Complex lipids, synthesis proteins

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

Oxidative phosphorylation is central to the metabolism of all higher organisms, because the free energy of hydrolysis of the ATP so generated is used in the synthesis of, inter alia, nucleic acids (Chaps. 7 and 16), proteins (Chaps. 4,9, and 17), and complex lipids (Chap. 6), as well as in processes as diverse as muscle contraction (Chap. 5) and the transmission of nerve impulses. 

The TCA cycle is the major final common pathway of oxidation of carbohydrates, lipids, and proteins, since their oxidation yields acetyl-CoA. Acetyl-CoA also serves as precursor in the synthesis of fatty acids, cholesterol, and ketone bodies. All enzymes of the cycle are located in the mitochondrial matrix except for succinate dehydrogenase, which is embedded in the inner mitochondrial membrane. Thus, the reducing equivalents generated in the cycle have easy access to the electron transport chain. TCA cycle enzymes, with the exception of a-ketoglutarate dehydrogenase complex and succinate dehydrogenase, are also present outside the mitochondria. The overall TCA cycle is shown in Figure 13-12. 

Acetyl-CoA carboxylase is recognised generally as being a key enzyme in acyl lipid formation. In plants it has been shown to be important as a regulatory enzyme in light-driven lipid synthesis [1] and has a high flux control coefficient under such conditions [2]. Recently, characterisation of different isoforms of acetyl-CoA carboxylase from plants has been made. In Poaceae such as maize there are two multifunctional proteins [3]. By contrast, the dicotyledon pea contains a multienzyme complex form of acetyl-CoA carboxylase in the mesophyll chloroplasts but a multifunctional protein Isoform In the cytosol of epidermal cells [4]. 

In recent years it has become apparent that dicotyledonous plant species contain two distinctive forms of the enzyme acetyl CoA carboxylase, type I a single polypeptide of >220 kDa, and, type II a complex of smaller proteins BC, BCCP and CTa and p. To fully achieve the potential for the genetic manipulation of lipid products from crops a detailed understanding of fatty acid synthesis is required. This is dependent upon a full characterisation of the proteins involved, and more specifically on the pivotal enzyme ACCase. 

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