Homogentisic acid biosynthesis

The NIH-shift is based on the translocation of a hydride ion or another substituent in its anionic form, which is located at the hydroxylated C-atom, e.g., a chloride or a bromide anion, a negatively charged alkyl group, or another carbon chain as in homogentisic acid biosynthesis (D 22). It takes place when the intermediate I in Fig. 20 is stabilized by the shift of electrons. 

Incorporation of DL-[2- C]tyrosine was to give specific labelling of C-1, whilst the results with L-[C/- C]tyrosine confirmed the utilization of this ammo-acid as the source of a Cg unit for rings c and D. (Tyramine, dopa, and homogentisic acid were much less effective precursors " for securinine, and homogentisic acid label was randomized.) The results with phenylalanine exclude it as a precursor and provide yet another example of the non-equivalence of phenylalanine and tyrosine in biosynthesis in higher plants. 

Collective name for polyprenylated 2,3-dimethylben-zoquinone derivatives, [e.g., PQ-9 (n=9), C5jH ,02, Mr 749.22, bright yellow platelets, mp. 48-49 C, u max 314 nm (petroleum ether)], isolated from chloro-plasts. The P. play a role as redox substrates in photosynthesis for cyclic and non-cyclic electron transport where they are converted reversibly into the corresponding hydroquinones (plastoquinols). The biosynthesis of P. proceeds from homogentisic acid, a product of L-tyrosine degradation, through prenylation and methylation (methyl group from L-methionine) to the plastoquinols which are dehydrated to the P.. ... 

With carotenoid biosynthesis, plastoquinone is involved as an electron acceptor, which we encounter further in photosynthetic electron transport [2]. An important precursor in the synthesis of plastoquinone, which also serves as a cofactor for the PDS enzyme, is homogentisic acid, which is formed from 4-hydroxyphenyl-pyruvate by 4-hydroxyphenylpyruvate dioxygenase (HPPD)... 

Vitamin E, as well as vitamin K, plastoquinones and ubiquinones, is formed, in principle, from shikimic acid via 4-hydroxyphenylpyruvic and homogentisic acid. The terpenoid side chain k synthesised from isopentenyl diphosphate, or more precisely from geranylgeranyl diphosphate, which is gradually reduced to phytyl diphosphate. The primary product of biosynthesis is 5-tocopherol other tocopherok are products of its methylation. The biosynthesis of tocotrienok lies in the condensation of homogentisic acid with geranylgeranyl diphosphate. Vitamin E k only synthesised by plants and some cyanobacteria. 

---