barrel In protein structures, a beta barrel is a beta sheet composed of tandem repeats that twists and coils to form a closed toroidal structure in which the first strand is bonded to the last strand (hydrogen bond). Beta-strands in many beta-barrels are ...

structures composed on a single circular beta sheet #

Beta propeller In structural biology, a beta-propeller (β-propeller) is a type of all-β protein architecture characterized by 4 to 8 highly symmetrical blade-shaped beta sheets arranged toroidally around a central axis. Together the beta-sheets form a funnel- ...

structures composed of beta sheets formed by individual repeat units arranged in a circle, in particular the

and

Kelch motif The Kelch motif is a region of protein sequence found widely in proteins from bacteria and eukaryotes. This sequence motif is composed of about 50 amino acid residues which form a structure of a four stranded beta-sheet "blade". This sequen ...

families

External links

RepeatsDB
classification

References

{{Protein tandem repeats Protein tandem repeats Protein domains