Photosynthesis transketolase

Transketolase and transaldolase appear to play a significant part in photosynthesis.86 Photosynthesizing plants have been exposed to C14C>2 for short periods (up to 15 seconds) and the radioactive products have been... 

In the second phase, transaldolase (with TPP as cofactor) and transketolase catalyze the interconversion of three-, four-, five-, six-, and seven-carbon sugars, with the reversible conversion of six pentose phosphates to five hexose phosphates. In the carbon-assimilating reactions of photosynthesis, the same enzymes catalyze the reverse process, called the reductive pentose phosphate pathway conversion of five hexose phosphates to six pentose phosphates. 

An example of an a-ketol formation that does not involve decarboxylation is provided by the reaction catalyzed by transketolase, an enzyme that plays an essential role in the pentose phosphate pathway and in photosynthesis (equation 21) (B-77MI11001). The mechanism of the reaction of equation (21) is similar to that of acetolactate synthesis (equation 20). The addition of (39) to the carbonyl group of (44) is followed by aldol cleavage to give a TPP-stabilized carbanion (analogous to (41)). The condensation of this carbanionic intermediate with the second substrate, followed by the elimination of (39), accounts for the observed products (B-7IMIHOO1). 

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). 

TPP involved in reactions catalysed by pyruvate decarboxylase (alcoholic fermentation), pyruvate dehydrogenase a-ketoglutarate dehydrogenase (TCA cycle), transketolase (photosynthesis Calvin cycle) acetolactate synthetase (Val, Leu biosynthesis)... 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

Part of the dark reactions of photosynthesis is interconversion of sugars with an enzyme called transketolase using thiamine pyrophosphate, TPP, as a catalyst (Section 8.12.8). Provide a reasonable mechanism for this enzymatic reaction. In addition to water, there are weak general acids and general bases present in the active site at pH 7. 

Transketolase is also involved in the Calvin-Benson cycle of photosynthesis by plants and some bacteria. 

A most important clue to the nature of the steps between pentose phosphate and hexosemonophosphate, and thus to the role of the pentose phosphate pathway in photosynthesis, came from our discovery in 1953 of sedoheptulose 7-phosphate as the first product formed from pentose phosphate. The enzyme transketolase had been purified from rat liver and spinach in my laboratory and crystallized from yeast by Racker and his coworkers and the two laboratories simultaneously discovered that this enzyme contained thiamine pyrophosphate as its functional group, f Isotope studies in my laboratory showed that sedoheptulose was formed by the transfer of a C2 group ( active glycolaldehyde ) from one molecule of pentose phosphate to another, and that the reaction was fully reversible thus sedoheptulose 7-phosphate was also a Ca-donor. In addition, Racker s laboratory made the important finding that fructose 6-phosphate would also yield active glycolaldehyde, and Arturo Bonslgnore and his coworkers discovered that rat liver extracts catalyzed the rapid non-oxidative conversion of hexose phosphate to sedoheptulose phosphate. ... 

---