Desaturation hydrocarbon

FIGURE 63.1 Starting with mevalonate, carotenoids are biosynthesized by a special branch of the terpenoid pathway. The first C-40 hydrocarbon unit formed is phytoene, a carotenoid with three conjugated double bonds, which then is enzymatically desaturated to successively yield (3-carotene, neurosporene, and lycopene. Other carotenoids such as (3-carotene and oxocarotenoids are produced from lycopene following cyclization and hydroxylation reactions. Thus, lycopene is a central molecule in the biosynthesis pathway of carotenoids. 

Sex pheromones in the Lepidoptera are multi-component mixtures consisting mostly of olefinic compounds possessing a terminal aldehyde, alcohol, or acetate moiety. Besides functional group differences, the constituents of lepidopteran sex pheromones vary in hydrocarbon chain length and in the specific number, location, and geometry of double bonds. These chemical structures are formed in biosynthetic pathways involving a limited number of enzymatic steps believed to use fatty-acyl thioesters of coenzyme A (acyl-CoA) as substrates. Key reactions are desaturation, limited [3-oxidation, and a small number of terminal functional group modifications (reviewed in Chapter 3). 

The active site similarities listed above belie a remarkable functional diversity, which includes phosphate ester hydrolysis, dioxygen and NO reduction, reversible O2 binding, and O2 activation, the last of which includes enzymes involved in ribonucleotide reduction, hydrocarbon monooxygenation, and fatty acyl desaturation. At the overall protein level, the purple acid phosphatases (PAPs) seem to be completely unrelated, both structurally and functionally, to any of the others in this class. Similarly, the flavo-diiron enzymes form a structurally and probably functionally distinct family of proteins, catalyzing both dioxygen and NO reduction. These last two examples illustrate that attempts to shoehorn all of these enzymes into a single class can sometimes provide a simplistic and misleading view of their chemistry and biochemistry. 

Scheme 1 Typical reactions catalyzed by cytochromes P450 (a) hydroxylation of saturated hydrocarbons (b) heteroatom oxidation and dealkylation (c) olefin epoxidation (d) aromatic hydroxylation (e) desaturation (f) aldehyde deformylation.

Fig. 1. The n-3/n-6 metabolic pathways. Precursors of the n-3 (18 3n-3, linolenic acid) and n-6 (18 2n-6, (/.-linoleic acid) are converted by a series of desaturation and (adding double bonds) and elongation (adding carbon atoms to the hydrocarbon backbone) reactions. Note that the same enzymes catalyze n-3 and n-6 desaturation and elongation reactions. Major metabolites are indicated. PUFAs with 20-carbon backbones (20 4n-6, arachidonic acid, and 20 5n-3, eicosapentaenoic acid) are precursors to the eicosanoids (prostaglandins, leukotrienes, thromboxanes). Docosahexaenoic acid (22 6n-3) is also indicated. Note that only a limited part of the metabolic pathway is shown in this figure.

Cytochrome P450 enzymes catalyze dehydrogenation as well as oxygenation reactions, including the oxidation of saturated to unsaturated hydrocarbons, alcohols to carbonyl compounds, and amines to imines or other unsaturated products. The most extensively investigated of these reactions in terms of mechanism is the desaturation of valproic acid to 2-/i-propyl-4-pentenoic... 

The past decade has shown that hydrocarbon desaturation is not uncommon but, except in cases such as the biosynthesis of ergosterol, it generally accounts for a minor proportion of the metabolic products. The earliest reported example of P450-mediated hydrocarbon desaturation appears to be the conversion of lindane (1,2,3,4,5,6-hexachloro-cyclohexane) to 1,2,3,4,5,6-hexachlorocyclohex-ene, but the known hydrocarbon desaturation reactions now include the A -desaturation of androstenedione and deoxycorticosterone by adrenal mitochondria, the oxidation of dihydronaphthalene to naphthalene and 7,8-dihydrobenzo[a] pyrene to benzo[a]pyrene the conversion of warfarin to dehydrowarfarin ", the desaturation of lovostatin and simvastatin to the 6-exo-methylene... 

In MeCN at —40°C, low-spin iron(III)iron(IV) compounds [Fe2(/i-0)2(TPA)2] + (39b) and (40b) quantitatively oxidize 2,4-di-tert-butylphenol to the corresponding phenoxyl radical, which dimerizes to form the 2,2 -biphenol product. Hydroxylation or desaturation of hydrocarbons can also be achieved by these high-valent species, which act as one-electron oxidants. A KIE of 20 was obtained for the second-order reaction between (39b) and ethylbenzene. Activation of the benzylic C—bond is a major component of the rate-determining step in both hydroxylation and desaturation, as demonstrated by product analysis. A mechanistic model was proposed... 

The degree of unsaturation in the molecule, i.e. the length of the conjugated polyene chromophore, has a substantial effect on retention times. Thus the acyclic hydrocarbons of the biosynthetic desaturation series, namely phytoene, phytofluene, tetrahydrolycopene, neurosporene and lycopene are well resolved, as are compounds having the same substituent group but different levels of desaturation, e.g. the 1-hydroxy-derivatives of these hydrocarbons. The 1,2-dihydrocarotenes, found in Rps. viridis are eluted later than their parent hydrocarbons e.g. 1,2-dihydroneurosporene after neurosporene. 

ABA-induced chilling resistance may be due to an increase in the mobility of the membrane hydrocarbon acyl chains [18]. In intact seedlings and isolated cotyledonary discs of cotton, ABA reduced chilling injury by preventing a decrease in the content of reduced glutathione and this was closely correlated with membrane stabilization [28]. At an injurious chilling temperature desaturation of leaf polar lipids fatty acids in cucumber leaves was reduced. These observations support the suggestion that ABA stabilizes membranes [35]. 

Fatty acids (FAs) have a -COOH group on the end of their hydrocarbon chain which ionizes at physiological pH to give them a hydrophihc region as well as a hydrophobic region (they are amphipathic). Long-chain FAs are predominantly hydrophobic and thus require a carrier in circulation - albumin. Saturated FAs have no double bonds. Double bonds can be inserted by desaturating enzymes at positions up to nine carbon atoms from the methyl end, in which case they become unsaturated. Monounsaturated implies just one double bond polyunsaturated implies two, three or four double bonds. 

The desaturation of positions 1 and 2 of the alkyl chain to form an alkenyl chain is catalysed by cell free extracts of intestinal epithelial cells, tumour cells and brain. The enzyme is present in the microsomal fraction but the reaction is stimulated by a high molecular weight, heat-labile factor in the soluble cytosol. The fact that a reduced pyridine nucleotide and molecular oxygen are absolute requirements and that the reaction is inhibited by cyanide but not by CO, strongly suggest that this enzyme is very similar to the fatty acyl-CoA desaturase described in section 3.2.4. This provides another interesting example of an enzyme catalysing a modification of a hydrocarbon chain in the intact lipid molecule. 

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