Biosynthesis hypothetical biosynthetic pathway

Scheme 5. Various Glycosyltransfer Reactions Related to the Biosynthesis of Glycosphingolipids having Globo-series Sugar Chains. (Broken arrows indicate the hypothetical, biosynthetic pathway.)...

In a branched biosynthetic pathway, the initial reactions participate in the synthesis of sevetal products. Figure 9—4 shows a hypothetical btanched biosynthetic pathway in which cutved attows lead from feedback inhibitors to the enTymes whose activity they inhibit. The sequences S3 —> A, S4 —> B, S4 —> C, and S3 — > D each represent hneat teaction sequences that are feedback-inhibited by theit end products. The pathways of nucleotide biosynthesis (Chaptet 34) provide specific examples. 

The biosynthesis of [6]-gingerol was investigated by administration of labelled precursors to whole Z. officinale plants [303, 305]. The hypothetical intermediates [6]-dehydrogingerdione (335), [6]-gingerdione (337) and [6]-dehydrogingerol (339), were synthesized and shown to be incorporated into [6]-gingerol [304]. A biosynthetic pathway for [6]-gingerol was then proposed as shown in "Fig. (4)". 

Musca domestica, with its female-specific Muscalure - (Z)-9-tricosene - was first used as a model for biosynthetic studies (Blomquist and Jackson, 1979 Blomquist et al., 1987). Inspired by these model studies, Jallon (1984) proposed and discussed the first hypothetical pathway for the biosynthesis of major cuticular... 

The de novo biosynthesis of the BH4 cofactor is carried out by three classical enzymes as described below. In addition, a hypothetical alternative way to circumvent the last enzymatic step by involving other reductases is presented (from 41 to 45 in Figure 11). Part of this alternative pathway is also called salvage pathway, which feeds BH4 from dihydrobiopterin through the NADPH-dependent dihydrofolate reductase. As will be discussed later, this alternative pathway seems to compensate for a genetic defect in the last biosynthetic step at least in peripheral tissues. 

Adelberg et oZ. (148) showed that the pathways for isoleucine and valine biosynthesis were independent of each other. Previously Tatum and Adel-bei (148) had proposed a common biosynthetic origin for these two amino acids from a hypothetical six-carbon dicarboxylic acid. Proof for the independent synthesis was obtained by combining a Neurospora mutant which was blocked between the ,/8-dihydroxy acids (see Fig. 7, IV) and valine and isoleucine and another mutant that was blocked prior to L-threonine in isoleucine biosynthesis. If a common intermediate was involved, it was reasoned that the double mutant should show a reduced accumulation of both dihydroxy acids, and the accinnulation of both should be specifically stimulated by threonine or 7-ketobutyrate. These predictions were not borne out. 

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