Bizarre Thiamine Reaction Mechanism From An Obscure 1970 PhD Thesis

[Travis]

The orthodox reaction model (Brezlow) is shown below from Wikipedia. Coenzyme A can be seen taking the acetyl group from the thiamine complex. Notable is that there is no opening of the thiazole ring in the Breslow mechanism.

However, A.J. Knell has some things to say about this mechanism:

6. Breslow's theory is unsatisfactory for the following reasons.
(a) Intermolecular reaction at the 2'-position is stercially hindered, and it is difficult or impossible to build space-filling models of the proposed intermediates.
(b) It accounts for only the first of the structural features essential for activity
(c) It does not easily explain the oxidative functions of thiamine.
(d) 2'-(1"-Hydroxyethyl)thlamine pyrophosphate Is not a cofactor for apo-pyruvate decarboxylase (table 1), and the 1"-proton exchanges slowly. 2'-Acetyl thiazollum salts are notacetylating agents in water.
(e) It is difficult to show that thiamine acts catalytically in the model reaction, and the results obtained using thiamine analogues differ qualitatively and quantitatively from the results of enzyme studies

He goes to show his proposed mechanism of thiamine decarboxylation. This is radically different and interesting:

He justifies it with evidence and logic:

11. The advantages of this theory are as follows:
(a) There are no steric objections.
(b) It accounts for most of the observations relating to structure and function.
(c) The oxidative function of thiamine is explained (figure 8). The intermediate (15) should be easily oxidised, by analogy with the oxidation of xantho-thiamine and dihydrothiochrome to thiochrome. The product is an S-acyl xantho-thiamine (16). Thiol esters are usually reactive, and S-acyl leuco-thiamine (17) will transfer the acyl group to the S'-(2''-hydroxy)ethyl group or to solvent.
(d) It is consistent with the results of studies of the active sites.

This is a neat 144-page thesis on thiamine with a wealth of information. The author is racy and is not afraid to b****-slap other people's conclusions.

I looked-up the author and he (ostensibly) got his PhD for writing this. He later did studies on billirubin and disappeared from the public record.

Nonetheless, this is a cool thesis with 191 references. Besides chemical and structural data, there are sections on enzymes.

THIAMINE: A Study of its Chemistry, Biochemistry and Mechanism of Action

 

[goodandevil]

Does he expand on what thiamine is necessary for, in the more abstract sense i mean. A novel mechanism suggest novel interpretation.

 

[Travis]

Nothing that this guy says is unusual except for the reaction mechanism.

Even Breslow, who created the boring orthodox reaction mechanism, said this:

Karrer has suggested that thiamine may act in a form in which the thiazole ring has opened to free a sulfhydryl group. In view of the known ring-opening reactions of thiamine, the possibility must of course be kept in mind that an open form of the molecule could be the actual catalytic species.

On the Mechanism of Thiamine Action. IV. Evidence from Studies on Model Systems

I think the Karrer ring-opening mechanism is more realistic and fun.

Ronald Breslow was/is a very influential American chemist who was a student of Woodward and eventually became president of the ACS.

Paul Karrer was a Nobel Prize winner for Switzerland.

In terms of prestige, they could be considered equals. Maybe in American textbooks we get the Breslow mechanism and the Karrer mechanism is taught in Europe?

Not sure.

Reaction mechanisms like this take some of the magic out of enzymes and put them back in the small molecules, the coenzymes.

 

[Travis]

X-ray crystallographic data confirm ring opening:

During the refinement, it became clear that the ThDP molecule bound to the ZmPDC crystals was a chemically modified analogue of ThDP rather than the cofactor itself. The electron density maps showed no electron density between the assumed position of the C2 carbon atom of the thiazolium ring and the neighboring sulfur and nitrogen atoms, indicating that the bond between these atoms was broken (Fig. 1,bottom). In addition, negative difference electron density was observed at the position of the C2 carbon atom, indicating that the thiazolium ring had been opened and that the C2 carbon atom had been lost.

High Resolution Crystal Structure of Pyruvate Decarboxylase from Zymomonas mobilis
IMPLICATIONS FOR SUBSTRATE ACTIVATION IN PYRUVATE DECARBOXYLASES
Journal of Biological Chemistry

 

[Travis]

Here the Knell/Karrer mechanism drawn by modern software:

Thiochrome is fluorescent and can be detected by light.

Simple resonance forms.

Here thaimine attacks the carbonyl carbon of pyruvate. The sulfur would be expected to have a partial negative change and the carbonyl carbon of pyruvate would be expected to have a partial positive charge.

An histidine is added in this figure. It has been shown by x-ray crystallography that there is an unusual amount of histidine residues in the catalytic site of decarboxylase.

Carbon dioxide is liberated because of sulfur's electronegativity.

Two electrons flow through the thiamine molecule into the enzyme. This action releases acetaldehyde from thiamine. This is what A.J. Knell was referring to when speaking about thiamine's oxidative function that cannot be explained by the Breslow mechanism.

 

[BigYellowLemon]

I wish I understood the terms you use.