Domain Organization in Candida glabrata THI6, a Bifunctional Enzyme Required for Thiamin Biosynthesis in Eukaryotes,

Abstract

THI6 is a bifunctional enzyme found in the thiamin biosynthetic pathway in eukaryotes. The Nterminal domain of THI6 catalyzes the ligation of the thiamin thiazole and pyrimidine moieties to form thiamin phosphate and the C-terminal domain catalyzes the phosphorylation of 4-methyl-5-hydroxyethylthiazole in a salvage pathway. In prokaryotes, thiamin phosphate synthase and 4-methyl-5-hydroxyethylthiazole kinase are separate gene products. Here we report the first crystal structure of a eukaryotic THI6 along with several complexes that characterize the active sites responsible for the two chemical reactions. THI6 from Candida glabrata is a homohexamer in which the six protomers form a cage-like structure. Each protomer is composed of two domains, which are structurally homologous to their monofunctional bacterial counterparts. Two loop regions not found in the bacterial enzymes provide interactions between the two domains. The structures of different protein-ligand complexes define the thiazole and ATP binding sites of the 4-methyl-5-hydroxyethylthiazole kinase domain, and the thiazole phosphate and 4-amino-5-hydroxymethyl-2-methylpyrimidine pyrophosphate binding sites of the thiamin phosphate synthase domain. Our structural studies reveal that the active sites of the two domains are 40 Å apart and are not connected by an obvious channel. Biochemical studies show 4-methyl-5-hydroxyethylthiazole phosphate is a substrate for THI6; however, adenosine diphospho-5-β-ethyl-4-methylthiazole-2-carboxylic acid, the product of THI4, is not a substrate for THI6. This suggests that unidentified enzyme is necessary to produce the substrate for THI6 from the THI4 product.Thiamin (vitamin B 1 ) is an essential component of all living systems. The active form of thiamin, thiamin pyrophosphate (ThDP), acts as a cofactor for several important enzymes in carbohydrate and amino acid metabolism. The mechanistic role of the cofactor is stabilization of an acyl carbanion intermediate (1). Thiamin biosynthetic pathways are found in prokaryotes and some eukaryotes (yeast, fungi and plants); however, vertebrates cannot synthesize thiamin. Therefore, thiamin is an essential component of the human diet with a daily requirement of 1.0-1.2 mg (2). Deficiency of thiamin leads to diseases such as beriberi † This work was supported by NIH grant DK44083 to T.P.B. and DK67081 to S.E.E. || This work is based upon research conducted at the Advanced Photon Source on the Northeastern Collaborative Access Team beamlines, which are supported by award RR-15301 from the National Center for Research Resources at the National Institutes of Health. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. ‡ The coordinates of the THI6 structures have been deposited in the Protein Data Bank under accession number 3NL2, 3NL3, 3NM3, 3NL5, 3NL6 and 3NM1 for THI6, THI6/ThMP, THI6/ThMP/PP, THI6/Thz/AMP-PCP, THI6/ThMP/AMP-PCP and THI6/CF 3 HMP-PP/carboxy-Thz-P, respec...

Structure of trifunctional THI20 from yeast

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“…The C-terminal and N-terminal domains of Thi20 are homologous to TenA (thiaminase II) and ThiD (HMP/ HMP-P kinase) from B. subtilis, respectively (53)(54)(55). THI6 has N-and C-terminal homology with ThiE (thiamin phosphate synthase) and ThiM (4-methyl-5-hydroxyethylthiazole kinase) from B. subtilis, respectively (56). This fusion of enzymatic activities that are key to thiamin salvage may be an emerging theme in eukaryotes.…”

Essential Metabolic Routes as a Way to ESKAPE From Antibiotic Resistance

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