4-oxalocrotonate tautomerase, bacteria/archaea <p>4-Oxalocrotonate tautomerase (4-OT) catalyzes the isomerisation of beta,gamma-unsaturated enones to their alpha,beta-isomers. The enzyme is part of a plasmid-encodedpathway, which enables bacteria harbouring the plasmid to use various aromatic hydrocarbons as their sole sources of carbon and energy. Theenzyme is a barrel-shaped hexamer, which can be viewed as a trimer of dimers. The hexamer contains a hydrophobic core formed by three beta-sheets andsurrounded by three pairs of alpha-helices. Each 4-OT monomer of 62 amino acids has a relatively simple beta-alpha-beta fold as described by the structure of the enzyme from <taxon tax_id="303">Pseudomonas putida</taxon> [<cite idref="PUB00007676"/>]. The monomer beginswith a conserved proline at the start of a beta-strand, followed by an alpha-helix and a 3₁₀ helix preceding a second parallel beta-strand, and ends witha beta-hairpin near the C terminus. The dimer results from antiparallel interactions between the beta-sheets and alpha-helices of the two monomers, forming afour-stranded beta-sheet with antiparallel alpha-helices on one side, creating two active sites, one at each end of the beta-sheet. Three dimers furtherassociate to form a hexamer by the interactions of the strands of the C-terminal beta-hairpin loops with the edges of the four-stranded beta-sheets of neighbouringdimers, creating a series of cross-links that stabilise the hexamer</p><p>Pro-1 of the mature protein functions as the general base while Arg-39 and an ordered water molecule each provide a hydrogen bond to the C-2 oxygen of substrate. Arg-39plays an additional role in the binding of the C-1 carboxylate group. Arg-11 participates both in substrate binding and in catalysis. Itinteracts with the C-6 carboxylate group, thereby holding the substrate in place and drawing electron density to the C-5 position. The hydrophobic nature ofthe active site, which lowers the pKa of Pro-1 and provides a favourable environment for catalysis, is largely maintained by Phe-50.</p><p>Because several Arg residues located near the active site are not conserved among all members of this family and because of the presence of fairly distantly related paralogs in <taxon tax_id="197">Campylobacter jejuni</taxon>, the family is regarded as not necessarily uniform in function.</p>