Publisher Summary This chapter focuses on the structure, mechanism of action, and regulation of the enzymes responsible for the biosynthesis of saturated fatty acids de novo in eukaryotes. Fatty acids fulfill a number of crucial roles in animals. They represent a major storage form of energy, they constitute essential structural components of membranes, they are used to modify and regulate the properties of many proteins through direct covalent linkage, and they perform important roles as signaling molecules in metabolic regulation. Cytosolic fatty acid synthase (FAS) is responsible for the bulk synthesis of fatty acids and in animals is most active in the “lipogenic tissues” such as liver, adipose, and lactating mammary glands. The exact role of the mitochondrial FAS is unclear, but it may provide the octanoyl precursor required for the de novo biosynthesis of lipoyl moieties that are utilized for the covalent modification of several mitochondrial enzymes, and/or fatty acids required for the remodeling of mitochondrial lipids. In the mitochondrial system, the enzymes exist as separate, freestanding proteins (type II) whereas in the cytosolic system, the enzymes are covalently linked in large multifunctional polypeptides (type I). The mitochondrial proteins closely resemble their prokaryotic type II counterparts, consistent with the hypothesis that mitochondria originated from free-living bacteria, but are nuclear encoded and possess N-terminal targeting sequences that direct them into the mitochondrial compartment.
This chapter provides an overview of lipoproteins, describes lipoprotein triglyceride and lipolysis, high-density lipoprotein (HDL) and plasma cholesterol metabolism, and reactions linking the metabolism of apo A1 and apo B lipoproteins. The apolipoprotein content of lipoprotein particles alters during recirculation, as changes in the lipid composition of the particles modify the affinity of apolipoproteins for their surface. Lipoprotein lipase (LPL), synthesized in adipocytes and myocytes, is transported out of the parenchymal cells, through the pericyte layer, and across the endothelium, before binding to functional sites on the vascular endothelial surface. The efflux of cholesterol from cells to plasma lipoproteins, previously considered the result of passive diffusion, is now recognized to be highly regulated by cell membrane proteins and on a par with influx as a key determinant of cellular cholesterol homeostasis. The availability of mice overexpressing or deficient in almost all known factors of plasma lipid transport has provided key insights into regulatory pathways.
Publisher Summary This chapter provides an overview of eukaryotic phospholipid biosynthesis. Phospholipids make up the essential milieu of cellular membranes and act as a barrier for entry of compounds into cells. Phospholipids also function as precursors of second messengers such as diacylglycerol (DG) and inositol-1,4,5-P3. A third, and usually overlooked, function of phospholipids is the storage of energy in the form of fatty acyl components. This function is probably quantitatively important only under extreme conditions such as starvation. Phosphatidic acid (PA) is an intermediate that occurs at a branchpoint in glycerolipid biosynthesis. An important step in PA biosynthesis is the activation of fatty acids by acyl-CoA synthetases to yield acyl-CoA. Five different forms of rat acyl-CoA synthetase have been identified, each encoded by a separate gene. Distinct forms of the enzyme have been found on the endoplasmic reticulum (ER), mitochondria, and mitochondria-associated membranes (MAM), a sub-fraction of the ER. Hence, these synthetases might provide distinct pools of acyl-CoA substrates for the biosynthesis of phospholipids and triacyglcyerols (TGs).
Publisher Summary Cholesterol's structure, biosynthetic pathway, and metabolic regulation have tested the ingenuity of chemists, biochemists, and cell biologists. The cholesterol biosynthetic pathway was understood to be a complex pathway of over 40 cytosolic and membrane-bound enzymes, at least two of which are subject to feedback regulation by the end product, cholesterol, and oxygenated forms of cholesterol (called oxysterols). Genes encoding the key enzymes were cloned, which revealed the transcriptional and post-translational control of these enzymes. Subsequently, the mechanisms of regulation were elucidated on a molecular level, although it was still not clear how cholesterol elicits all of the regulation. Factors that regulate cholesterol synthesis have been found to also control the synthetic pathways for fatty acids, triacylglycerols, and NADPH. Furthermore, the evidence is rapidly building that cholesterol's precursors and metabolites might serve as biologically active signaling molecules. Cholesterol is synthesized from acetyl-CoA via the isoprenoid pathway. Cellular cholesterol levels regulate at least 14 enzymes in the biosynthetic pathway.
This chapter provides an overview of the lipoprotein receptors, biochemical and physiological properties of the low-density lipoprotein (LDL) receptor and discusses the molecular basis for the genetic disease, familial hypercholesterolemia, removal of triacylglycerol-rich lipoproteins from the plasma, multifunctional receptors in the chicken, very-low-density lipoprotein (VLDL) receptor and apolipoprotein E (apoE) receptor type 2 (apoER2) as signal transducers, other relatives of the LDL receptor family, and scavenger receptors (SRs). The functional redundancy of LDL receptor relatives can be because of their simultaneous expression on the same cells in a given organ. SRs are a widely expressed and highly diverse group of proteins that recognize a broad array of ligands; at least some of the intriguing group of receptors may play roles in the metabolism of modified lipoproteins, and thus, may well be related to lipid anomaly disorders. Specific functions of the gene family must be defined at two levels: the cellular level, to delineate molecular events and in physiological studies, including state-of-the-art genetic manipulations, which should reveal the functional relevance of receptor redundancy.
Publisher Summary This chapter focuses on the biochemistry of lipid metabolism in the adipocyte. Adipocytes make up approximately one-half of the cells in adipose tissue, the remainder being blood and endothelial cells, adipose precursor cells of varying degrees of differentiation, macrophages, and fibroblasts. In humans, small clusters of adipocytes are present that increase in size during gestation. Larger clusters of fat cells are associated with tissue vascularization and a general increase in cluster size is positively correlated with larger blood vessels. Paracrine/autocrine factors play a significant role in both capillary growth and adipose conversion. Recent advances have demonstrated that the adipocyte is not a passive lipid storage depot but a dynamic cell that plays a fundamental role in energy balance and overall body homeostasis. Moreover, the fat cell functions as a sensor of lipid levels, transmitting information to a neural circuit affecting major biological processes including hunger, sleep, and reproduction.
Publisher Summary Plants produce the majority of the world's lipids, and most animals, including humans, depend on these lipids as a major source of calories and essential fatty acids. Like other eukaryotes, plants require lipids for membrane biogenesis, as signal molecules, and as a form of stored carbon and energy. In addition, leaves and other aerial surfaces, bark, herbaceous shoots, and roots each have distinctive protective lipids that help prevent desiccation and infection. The presence of chloroplasts and related organelles in plants has a profound effect on both gross lipid composition and the flow of lipid within the cell. Fatty acid synthesis occurs not in the cytosol as in animals and fungi, but in the chloroplast and other plastids. On a whole organism basis, plants store more carbon as carbohydrate than as lipid. Since plants are not mobile, and since photosynthesis provides fixed carbon on a regular basis, plant requirements for lipid storage as an efficient, light weight energy reserve are less acute than those of animals. Plant lipids have a substantial impact on the world economy and human nutrition. More than three-quarters of the edible and industrial oils marketed annually are derived from seed and fruit triacylglycerols.
Publisher Summary The term “eicosanoids” is used to denote a group of oxygenated, 20-carbon fatty acids. The major precursor of these compounds is arachidonic acid, and the pathways leading to the eicosanoids are known collectively as the “arachidonate cascade.” The major eicosanoids are products of ω-6 essential fatty acids, and the essentiality of this group of fatty acids relates primarily to their functions as eicosanoids. Because eicosanoid overproduction is associated with a number of pathologies, potent enzyme inhibitors and receptor antagonists have been developed that are widely used therapeutically. Both the prostanoids and the leukotrienes (LTs) were discovered as vasoactive substances active on reproductive and pulmonary smooth muscle, respectively. The LTs were known initially as slow-reacting substances of anaphylaxis. The conversion of LTs into alternative structural entities is an important feature of the inactivation of these potent biologically active eicosanoids. The metabolism of LTs is rapid and the exact pathway depends upon whether the substrate is LTB4 or LTC4. LTB4 is rapidly metabolized through both oxidative and reductive pathways.
Atherosclerotic vascular disease is the cause of heart attacks, stroke, aortic aneurysms, and peripheral vascular disease, which together represent the most frequent causes of death in the industrialized world. Indeed, the aging of the population and the "westernization" of world diet are predicted to increase the impact of atherosclerosis worldwide despite continuing advances in plasma lipid-lowering therapy. Atherosclerosis progresses in a series of stages, although some lesions at each stage may not progress further or may even regress if inciting events such as hypercholesterolemia, diabetes, smoking, or hypertension are controlled. The initial stage involves the accumulation of subendothelial lipoproteins in focal areas of the arterial tree, usually at branch points with disturbed laminar flow. In response to this retention, a series of biological responses ensue, including lipoprotein oxidation, endothelial alterations, inflammatory responses including T cell recruitment, cytokine secretion, monocyte chemotaxis, subendothelial macrophage accumulation, and intracellular cholesterol accumulation in macrophages. Much of the cholesterol is stored as cholesteryl fatty acid esters (CE) in cytoplasmic lipid droplets surrounded by a monolayer of phospholipid. These cytoplasmic droplets give the macrophages a foamy appearance when viewed by microscopy, and thus these cells are referred to as "foam cells." The presence of macrophage foam cells defines the earliest pathological lesion, referred to as the "fatty streak."
A fundamental problem of cell biology and biochemistry is the elucidation of the mechanisms by which the specific components of subcellular membranes are assembled into mature organelles. The major components of all cell membranes are lipids and proteins. The presence of discrete structural motifs contained in the primary sequence of proteins directs a large number of post-translational processes that enable their sorting among different membrane compartments. The sorting process for proteins is essentially absolute such that plasma membrane proteins are never found in the mitochondria or vice versa. In contrast, lipid molecules do not contain discrete structural subdomains that exclusively direct their movement to specific membranes. The distribution of lipids among different organelles is heterogeneous, but is not usually absolute. These observations indicate that specialized sorting and transport machinery must exist for lipid assembly into different membranes, but the mechanisms governing these processes are only now beginning to be understood at the molecular level. A multiplicity of individual lipids can contribute to membrane formation. The biological role of this lipid heterogeneity is not completely understood and the list of significant actions continues to grow. Some of the diversity contributes to membrane fluidity.
Lipid modifications of proteins are widespread and functionally important in eukaryotic cells. This chapter discusses the structure of posttranslational lipid modifications, describes their biosynthesis, and provides a survey of their functional significance. Although the functional diversity of lipid-modified proteins makes it difficult to arrive at generalizations about the evolutionary impetus for lipid anchoring compared to the use of conventional protein transmembrane domains, the observation that many lipid-modified proteins are associated with cholesterol and sphingolipid-rich membrane domains is suggestive. Analyses of the biophysical characteristics of the membrane association of lipid-modified proteins, the role of raft-associated proteins in cell signaling, and the regulation of protein function by lipid modification are likely to remain fruitful areas of investigation. Despite the identification of numerous genes and gene products associated with the glycosylphosphatidylinositol (GPI) biosynthetic pathway, the enzymology of GPI biosynthesis, including analyses of enzyme structure, basis for enzyme localization to the endoplasmic reticulum (ER) and ER domains, and transbilayer distribution of the pathway remains open to new investigation.
Bacteria are a model system to study membrane lipid biosynthesis and metabolism. The chapter reviews the major events in bacterial membrane lipid synthesis. The most energy intensive part of membrane lipid synthesis is the dissociated type II fatty acid synthase. The initiation module consists of the enzymes required to initiate type II fatty acid biosynthesis and is the focus for the regulatory biochemistry that governs the total amount of cellular lipid. The elongation module is a conserved set of proteins that iteratively elongate the growing acyl chain by 2-carbon units at a time. The end products of the elongation module are used by the acyltransfer module to generate phosphatidic acid, the universal intermediate in phospholipid synthesis. The phospholipid module of enzyme then diversifies the membrane phospholipid composition. Biochemical regulation of each of the four enzyme modules function together to maintain membrane lipid homeostasis.
Fatty acids (FA) synthesised de novo and those obtained from the diet are modified through desaturation and elongation on the endoplasmic reticulum membrane. These reactions produce a variety of long-chain and very long-chain saturated, monounsaturated, and polyunsaturated fatty acids that have a variety of fates and functions. FA serve as substrates in the synthesis of complex lipid classes, such as triacylglycerols and phospholipids, through which these FA facilitate energy storage and membrane fluidity, respectively. Nonesterified FA also exert unique signalling properties. Here, we describe the function and regulation of mammalian fatty acid desaturases (stearoyl-CoA desaturase, Δ5 desaturase, and Δ6 desaturase) and elongases (elongase 1–7). Additionally, we illustrate the diverse physiological roles of specific MUFA and PUFA and their relation to biochemical processes and disease.
1. Physical Properties and Functional Roles of Lipids in Membranes (Pieter R. Cullis and Michael J. Hope). 2. Lipid Metabolism in Procaryotes (Suzanne Jackowski, John E. Cronan, Jr. and Charles O. Rock). 3. Oxidation of Fatty Acids (Horst Schulz) 4. Fatty Acid Synthesis in Eucaryotes (Alan G. Goodridge). 5. Fatty Acid Desaturation and Chain Elongation in Eucaryotes (Harold W. Cook). 6. Metabolism of Triacylglycerols (David N. Brindley) 7. Phospholipid Metabolism and Cell Signaling in Eucaryotes (Dennis E. Vance) 8. Metabolism, Regulation and Function of Ether-linked Glycerolipids and their Bioactive Species (Fred Snyder). 9. Phospholipases (Moseley Waite). 10. The Eicosanoids: Cyclooxygenase, Lipoxygenase and Epoxygenase Pathways (William L. Smith, Pierre Borgeat and Frank A. Fitzpatrick). 11. Sphingolipids (Charles C. Sweeley). 12. Cholesterol: Evolution of Structure and Function (Konrad Bloch). 13. Regulation of Sterol Biosynthesis and Isoprenylation of Proteins (Peter A. Edwards). 14. Lipoprotein Structure and Secretion (Roger Davis). 15. Dynamics of Lipoprotein Transport in the Circulatory System (Christopher J. Fielding and Phoebe E. Fielding). 16. Removal of Lipoproteins from Plasma (Wolfgang J. Schneider). 17. Lipid Assembly in Cell Membranes (Dennis R. Voelker). 18. Assembly of Proteins into Membranes (Reinhart A.F. Reithmeier).
This chapter provides a general overview of bile acid biochemistry and reviews recent discoveries that have advanced the understanding of bile acid metabolism and function in mammals. Bile acids make up a group of sterol-derived compounds that act as detergents to facilitate the digestion and absorption of fats and fat-soluble molecules in the intestine, and to keep cholesterol from precipitating in bile. In mammalian species, the cholesterol side chain is trimmed to yield C24-sterol derivatives. In other vertebrate species, the hydroxylation of the side chain does not lead to its removal and the products of the biosynthetic pathway are referred to as bile alcohols. Invertebrate species do not synthesize sterol bile acids. Over the last few years, much information has been gained about the function of bile acids and the mechanisms that regulate their synthesis. Bile acids are the major solutes in bile. Typical mammalian bile consists of about 82% water, 12% bile acids, 4% phospholipids (mostly phosphatidylcholines), 1% unesterified cholesterol, 0.3% bilirubin, and the remainder as assorted solutes (including proteins).
Fatty acids are major sources of energy in animals. Fatty acids are transported between organs either as unesterified fatty acids complexed to serum albumin or in the form of triacylglycerols associated with lipoproteins. Triacylglycerols are hydrolyzed outside cells by lipoprotein lipase to yield free fatty acids. The mechanism by which fatty acids enter cells remains poorly understood despite a number of studies performed with isolated cells from various tissues. Kinetic evidence has been obtained for both a saturable and a non-saturable uptake of fatty acids. The saturable uptake predominates at nanomolar concentrations of fatty acids and is thought to be mediated, or assisted, by proteins. In contrast, the non-saturable uptake that is effective at higher concentrations of fatty acids has been attributed to passive diffusion of fatty acids across the membrane. Several suspected fatty acid transport proteins have been identified. Although their specific functions in fatty acid uptake remain to be elucidated, these proteins may assist in the desorption of fatty acids from albumin and function in uptake coupled to the esterification of fatty acids with CoA, in a process referred to as vectorial acylation. Once long-chain fatty acids have crossed the plasma membrane, they either diffuse or are transported to mitochondria, peroxisomes, and the endoplasmic reticulum where they are activated by conversion to their CoA thioesters.
Inositol 1,4,5-trisphosphate receptor (IP3R) is a Ca2+ release channel localized on the endoplasmic reticulum (ER) and plays an important role in various cell functions. IP3R was discovered as a developmentally regulated glyco-phosphoprotein missing in cerebellar mutant mice. Recent studies using loss of function analysis indicate that IP3R is involved in fertilization, early development, neuronal plasticity, and other cell functions. IP3R works like a scaffold protein associating with various molecules that may regulate the function of IP3R. IP3 works not only to release Ca2+ through the channel pore of IP3R but also to release IP3R-binding protein released with IP3 (IRBIT) from the IP3-binding core. IRBIT binds to and activates pancreas-type Na+/HCO3(-) cotransporter 1 that is important for regulating acid-base balance. Electron microscopic (EM) studies show the IP3R has allosteric property to change reversibly its form from square to windmill in the presence of Ca2+. Cryo-EM analysis of IP3R shows a balloon-like structure with holes on the surface and a large cavity inside, this structure is convenient for IP3R to associate various molecules to be regulated. It is also found that ER carrying IP3R moves along micro-tubules in addition to reticular ER. All these data suggests that the IP3R/Ca2+ channel works as a "signaling center'' inside cells by associating with many molecules like a scaffolding protein presumably forming a "calcio-signalosome.''
