Cytosol thiamin diphosphate

The terpenes, carotenoids, steroids, and many other compounds arise in a direct way from the prenyl group of isopentenyl diphosphate (Fig. 22-1).16a Biosynthesis of this five-carbon branched unit from mevalonate has been discussed previously (Chapter 17, Fig. 17-19) and is briefly recapitulated in Fig. 22-1. Distinct isoenzymes of 3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase) in the liver produce HMG-CoA destined for formation of ketone bodies (Eq. 17-5) or mevalonate.7 8 A similar cytosolic enzyme is active in plants which, collectively, make more than 30,000 different isoprenoid compounds.910 However, many of these are formed by an alternative pathway that does not utilize mevalonate but starts with a thiamin diphosphate-dependent condensation of glyceraldehyde 3-phosphate with pyruvate (Figs. 22-1,22-2). 

Two percent to 3% of the thiamin in nervous tissue is present as the triphosphate, which also occurs in significant cunounts in skeletal muscle, especially in fast-twitch muscle fibers. In the nervous system, the triphosphate is found exclusively in the membreme fraction muscle thicunin triphosphate is mainly cytosolic. There eue two pathways for formation of thiamin triphosphate from the diphosphate ... 

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