|Lecture Notes: 5 April||
In the non-oxidative portion of the Pentose Phosphate Pathway a series of sugar interconversions takes the RU-5-P to intermediates of other pathways: Ribose-5-P for nucleotide biosynthesis, and F-6-P and Ga-3-P for glycolysis/gluconeogenesis. All of these reactions are near equilibrium, with fluxes driven by supply and use of the three intermediates listed above.
In the first two reactions of this phase Ribulose-5-phosphate is converted either to Ribose-5-P via a 1,2-enediol intermediate, or to Xylulose-5-P via a 2,3-enediol intermediate.
These two 5-C sugars, R-5-P and Xu-5-P, are now interconverted to a 7-C sugar, Sedoheptulose-7-P, and a 3-C sugar, Glyceraldehyde-3-P. This reaction is catalyzed by Transketolase, a Thiamine pyrophosphate dependent enzyme which catalyzes the transfer of C2 units. In the first part of this reaction the TPP carbanion (ylid form) makes a nucleophilic attack on the carbonyl group of xylulose. In the resulting intermediate the C2-C3 bond is destabilized and cleavage takes place to yield the enzyme bound 2-(1,2-dihydroxyethyl)-TPP resonance stabilized carbanion:
This first part of the reaction is very similar to the first part of the Pyruvate DH catalyzed reaction in the Pyruvate DH Complex. (Ga-3-P is the leaving group instead of carbon dioxide; there is a 1,2-dihydroxyethyl instead of a 1-hydroxyethyl carbanion intermediate.) In the second part of the reaction the carbanion then attacks the aldehyde of R-5-P to give Su-7-P and regenerate the TPP catalyst:
This is similar to the second part of the Pyruvate DH reaction where the hydroxyethyl group attacks the disufide of the lipoamide. (In this case, of course, the redox catalyzed by the lipoamide does not take place.)
Transaldolase catalyzes the transfer of a C3 unit. The reaction occurs via an aldol cleavage similar to that seen with aldolase: there is a schiff base intermediate formed with an active site lysine. The difference between aldolase and transaldolase is in the acceptor groups: in aldolase the acceptor is a proton, in transaldolase it is another sugar. This reaction yields a F-6-P, which can go to Glycolysis, and an E-4-P which reacts with Xu-5-P catalyzed by the same transketolase seen above. This second transketolase reaction yields F-6-P and Ga-3-P, both intermediates of Glycolysis and the end products of the Pentose-P pathway.
The interconversions of the sugars in this pathway are summarized in the flow diagram below:
Note that the principle products of this pathway are R-5-P and NADPH. Under reductive biosynthetic conditions where R-5-P is not needed the Pentose-P pathway can be used to completely oxidize G-6-P to 6 carbon dioxide molecules with the concomitant production of 12 NADPH's. Note also that when R-5-P is needed and NADPH is not needed for reductive biosynthesis it can be made from F-6-P and Ga-3-P.
Overview of Glucose Metabolism in the Tissues: Diagram in packet [overhead]
Metabolism of hexoses other than glucose: Looked at Fructose, Mannose and Galactosse on Glyc/Gluconeo Overview.
The electron transport system involves a variety of redox reactions, so it is useful to review some electrochemical relationships. First, the free energy of a reaction is related to the reduction potential for electron transfer by the equation:
where n= moles of electrons transferred, and F is Faraday's constant (96,485 J V-1mol-1).
Last modified 5 April 2007