Homogentisic acid derivatives

Homogentisic acid-derived hydroquinone and benzoquinone derivatives (D 22.4)... 

D 22.4 Homogentisic Acid-Derived Hydroquinone and Benzoquinone Derivatives... 

Homogentisic acid derivatives 2-Methyl and 2-ethyl-benzoquinones (D 22.4) Synthesized in several beetles... 

The amino acids phenylalanine and its hydroxylated derivative, tyrosine, are both catabolised in the livers of animals to fumaric acid and acetoacetic acid via homogentisic acid. This is formed by the oxidation of 4-hydroxyphenylpyruvate, catalysed by the copper containing enzyme 4-hydroxyphenylpyruvate dioxygenase, which requires vitamin C for its activity. The complete sequence is shown in Figure 5.13. The dioxygenase is so called because both the atoms of the... 

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.. ... 

The aromatic nucleus and one of the methyl groups are derived from homo-gentisic acid, a product of L-tyrosine degradation (D 22). Homogentisic acid reacts with prenylpyrophosphates of different chain length (D 6). The quinols formed are reversibly dehydrogenated to the corresponding quinones, e.g.,... 

The next metabolic step involves the transformation of P-hydroxyphenylpyruvate into homogentisic acid. This reaction is accomplished in two steps (1) side chain migration and the addition of the hydroxyl group to the ring and (2) oxidative decarboxylation of the side chain. Ascorbic acid or ascorbic acid derivatives activate the system in an unknown manner. A study of the enzymes involved in parahydroxyphenylpyruvic oxidation demonstrated that two protein fractions, neither of which is active alone, are needed. One of the protein fractions was identified as catalase however, the formation of hydrogen peroxide during the reaction was not established. 

Q5. Both homogentisic acid (Figure 8.11) and / -hydroxybenzaldehyde can be derived from tyrosine. Marginalin is a eondensation product of homogentisic acid and p-hydroxybenzaldehyde. 

Figure 4. Chromatogram of a mixture of carboxylic acids as the t-butyidimethylsilyl derivatives. GC conditions DB-1 fused-silica capillary column (30 m x 0.32 mm i.d, 0.25 pm), initially at 60 "C for 2 min, then programmed to 280 °C at 4°C/min 0.8 pi sample, injected with split ratio of 15 1 both injector and detector temperatures at 300 °C nitrogen as the carrier gas at 0.9ml/min. Peaks l = formic, 2 = acetic, 3 = propionic, 4 = isobutyric, 5 = butyric, 6 — isovaleric, 7 = valeric, 8 = caproic. 9 = enanthic, 10 = benzoic, 11= caprylic, 12 = lactic, 13 = phenylacetic, 14 = glycollic, 15 = oxalic, 16 = pelargonic. 17 = malonic, 18 = capric, 19 - succinic, 20 - methylsuccinic, 21 = undecanoic, 22 = fumaric, 23 = 5-phenylvaleric, 24 = p-aminobenzoic, 25 = lauric. 26 = mandelic, 27 = adipic, 28 = 3-methyladipic, 29 = tridecanoic, 30 = phenyllactic. 31 = hippuric, 32 = myristic, 33 = p-hydroxybenzoic, 34 = malic, 35 = suberic, 36 = pentadecanoic, 37 = vanillic, 38 = palmitic. 39 = syiingic, 40 = tartaric, 41 — margaric, 42 = a-resorcylic, 43 = p-hydroxymandelic, 44 = y-resorcylic, 45 = stearic. 46 = homogentisic, 47 = protocatechuic 48 = nonadecanoic, 49 = citric 50 = cirachidic acid. (Reproduced from Ref. 299 with permission).

Fig. 4.2 Chromatogram of standard phenolic acids separated as their methyl ester-trimethylsilyl ether derivatives on a 6 ft column (0.004 m i.d.) packed with 10 per cent F-60 on silanized Gas Chrom P (80-100 mesh) using temperature programming from 100°C to 240°C at 2°C min" Peak identifications are 1, 2-hydroxyphenylacetate 2, 3-hydroxyphenylacetate 3, 4-hydroxyphenylacetate 4, indoleacetate 5, homovanil-late 6, homogentisate 7, vanillylmandelate 8, 5-hydroxyindoleacetate 9, nonadecanoate (standard). (Redrawn with modifications from Horning etal, 1966)...

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