3D-structural models of archaeal THI4 family proteins compared to the X-ray structure of Neurospora crassa (Nc) THI4p (PDB: 3JSK). Proteins are represented in ribbon diagram including HVO_0665 (HvThi4, dark blue), MA_2851 (cyan), TK0434 (purple) and NcTHI4p (light brown), with the latter in octameric (A) and monomeric (B) configuration. For clarity in panel B, N- and C-terminal amino acid extensions of HvThi4 (residues 1–9 and 298–307) and NcTHI4p (residues 35–57) are hidden. (C) NcTHI4p residues bound or in close proximity to adenosine diphosphate 5-(beta-ethyl)-4-methyl-thiazole-2-carboxylic acid (AHZ) are indicated with structurally analogous residues of HvThi4 highlighted (where .a and .b indicate residues of chains a and b at the dimer interface). The conserved catalytic cysteine residue of NcTHI4p (Cys232) that is essential for thiamine biosynthesis is in the sulfur minus 2,3-didehydroalanine (DHA) form and is structurally analogous to HvThi4 Cys165 as indicated in pink.