Fatty-acid chain

Fatty acids are susceptible to oxidative attack and cleavage of the fatty acid chain. As oxidation proceeds, the shorter-chain fatty acids break off and produce progressively higher levels of malodorous material. This condition is known as rancidity. Another source of rancidity in fatty foods is the enzymatic hydrolysis of the fatty acid from the glycerol. The effect of this reaction on nutritional aspects of foods is poorly understood andhttie research has been done in the area. 

Fatty acids derived from animal and vegetable sources generally contain an even number of carbon atoms siace they are biochemically derived by condensation of two carbon units through acetyl or malonyl coenzyme A. However, odd-numbered and branched fatty acid chains are observed ia small concentrations ia natural triglycerides, particularly mminant animal fats through propionyl and methylmalonyl coenzyme respectively. The glycerol backbone is derived by biospeciftc reduction of dihydroxyacetone. 

Proton chemical shift data from nuclear magnetic resonance has historically not been very informative because the methylene groups in the hydrocarbon chain are not easily differentiated. However, this can be turned to advantage if a polar group is present on the side chain causing the shift of adjacent hydrogens downfteld. High resolution C-nmr has been able to determine position and stereochemistry of double bonds in the fatty acid chain (62). Broad band nmr has also been shown useful for determination of soHd fat content. 

Hydroxyl tion. Commercial lecithin can be hydroxylated at the unsaturated fatty acid chains by treatment with concentrated hydrogen peroxide and acids like lactic or acetic acid. 

Fig. 3. The stmcture of the nodulation (Nod) factors of i bium meliloti 2011 (44), where is 2 or 3, R is —H or—COCH, and R is C 2 as shown, C gT, or ie, a fatty acid chain having from 1 to 3 double bonds. The A/-acetyl glucosamine residues and an acyl moiety, R, are present ia all...

Free rotation around each of the carbon-carbon bonds makes saturated fatty acids extremely flexible molecules. Owing to steric constraints, however, the fully extended conformation (Figure 8.1) is the most stable for saturated fatty acids. Nonetheless, the degree of stabilization is slight, and (as will be seen) saturated fatty acid chains adopt a variety of conformations. 

The double bonds found in fatty acids are nearly always in the cis configuration. As shown in Figure 8.1, this causes a bend or kink in the fatty acid chain. This bend has very important consequences for the structure of biological membranes. Saturated fatty acid chains can pack closely together to form ordered, rigid arrays under certain conditions, but unsaturated fatty acids prevent such close packing and produce flexible, fluid aggregates. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Fatty acid chains are constructed by the addition of two-carbon units derived from acctyl-CoA. 

The individual steps in the elongation of the fatty acid chain are quite similar in bacteria, fungi, plants, and animals. The ease of purification of the separate enzymes from bacteria and plants made it possible in the beginning to sort out each step in the pathway, and then by extension to see the pattern of biosynthesis in animals. The reactions are summarized in Figure 25.7. The elongation reactions begin with the formation of acetyl-ACP and malonyl-ACP, which... 

Both prokaryotes and eukaryotes are capable of introducing a single cis double bond in a newly synthesized fatty acid. Bacteria such as E. coli carry out this process in an Og-independent pathway, whereas eukaryotes have adopted an Og-dependent pathway. There is a fundamental chemical difference between the two. The Og-dependent reaction can occur anywhere in the fatty acid chain. 

FIGURE 25.13 Double bonds are introduced into the growing fatty acid chain in E. coli by specific dehydrases. Palmitoleoyl-ACP is synthesized by a sequence of reactions involving four rounds of chain elongation, followed by double bond insertion by /3-hydroxydecanoyl thioester dehydrase and three additional elongation steps. Another elongation cycle produces cA-vaccenic acid. 

The four steps of the /3-oxidation pathway, resulting in the cleavage of an acetyl group from the end of the fatty-acid chain. The key chain-shortening step is a retro-Claisen reaction of a /3-keto thioester. Individual steps are explained in the text. 

Step 5 of Figure 29.5 Condensation The key carbon-carbon bond-forming reaction that builds the fatty-acid chain occurs in step 5. This step is simply a Claisen condensation between acetyl synthase as the electrophilic acceptor and malonyl ACP as the nucleophilic donor. The mechanism of the condensation is thought to involve decarboxylation of malonyl ACP to give an enolate ion, followed by immediate addition of the enolate ion to the carbonyl group of acetyl... 

Problem 29.5 Evidence for the role of acetate in fatty-acid biosynthesis comes from isotope-labeling experiments. If acetate labeled with 13C in the methyl group ( CFtyCC H) were incorporated into fatty acids, at what positions in the fatty-acid chain would you expect the, 3C label to appear ... 

LPA, i.e. monoacyl-glycerol-3-phosphate, can be formed and degraded by multiple metabolic pathways (Fig. 1). Depending on the precursor molecule and respective pathway, the fatty acid chain in LPA differs in length, degree of saturation and position (sn-1 or sn-2), which has an influence on biological activity. LPA... 

Olestra s manufacturing process creates many different molecules, some with fewer than eight fatty acids, and with many different fatty acid chains other than those pictured above. 

For long-chain alcohol esters it is interesting to see that the interfacial tension between a 0.01 wt % aqueous solution and octane or xylene has a minimum for ester sulfonates with a total 22 carbon atoms in the fatty acid chain and the ester chain [60]. The balance in length between the two chains has only a poor effect. Thus, a-sulfonated fatty acid esters with a total number of 22-26 carbon atoms in the molecule have excellent interfacial activities. To attain the same magnitude in the interfacial tension between linear alkylbenzenesulfonate (LAS) solution and octane, the required concentration of LAS is 0.1 wt %. This is 10 times the concentration needed for a-sulfonated fatty acid esters [60]. 

Special mention must be made of the control of the regioselectivity of the ring opening of AT-acylaziridines 38 at an organic-aqueous interface (Scheme 26) [36]. The fatty acid chains and the phenoxy substituent will orient the substrate such that the unsubstituted aziridine carbon atom points to the aqueous layer... 

Figure 21-S. Microsomal elongase system for fatty acid chain elongation. NADH is also used by the reductases, but NADPH is preferred.

Scheme 2.1 The key reactions that occur during lipid peroxidation, in this scheme, X represents the initiating species, which must be a highiy reactive oxidant, in order to abstract a H atom from a poiyunsaturated fatty-acid chain LH, the iipid substrate LO2, the peroxyi radicai L, the alkyl radical LOOH, the lipid hydroperoxide.

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