Thiamin cleavage

Thiaminolytic enzymes are found in a variety of microorganisms and foods, and a number of thermostable compounds present in foods (especially polyphenols) cause oxidative cleavage of thiamin, as does sulfite, which is widely used in food processing. The products of thiamin cleavage by sulfite and thiaminases are shown in Figure 6.1. 

Monodeuteriothiamine 163 was prepared for use in determining the stereochemistry of both the thiaminase I and the bisulfite-catalyzed thiamin-cleavage reactions (Scheme 61) <1996JOC4172>. 

Leichter, J., and Joslyn, M.A., 1969. Kinetics of thiamin cleavage by sulphite. Biochemical Journal. 113 611-615. 

Thiazole, 4-methyl-5-(2-hydroxyethyl)-in thiamine biosynthesis, 1, 97 Thiazole, 4-methyl-2-methylami nosynthesis, 6, 300 Thiazole, 4-methyl-2-phenyl-alkylation, 6, 256 mercuration, 6, 256 Thiazole, 2-(methylthio)-methylation, 6, 290 thermodynamic values, 6, 291 Thiazole, 2-methylthio-5-phenyl-synthesis, 5, 153 Thiazole, 4-methyl-5-vinyl-occurrence, 6, 327 Thiazole, 2-phenyl-acetylation, 6, 270-271 Conformation, 6, 237 synthesis, 5, 113, 6, 306 Thiazole, 4-phenyl-conformation, 6, 237 2,5-disubstituted synthesis, 6, 304 Thiazole, 5-phenyl-conformation, 6, 237 Thiazole, 2-phenyl-5-triphenylmethyl-synthesis, 6, 265 Thiazole, 2-(2-pyridyl)-metal complexes, 5, 51 6, 253 Thiazole, 4-(2-pyridyl)-metal complexes, S, 51 6, 253 Thiazole, tetrahydro-ring cleavage, 5, 80 Thiazole, 2,4,5-trimethyl-occurrence, 6, 327... 

FIGURE 18.18 Thiamine pyrophosphate participates in (a) the decarboxylation of n-keto acids and (b) the formation and cleavage of n-hydroxyketones. 

Scheme 4.—Cleavage of deaminated thiamine with thioglycolic acid.

These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove specific functional groups. Examples include decarboxylases, which remove carboxylic acid groups as carbon dioxide, dehydrases, which remove water, and aldolases. The decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine pyrophosphate and magnesium ions for activity. 

Benzoylformate decarboxylase (BFD EC 4.1.1.7) belongs to the class of thiamine diphosphate (ThDP)-dependent enzymes. ThDP is the cofactor for a large number of enzymes, including pyruvate decarboxylase (PDC), benzaldehyde lyase (BAL), cyclohexane-1,2-dione hydrolase (CDH), acetohydroxyacid synthase (AHAS), and (lR,6] )-2-succinyl-6-hydroxy-2,4-cyclohexadiene-l-carboxylate synthase (SHCHC), which all catalyze the cleavage and formation of C-C bonds [1]. The underlying catalytic mechanism is summarized elsewhere [2] (see also Chapter 2.2.3). 

Various thiamine diphosphate (ThDP)-dependent a-keto acid decarboxylases have been described as catalyzing C-C bond formation and/or cleavage [48]. Extensive work has already been conducted on transketolase (TK) and pyruvate decarboxylase (PDC) from different sources [49]. Here attention should be drawn to some concepts based on the investigation of reactions catalyzed by the enzymes... 

Thiamine pyrophosphate plays an important role in the cleavage of bonds adjacent to a carbonyl group, such as the decarboxylation of a-lceto acids, and in chemical rearrangements in which an activated acetaldehyde group is transferred from one carbon atom to another (Table 14-1). The functional part of TPP, the thiazolium ring, has a relatively acidic proton at C-2. Loss of this... 

The following are topics that may be especially valuable to the student and which might be read initially in Chapter 12, lysozyme (Section B,5), chymo-trypsin (Section C,l), kinases (Section D,9), multiple displacement, reactions (Section G) in Chapter 13, imines (Section A,2), addition to C=C bonds (Section A, 4,5), beta cleavage and condensation (Section C) in Chapter 14, thiamin diphosphate (Section D), pyridoxal phosphate (Section E) in Chapter 15, NAD (Section A). 

A similar cleavage is catalyzed by thiamin-degrading enzymes known as thiaminases which are found in a number of bacteria, marine organisms, and plants. 

Below the structures of the adducts in Eq. 14-20 are those of a 2-oxo acid and a (3-ketol with arrows indicating the electron flow in decarboxylation and in the aldol cleavage. The similarities to the thiamin-dependent cleavage reaction are especially striking if one remembers that in some aldolases and decarboxylases the substrate carbonyl group is first converted to an N-proto-nated Schiff base before the bond cleavage. 

We see that the essence of the action of thiamin diphosphate as a coenzyme is to convert the substrate into a form in which electron flow can occur from the bond to be broken into the structure of the coenzyme. Because of this alteration in structure, a bond breaking reaction that would not otherwise have been possible occurs readily. To complete the catalytic cycle, the electron flow has to be reversed again. The thiamin-bound cleavage product (an enamine) from either of the adducts in Eq. 14-20 can be reconverted to the thiazolium dipolar ion and an aldehyde as shown in step b of Eq. 14-21 for decarboxylation of pyruvate to acetaldehyde. 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each halfreaction is an a cleavage which leads to a thiamin- bound enamine (center, Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step b followed by a in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an a4 tetramer and one with an ( P)2 quaternary structure. The isolation of ohydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate52 provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase126 in the biosynthesis of the plant hormone indoIe-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25).1243/127... 

Figure 14-3 Half-reactions making up the thiamin-dependent a cleavage and a condensation reactions.

Ketols can also be formed enzymatically by cleavage of an aldehyde (step a, Fig. 14-3) followed by condensation with a second aldehyde (step c, in reverse). An enzyme utilizing these steps is transketolase (Eq. 17-15),132b which is essential in the pentose phosphate pathways of metabolism and in photosynthesis. a-Diketones can be cleaved (step d) to a carboxylic acid plus active aldehyde, which can react either via a or c in reverse. These and other combinations of steps are often observed as side reactions of such enzymes as pyruvate decarboxylase. A related thiamin-dependent reaction is that of pyruvate and acetyl-CoA to give the a-diketone, diacetyl, CH3COCOCH3.133 The reaction can be viewed as a displacement of the CoA anion from acetyl-CoA by attack of thiamin-bound active acetaldehyde derived from pyruvate (reverse of step d, Fig. 14-3 with release of CoA). 

A reaction that is related to that of transketolase but is likely to function via acetyl-TDP is phosphoketolase, whose action is required in the energy metabolism of some bacteria (Eq. 14-23). A product of phosphoketolase is acetyl phosphate, whose cleavage can be coupled to synthesis of ATP. Phosphoketolase presumably catalyzes an a cleavage to the thiamin-containing enamine shown in Fig. 14-3. A possible mechanism of formation of acetyl phosphate is elimination of HzO from this enamine, tautomerization to 2-acetylthiamin, and reaction of the latter with inorganic phosphate. 

The oxidative cleavage of an a-oxoacid is a major step in the metabolism of carbohydrates and of amino acids and is also a step in the citric acid cycle. In many bacteria and in eukaryotes the process depends upon both thiamin diphosphate and lipoic acid. The oxoacid anion is cleaved to form C02 and the remaining acyl group is combined with coenzyme A (Eq. 15-33). 

Tire enzyme does not require lipoic acid. It seems likely that a thiamin-bound enamine is oxidized by an iron-sulfide center in the oxidoreductase to 2-acetyl-thiamin which then reacts with CoA. A free radical intermediate has been detected318 321 and the proposed sequence for oxidation of the enamine intermediate is that in Eq. 15-34 but with the Fe-S center as the electron acceptor. Like pyruvate oxidase, this enzyme transfers the acetyl group from acetylthiamin to coenzyme A. Cleavage of the resulting acetyl-CoA is used to generate ATR An indolepyruvate ferredoxin oxidoreductase has similar properties 322... 

The mechanism of the cleavage of the pyruvate in Eq. 15-37 is not obvious. Thiamin diphosphate is not involved, and free C02 is not formed. The first identified intermediate is an acetyl-enzyme containing a thioester linkage to a cysteine side chain. This is cleaved by reaction with CoA-SH to give the final product. A clue came when it was found by Knappe and coworkers that the active enzyme, which is rapidly inactivated by oxygen, contains a long-lived free radical.326 Under anaerobic conditions cells convert the inactive form E to the active form Ea by an enzymatic reaction with S-adenosylmethionine and reduced flavodoxin Fd(red) as shown in Eq. 15-38.327-329 A deactivase reverses the process.330... 

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