Thiaminase

Thiaminase is an enzyme that metabolizes or breaks down thiamine into pyrimidine and thiazole. It is an antinutrient when consumed.

The old name was "aneurinase".^[1]

There are two types with different Enzyme Commission numbers:^[2]

• Thiamine pyridinylase, Thiaminase I (EC 2.5.1.2, InterPro: IPR030901)
  • pyridine + thiamine <=> 5-(2-hydroxyethyl)-4-methylthiazole + heteropyrithiamine^[3]
  • Secreted by Paenibacillus thiaminolyticus,^[4] an anaerobic organism that occurs in the human small intestine
• Aminopyrimidine aminohydrolase, Thinaminase II (EC 3.5.99.2, InterPro: IPR027574, IPR004305)
  • 4-amino-5-aminomethyl-2-methylpyrimidine + H₂O <=> 4-amino-5-hydroxymethyl-2-methylpyrimidine + NH₂^+[5]
  • H₂O + thiamine <=> 4-amino-5-hydroxymethyl-2-methylpyrimidine + 5-(2-hydroxyethyl)-4-methylthiazole + H^+[5]
  • Produced by a wide range of plants and bacteria. In these organisms, it is mainly responsible for salvage of thiamine pyrimidine from degradation products, rather than the breakdown of thiamine.^[5] In bacteria, it stays inside their cells.^[6]

Structure and function

Thiaminase I

Thiaminase I works to cleave the pyrimidine ring in thiamin from the thiazolium ring at the methylene bridge. From there it adds a base compound to the pyrimidine, creating an analogue inhibitor of thiamin. Thiaminase I has the ability to use a multitude of C-N cleaving nucleophilic substrates like cysteine, pyridine, aniline, veratrylamine, dithiothreitol, and quinoline.^[7]

When analyzing the structure of Thiaminase I it shows a fold similar to that of group II periplasmic binding proteins like maltose-binding protein.^[8] These periplasmic binding proteins have two domains that each contain an α/β fold. These two domains come together to form a deep cleft that are connected by three crossover segments. Due to this structure scientists proposed that Thiaminase I could have evolved from prehistoric periplasmic binding protein that had been responsible for up taking thiamin.^[8] Between the two domains, in the cleft, sit the active site for Thiaminase I. Along the cleft there are four acidic residues and six tyrosine residues. In order for Thiamin to interact with Thiaminase I it is positioned in the active site between the pyrimidine and Asp272 by two hydrogen bonds. The Glu241 the goes on to activate the Cys113 to attack C6 of the pyrimidine. This forms a zwitterionic intermediate.^[8] The Glu241 causes and protonation and nucleophilic attack that results in the split of the bond between the pyrimidine and the thiazole. When observing the crystalline structure, it has two ⍺/ꞵ-type domains separated by a large cleft. At room temperature the two molecules have a noncrystallographic twofold axis that are bridged by a sulfate ion. ^[9]

Thiaminase II

Thiaminase II cleaves but does not add a base compound. Thiaminase II can only use water as the nucleophile.^[10]

Thiaminase II has been found to be TenA. In order to cleave the C-N bond between the thiazole and pyrimidine Thiaminase only uses water as its nucleophile. When viewing Thiaminase II it is found to have a crystal structure that has 11 helices surrounding a deep acidic pocket.^[8] For each monomer present in the quaternary structure it interacts with two other monomers. There are several residues like Tyr112, Phe208, Tyr47, and Tyr163 that have some sort of contribution to the π- stacking environment surrounding the HMP ligand.^[8] The Glu205 side chain will form a hydrogen bond with the N1 nitrogen in the pyrimidine ring. Next the Tyr163 and the Asp44 side chain come together to form the hydrogen bonds with the N3 and N4'.^[8] Finally the Cys135 catalytic residue is positioned near the C2 in the pyridine ring to complete the split of thiamin into its heterocycles.^[8]

Sources

This enzyme can be found in a variety of different sources. It can be found in marine organisms, plants, and bacteria. Since Thiamine (vitamin B₁) is a very important substance required for metabolic pathways by almost all organisms, it can be very detrimental to introduce Thiaminase to a system. Frequently an organism gains this enzyme by ingesting another organism that carries it. In most cases, prey fish will contain one of the bacteria that produces this enzyme. When that prey fish is consumed raw without treatment the bacteria will transfer to the consumer.^[11] The consumer eventually will fall ill, even die, from a thiamine deficiency. This has been seen in different lab studies. Through these studies the enzyme has been found in zebra fish as well as red cornet fish.^[11] Cooking thiaminase-containing foods usually inactivates the enzyme.^[11]

Sources of thiaminase I include:

• Plants: Bracken (brake),^[12] nardoo,^[12] horsetail.^[13]
• Fish including zebra fish,^[11] carp and goldfish.^[14]
• Bacteria such as
  C. botulinum.^[7]
• An African silk worm, Anaphe venata; this enzyme is more heat-tolerant than other thiaminases and requires a longer cooking time^[16]

Sources of thiaminase II include:

• Bacillus aneurinolyticus^[6] and Bacillus subtilis.^[17]

Effects

Function

It is still unclear what thiaminase does for fish, bacterial cell or insects that contain it. In ferns, thiaminase I is thought to offer protection from insects^[18]

Studies have shown that thiamine hydrolase (thiaminase II), which was originally thought to be involved solely in the degradation of thiamine, has actually been identified as having a role in thiamine degradation with the salvage of the pyrimidine moiety. Thiamin hydrolysis product N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine is transported into the cell and deformylated by the amidohydrolase ylmB and hydrolyzed to 5-aminoimidazole ribotide.^[19]

When ingested

It was described in 1941 as the cause of highly mortal