Secondary alcohols biosynthesis

Table II shows (28) a correlation between the biosynthesis of veratryl alcohol and PAL activities, both of which are affected by initial glucose and ammonium salt levels. The HC-LN culture with a C/N ratio of 240, which is almost comparable to that of wood, shows the greatest amount of veratryl alcohol biosynthesis and, therefore, the highest PAL activity. As can be seen, depending upon the C N balance of the media used, the amounts of secondary metabolites formed and PAL activites can vary greatly (Tables I and II).

For a synthesis of the anti-cancer drug taxol TPAP/NMO was used in three steps, two for oxidation of primary alcohols to aldehydes (by TPAP/NMO/PMS/ CHjClj) and one for a secondary alcohol to ketone (by TPAP/NMO/PMS/CHjClj-CHjCN) [66], cf. also [111] and for the SERCA inhibitor thapsigargin (two primary alcohol and one secondary alcohol oxidation steps) [112], This system was also used during synthesis of the cholesterol biosynthesis inhibitor 1233A [52], the antibiotic and anti-parasitic ionophore tetronasin [113, 114] and for the cytotoxic sponge alkaloids motopuramines A and B [115]. 

The biosynthesis of many bis-indole alkaloids has been postulated to proceed by dimerisation of appropriate precursors, and there is now a substantial amount of experimental evidence to support this hypothesis. For example, treatment of the alcohol 1 with acid gives the alkaloid yuehchukene 2, and 1 could arise biogenetically by in vivo prenylation of indole followed by enzymatic oxidation. A study of related 2-prenylated indoles has confirmed the ease with which such molecules can "dimerise". Thus, treatment of the secondary alcohol 3 in benzene with silica gel impregnated with TsOH gave a complex mixture of products from which 4 (5.1%) and 5 (2.1%) were isolated (3 is very sensitive to acid, and is easily decomposed). Treatment of the isomeric tertiary alcohol 6 with a catalytic amount of TFA in anhydrous benzene gave much higher yields of the two "dimeric" products 7 (31%) and 8 (25%). 

Methods for the capillary gas chromatographic separation of optical isomers of chiral compounds after formation of diastereoisomeric derivatives were developed. Analytical aspects of the GC-separation of diastereoisomeric esters and urethanes derived from chiral secondary alcohols, 2-, 3-, 4- and 5-hydroxy-acid esters, and the corresponding jf- and -lactones were investigated. The methods were used to follow the formation of optically active compounds during microbiological processes, such as reduction of keto-precursors and asymmetric hydrolysis of racemic acetates on a micro-scale. The enantiomeric composition of chiral aroma constituents in tropical fruits, such as passion fruit, mango and pineapple, was determined and possible pathways for their biosynthesis were formulated. 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

Stereospecific reductases are involved in the reduction of keto groups to secondary alcohols the desired endproducts of a deoxysugar biosynthesis. This has been shown for the biosynthesis of L-rhamnose and CDP-ascarylose (see Sect. 3.1.4). The reduction of keto groups at C-4 have been proposed to be catalyzed by DnrV (daunorubicin 29), EryBIV (erythromycin 14), and StrL (streptomycin 24) [21]. 

Figure V. Possible pathway for the biosynthesis of secondary alcohols, their esters, and other typical constituents of passion fruit. Enzyme (El) is operative in yellow passion fruit. The antipodal reduction catalyzed by enzyme (E2) and the following esterification take place only in the purple variety.

Fig. 13. An elongation-decarboxylation pathway for the biosynthesis of alkanes, secondary alcohols, and ketones. The steps most probably affected by light, chemicals, and mutations are indicated gli, gU, and gU are mutants of B. oleracea, and wsp is a mutant of P. sativum.

Despite the sequence similarity between the CERl and GLl proteins, their biochemical function is unclear since mutations at each gene affect the respective cuticles differently. CERl has been predicted to code for an aldehyde decarbonylase (Aarts et al., 1995), an enzyme required for the production of the alkane fraction of the cuticular waxes of this species. Indeed, mutations at the CERl locus cause an enrichment of the aldehydes and a depletion of the alkanes and alkane-derived metabolites (ketones and secondary alcohols). In maize however, its unlikely that gll codes for an aldehyde decarbonylase. This conclusion is based upon the fact that alkanes account for a very small portion of the cuticular waxes of wild-type maize seedlings (about 1%), and because mutations at the gll locus qualitatively and/or quantitatively affect the accumulation of fatty aldehydes, alcohols, and the ester components of the maize cuticular waxes. Therefore, even though the GLl and CERl proteins share similar structures, they may in fact perform different functions in cuticular wax biosynthesis. Alternatively, both proteins may perform similar, but as yet unidentified, function(s). 

Kolattukudy P E 1970 Biosynthetic relationships among very long chain hydrocarbons, ketones, and secondary alcohols and the noninvolvement of alkenyl glyceryl ethers in their biosynthesis. Arch Biochem Biophys 141 381-383... 

Kolattukudy P E, Buckner J S, Liu T Y J 1973 Biosynthesis of secondary alcohols and ketones from alkanes. Arch Biochem Biophys 156 613-620... 

As a showcase the first steps in the biosynthesis of sora-phen are depicted in Figure 2.1. In module 1 the intermediate p-keto ester is reduced stereoselectively to generate the chiral secondary alcohol. 

However, there are two articles which report evidence against these relationships between biosynthesis of this secondary metabolite and lignin decomposition (43,44). For example, a mutant of P. chrysosporium, which does not produce veratryl alcohol, has ligninolytic activity (43). Another mutant, which lacks glucose oxidase, is unable to decompose lignin to CO2 and therefore is ligninase Mess. It is, however, able to produce about 30% of the amount of veratryl alcohol normally found in the fungus (44). 

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