TIM barrel
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Top view of a triosephosphateisomerase (TIM) barrel (PDB accession code 8TIM), colored from blue (N-terminus) to red (C-terminus). |
The
TIM barrel is an extremely common protein fold consisting of eight
α-helices and eight parallel
β-strands that alternate along the
peptide backbone. The structure is named after
triosephosphateisomerase, the first
protein discovered with this topology.
TIM barrels are considered
α/β protein folds because they include both helices and sheets in a single domain. The α-helices and β-strands form a solenoid that curves around to close on itself in a doughnut shape, topologically known as a
toroid. The parallel β-strands form the inner wall of the doughnut (hence, a
β-barrel), whereas the α-helices form the outer wall of the doughnut.
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Side view of the same TIM barrel (PDB code 8TIM). |
It should be emphasized that, although the ribbon diagram shows a hole in the protein's central core, the
amino acid side chains are not shown in this representation. The protein's core is actually tightly packed with bulky
hydrophobic amino acid residues. The packing interactions between the sheets and helices are also dominated by hydrophobicity and the branched
aliphatic residues
valine,
leucine, and
isoleucine comprise about 40% of the total residues in the β-strands.
Of the approximately 200 residues required to fully form a TIM barrel, about 160 are considered structurally equivalent between different proteins sharing this fold. The remaining residues are located on the loop regions that link the helices and sheets; the loops at the
C-terminal end of the sheets tend to contain the
active site, which is one reason this fold is so common: the residues required to maintain the structure and the residues that effect
enzymatic catalysis are for the most part distinct subsets. The linking loops can, in fact, be so long that they contain other protein domains.
# Carl Branden and John Tooze. 1999. Introduction to Protein Structure 2nd ed. Garland Publishing: New York, NY. pp 47-50.
*
triosephosphateisomerase*
protein folding*
SCOP list of proteins adopting the TIM barrel fold*
