One of the steps in the pentose phosphate pathway for...

One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of xylulose 5 -phosphate with ribose 5 -phosphate in the presence of a transketolase to give glyceraldehyde 3 -phosphate and sedoheptulose 7-phosphate. (EQUATION CAN'T COPY) (a) The first part of the reaction is nucleophilic addition of thiamin diphosphate (TPP) ylide to xylulose 5 -phosphate, followed by a retro-aldol cleavage to give glyceraldehyde 3 -phosphate and a TPP-containing enamine. Show the structure of the enamine and the mechanism by which it is formed. (b) The second part of the reaction is addition of the enamine to ribose 5-phosphate followed by loss of TPP ylide to give sedoheptulose 7-phosphate. Show the mechanism.

One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of xylulose 5 -phosphate with ribose 5 -phosphate in the presence of a transketolase to give glyceraldehyde 3 -phosphate and sedoheptulose 7-phosphate.
(EQUATION CAN'T COPY)
(a) The first part of the reaction is nucleophilic addition of thiamin diphosphate (TPP) ylide to xylulose 5 -phosphate, followed by a retro-aldol cleavage to give glyceraldehyde 3 -phosphate and a TPP-containing enamine. Show the structure of the enamine and the mechanism by which it is formed.
(b) The second part of the reaction is addition of the enamine to ribose 5-phosphate followed by loss of TPP ylide to give sedoheptulose 7-phosphate. Show the mechanism.

Key Concepts

Nucleophilic Addition and Retro-Aldol Cleavage

In many TPP-dependent reactions, the process begins with a nucleophilic attack by the TPP ylide on a carbonyl group, forming a tetrahedral intermediate. This is followed by a retro-aldol cleavage, a reaction in which a carbon–carbon bond is broken to yield smaller fragments. This two-step process is central to the mechanism, as it generates an enzyme-bound enamine intermediate that can further participate in subsequent carbon transfer reactions.

Thiamine Diphosphate (TPP) and TPP Ylide

Thiamine diphosphate (TPP) is a vitamin-derived cofactor critical for several enzymatic reactions in carbohydrate metabolism. It forms an ylide, a reactive nucleophilic species, which initiates the attack on carbonyl groups in substrates. The ylide’s formation and subsequent stabilization of carbanionic intermediates are essential for enabling the cleavage and recombination of carbon–carbon bonds during enzymatic transformations.

Addition Reaction and Enamine Intermediate

Following its formation, the enzyme-bound enamine intermediate acts as a nucleophile in an addition reaction with another sugar phosphate molecule. This step exemplifies a typical addition–elimination mechanism where the enamine attacks a carbonyl group to form a new carbon–carbon bond, and subsequent elimination of the TPP moiety regenerates the cofactor, completing the catalytic cycle. This covalent intermediate strategy is a hallmark of TPP-dependent enzymes.

Pentose Phosphate Pathway

The pentose phosphate pathway is a metabolic route parallel to glycolysis that generates NADPH and ribose sugars for nucleotide synthesis. Its non?oxidative branch includes a series of interconversions of sugar phosphates, facilitating flexibility in the production of various carbohydrate intermediates needed for biosynthetic reactions and energy metabolism.

Transketolase Mechanism

Transketolase is an enzyme that catalyzes the transfer of a two-carbon unit between sugar phosphates. Its mechanism involves the formation of a covalent intermediate with the substrate and the use of the cofactor thiamine diphosphate (TPP), making it a prototypical example of how enzymes use covalent catalysis to facilitate complex carbon rearrangements and bond cleavages in metabolic pathways.

Transcript

00:17 So just concerning ourselves with some more material of metabolic pathways.

00:22 These are the processes, the synthesis, the chemical transformations that occur within cells.

00:27 So we're essentially looking at organic chemistry in terms of how we will push arrows to generate new materials, intermediates, where all of our intermediate starting material and products are known as metabolites.

00:43 So we'll jump straight into the workload that we have here with the first step.

00:54 So firstly we need to understand that we have a retro aldol after the addition of our tpp.

01:06 This yields zucralose 5 phosphate.

01:11 So therefore we begin with adding ttp yield to the keto.

01:19 So that step looks as follows where we generate this product.

01:54 We have ch2 .o .h at the top here.

01:57 Oh h here another alcohol group proton another alcohol group one more hydrogen atom another bottom we have ch2 followed by o .p .o .3 to minus so then from this point we have a breakdown of this newly formed one molecule and then we form two intermediates so we can take a look at the arrow push so the density from, electron density from between the oxygen and the hydrogen here.

02:52 So then what we're left with after that arrow push is as follows.

02:55 We have two materials now.

03:04 Ch2o .p .o .3 to minus.

03:07 So that bottom portion remains unchanged.

03:09 Still have one alcohol group present in the molecule.

03:13 Now to move on to our heterocyclical molecule.

03:19 So we have a nitrogen and as well as the sulfur present here now...

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