C6 oxidation pathway

Nevertheless, there are many questions still open because of problems to detect enzyme activities corresponding to each step of the pathway. The model of biosynthesis pathway was put together by studying the metabolism of exogenously applied intermediates in cell cultures of various origins and combining these results with data of native brassinosteroid patterns. It is more or less accepted that there are three pathways in parallel, the early and the late C6 oxidation pathway, as well as the 24/ -epimers follow ing the same route. Some observations in the analysis of native brassinosteroid patterns suggest a possible connection between the pathways. It was shown that seeds of Arabidopsis contain castasterone and 24-epi-brassinolide [34]. Also members of both 24-epimers, brassinolide and 24-epi-brassinolide were detected in tomato seeds [Winter, unpublished]. 

Fig. 3. Biosynthesis of brassinolide from campesterol. Brassinolide is synthesized from castasterone which is derived from campestanol by either late C6-oxidation pathway or early C6-oxidation pathway. Campesterol is converted to campestanol by hydrogenation. Italic letters refer to the lesions in the biosynthesis and sensitivity mutants of Arabidopsis det2, dwf4, cpd and bril) and pea (Ik and Ika). The asterisked pathways and compounds are hypothetical.

Typhasterol is then hydroxylated at C2 to give castasterone having a pair of vicinal hydroxyls which is very frequently and abundantly found in plants. The pathway from teasterone to castasterone was also confirmed in the seedlings of C. roseus, tobacco and rice, indicating that the early C6 oxidation pathway is operative in intact plants [49]. 

In the ruminant mammary tissue, it appears that acetate and /3-hydroxybutyrate contribute almost equally as primers for fatty acid synthesis (Palmquist et al. 1969 Smith and McCarthy 1969 Luick and Kameoka 1966). In nonruminant mammary tissue there is a preference for butyryl-CoA over acetyl-CoA as a primer. This preference increases with the length of the fatty acid being synthesized (Lin and Kumar 1972 Smith and Abraham 1971). The primary source of carbons for elongation is malonyl-CoA synthesized from acetate. The acetate is derived from blood acetate or from catabolism of glucose and is activated to acetyl-CoA by the action of acetyl-CoA synthetase and then converted to malonyl-CoA via the action of acetyl-CoA carboxylase (Moore and Christie, 1978). Acetyl-CoA carboxylase requires biotin to function. While this pathway is the primary source of carbons for synthesis of fatty acids, there also appears to be a nonbiotin pathway for synthesis of fatty acids C4, C6, and C8 in ruminant mammary-tissue (Kumar et al. 1965 McCarthy and Smith 1972). This nonmalonyl pathway for short chain fatty acid synthesis may be a reversal of the /3-oxidation pathway (Lin and Kumar 1972). 

Side-chain oxidized derivatives of ascorbic acid are also implicated in the catabolism of ascorbic acid in plants. Loewus et al. (62) have established the intermediacy of ascorbic acid in the biosynthesis of tartaric acid in the grape. Labeling studies have established a metabolic pathway that must involve C5 and C6 oxidation of ascorbic acid. 

Which reaction pathway dominates is dependent on the O s) concentration and the relative rates of carbonate formation and desorption of the gaseous C4, C6 and C7 gaseous products some control of these is possible38 by varying the propene-to-oxygen ratio and also the oxidant, such as substituting N20 for 02. 

Figure 17-8 The pentose phosphate pathways. (A) Oxidation of a hexose (C6) to three molecules of C02 and a three-carbon fragment with the option of removing C3, C4, C5, and C7 units for biosynthesis (dashed arrows). (B) Non-oxidative pentose pathways 2 1/2 C6 —> 3 C5 or 2 C6 —> 3 C4 or 3 V2C6 —> 3 C7.

So in summary, three glucose-6-phosphate (3.1) molecules (3 x C6) are oxidized to three ribulose-5-phosphate (3.13) residues (3 x C5) and three molecules of C02 (3 x Ci) under generation of six molecules of NADPH. The three ribulose-5-phosphate residues are then converted to one glyceraldehyde-3-phosphate (3.14) molecule (lx C3) and two fructose-6-phosphate (3.2) molecules (2 x C6). Fructose-6-phosphate can be converted to glucose-6-phosphate and reenter the oxidative part of the pentose phosphate pathway. Fructose-6-phosphate and glyceraldehydes can also serve as intermediates in glycolysis (Section 5.1), which offers the cell considerable flexibility in terms of its metabolic flux. 

Some mammalian cells have the ability to metabolize glucose 6-phosphate in a pathway that involves the production of C3, C4, C5, C6, and C7 sugars. This process also yields the reduced coenzyme, NADPH, which is oxidized in the biosynthesis of fatty acids and steroids (Chap. 13). Consequently, this metabolic pathway is of major importance in those cells involved in fatty acid and steroid production, such as the liver, lactating mammary gland, adrenal cortex, and adipose tissue. The pentose phosphate pathway, which does not require oxygen and which occurs in the cytoplasm of these cells, has two other names the phosphogluconate pathway (after the first product in the pathway) and the hexose monophosphate shunt (since the end products of the pathway can reenter glycolysis). 

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