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Transketolase (abbreviated as TK) is an enzyme that, in humans, is encoded by the TKT gene. [1] It participates in both the pentose phosphate pathway in all organisms and the Calvin cycle of photosynthesis. Transketolase catalyzes two important reactions, which operate in opposite directions in these two pathways.
While the pentose phosphate pathway does involve oxidation of glucose, its primary role is anabolic rather than catabolic. The pathway is especially important in red blood cells (erythrocytes). The reactions of the pathway were elucidated in the early 1950s by Bernard Horecker and co-workers. [2] [3] There are two distinct phases in the pathway.
In enzymology, a formaldehyde transketolase (EC 2.2.1.3) is an enzyme that catalyzes the chemical reaction D-xylulose 5-phosphate + formaldehyde ⇌ {\displaystyle \rightleftharpoons } glyceraldehyde 3-phosphate + glycerone
transketolase and transaldolase: aldehyde or ketone groups EC 2.3: acyltransferase: acyl groups or groups that become alkyl groups during transfer EC 2.4: glycosyltransferase, hexosyltransferase, and pentosyltransferase: glycosyl groups, as well as hexoses and pentoses: EC 2.5: riboflavin synthase and chlorophyll synthase: alkyl or aryl groups ...
F6P has two carbons removed by transketolase, giving erythrose-4-phosphate (E4P). The two carbons on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P). [citation needed] E4P and a DHAP (formed from one of the G3P from the second CO 2 fixation) are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
In what is essentially the reverse of step two, the electrons push back in the opposite direction forming a new bond between the substrate carbon and another atom. (In the case of the decarboxylases, this creates a new carbon-hydrogen bond. In the case of transketolase, this attacks a new substrate molecule to form a new carbon-carbon bond.)
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In the non-oxidative branch of the pentose phosphate pathway, xylulose-5-phosphate acts as a donor of two-carbon ketone groups in transketolase reactions. [1] Xylulose-5-phosphate also plays a crucial role in the regulation of glycolysis through its interaction with the bifunctional enzyme PFK2/FBPase2.