Benzoic acid derivatives, biosynthesis

In this review we have only dealt with alkaloid biosynthesis in C. roseus the biochemistry of this plant has also been studied in detail for other aspects, such as anthocyanin production, phosphate metabolism, cell growth, and cell division cycle (e.g., ref. 362). Unfortunately, most of the studies concerning the primary metabolism are not linked with those of secondary metabolism. However, one may expect that in the future the studies on secondary metabolism, such as chorismate-derived products (an-thocyanins, benzoic acid derivatives, and alkaloids) and terpenoid-derived products such as the alkaloids, will be integrated. This will eventually allow us a much better insight into the overall biochemistry of the plant. All of the available information makes C. roseus an outstanding model system for the study of the regulation of plant metabolism. 

It is considered that the biosynthesis of cocaine resembles that of (—)-hyoscyamine and (—)-scopolamine. It was shown, through the feeding experiments using E. coca [2—5], that the P-ketothioester was the biosynthetic intermediate of cocaine, as in the case of (—)-hyoscyamine and (—)-scopolamine. It was also demonstrated that a thioester of benzoic acid derived from phenylalanie was the precursor of the benzoyl moiety of cocaine [6]. 

Fig. 5. Biosynthesis of cinnamic acid and some benzoic acid derivatives.

Figure 2. Biosynthesis of benzoic acid derivatives from Q-C3 compounds or dehydroshikimic acid.

On the basis of this and the other biosynthetic evidence, Kominek has identified the rate limiting steps in the biosynthesis of novobiocin as the formation of the coumarin (123) and p-hydroxybenzoic acid from (—)-shikimic acid. He proposed a pathway of biosynthesis (Figure 4.19) in which the final step is condensation of the noviosyl coumarin (124) and the p-hydroxy-benzoic acid derivative (125). 

Quinoline Alkaloids.—Anthranilic acid and a benzoylacetic acid derivative, derived from phenylalanine, are precursors for graveoline (124) cf. ref. 5. On the other hand, whilst the biosynthesis of (125) no doubt involves anthranilic acid, benzoic acid (carboxyl-labelled material), and not phenylalanine, was a precursor for (125) in Lunasia amara incorporation was specific, with labelling of C-2. The remaining carbons should derive from acetate, and a positive incorporation was recorded. It is clear that further work is required on the biosynthesis of this alkaloid before its origins can be described with certainty. 

There are no reports on the biosynthesis of this type of alkaloid. It is possible that the origin of these alkaloids is phenylalanine, as for other isoquinoline-type alkaloids. However, because this type of alkaloid possesses a methyl moiety attached to the carbon alpha to the nitrogen atom, like ephedrine and related alkaloids, it is also possible that these alkaloids are derived from a C6-Ci unit, such as benzoic acid or benzaldehyde, as in the case of ephedrine and related alkaloids. That is why these alkaloids are included in this chapter. Elucidation of the biosynthetic pathway for these alkaloids is awaited. 

A consideration of the biosynthesis of p-aminobenzoic acid (PABA) involves primarily the mechanism of formation of the ring structure of aromatic compounds. Studies with mutants of E. colt indicated that shikimic acid (Fig. 3) was the precursor of the aromatic ring as occurring in PABA and also in tyrosine, tryptophan, phenylalanine, and p-hydroxy-benzoic acid (12). Much is now known about the manner in which kimic acid is formed from intermediates of carbohydrate metabolism (IS, ISa). Certain E. colt mutants also accumulate shikimic acid-5 -phosphate (ISb). Recent work of Weiss and Srinivasan (13c) demonstrated that PABA could be formed from shikimic acid-5 -phosphate and L-glutamine in an enzyme system derived from baker s yeast. Free shikimic acid was utilized very poorly for PABA synthesis but was more active when incubated in the presence of ATP. Glutamine was a specific amino donor and could not be replaced by glutamic acid or asparagine. When uniformly C -labeled shikimic acid was used as substrate for PABA synthesis in the enzyme system, the PABA formed had the same molar specific activity as the initial shikimic acid 5 -phosphate. PABA synthesis was also dependent on small amounts of yeast or liver concentrates but the nature of the cofactor and the mechanism of the over-all reaction are not known. 

The economic importance of the tropane alkaloids has resulted in them being much studied. They are esters of the base tropine (or a derivative of this base) with organic acids such as tropic, cinnamic or benzoic acids. Progress has been made in working out the biosynthesis of these alkaloids, although the associated enzymology is still obscure some of the steps in biosynthesis may indeed be non-enzymic. 

The squalestatins, e.g. 6.28, also known as the zaragozic adds, have attracted considerable interest as inhibitors of squalene synthase and hence of cholesterol biosynthesis and lipid deposition in the circulatory system. They are also inhibitors of farnesyl protein transferase and thus they may have other potentially useful biological applications. They are formed by Phoma spedes and also by Setosphaeria khartoumensis. The squalestatins are characterized by a dioxabicyclo-octane core bearing three carboxyl groups and two polyketide chains, one of which is attached as an ester. The biosynthetic incorporation of succinic acid into part of the bicyclo-octane, together with its oxygenation pattern, indicate that it may be derived via oxaloacetic acid. Both the polyketide chains have several pendant methyl groups attached to them, which arise from methionine, whilst benzoic add ads as a starter unit for one of the chains. These complex structures are thus the summation of several biosynthetic pathways. 

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