Cleaving of ketosubstrates by transketolase and the nature of the productsformed

The interaction of transketolase ketosubstrates with the holoenzyme has bee n studied. On addition of ketosubstrates cleaving both irreversibly (hydrox ypyruvate) and reversibly (xylulose 5-phosphate), identical changes in the CD spectrum at 300-360 nm are observed. The changes in this, spectral regio n, as previously shown, are Clue to the formation of the catalytically acti ve holoenzyme from the apoenzyme and the coenzyme, and the cleavage of keto substrates by transketolase. The identity of the changes in transketolase C D spectrum caused by the addition of reversibly or irreversibly cleaving su bstrates indicates that in the both cases the changes are due to the format ion of an intermediate product of the transketolase reaction-a glycolaldehy de residue covalently bound tor the coenzyme within the holoenzyme molecule . Usually, in the course of the transferase reaction, the glycolaldehyde re sidue is transferred to an aldose (acceptor Substrate), resulting in the re cycling of the holoenzyme free of the glycolaldehyde residue. The removal o f the glycolaldehyde residue from the holoenzyme appears to proceed even in the absence of an. aldose. However, the glycolaldehyde can not be found th e free state because it condenses with another glycolaldehyde residue forme d in the course of the cleavage of another ketosubstrate molecule yielding erythrulose.