TIM barrel



The TIM barrel is a conserved protein fold consisting of eight α-helices and eight parallel β-strands that alternate along the peptide backbone. The structure is named after triosephosphateisomerase, a conserved glycolytic enzyme. Now, TIM barrels are known to be quite common among the conserved protein folds.

Structure and composition
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.

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.

Loop regions
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.