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C. Lipid and Protein Metabolism

The structure of lipids determines their function. For example, the very insoluble triacylglycerides are used as the predominant storage form of chemical energy in the body. In contrast to polysaccharides such as glycogen (a polymer of glucose), the Cs in the acyl-chains of the triacylglyceride are in a highly reduced state. The main source of energy to drive not only our bodies but also our society is obtained through oxidizing carbon-based molecules to carbon dioxide and water, in a reaction which is highly exergonic and exothermic. Sugars are already part way down the free energy spectrum since each carbon is partially oxidized. 9 kcal/mol can be derived from the complete oxidation of fats, in contrast to 4.5 kcal/mol from that of proteins or carbohydrates. In addition, glycogen is highly hydrated. For every 1 g of glycogen, 2 grams of water is H-bonded to it. Hence it would take 3 times more weight to store the equivalent amount of energy in carbohydrates as is stored in triacylglyceride, which are stored in anhydrous lipid "drops" within cells . The rest of this unit on lipids will focus not on triacylglycerides, whose main function is energy storage, but on fatty acids and phospholipids, and the structures they form in aqueous solution.

Functional Impact of Protein Modification by Lipid Electrophiles

Do all organisms use the same lipid building blocks to construct bilayers? It turns out they don't. Life can be divided into three separate domains, Bacteria, Archea, and Eukaryota. Studies of sequence similarities of the ribosomal RNA genes from the DNA of these cells show that archea and eukaryota are more closely related than bacteria (also called prokaryotes). Yet there are many similarities between archea and bacteria. Both bacteria and archea are single-celled organisms without nuclei and internal organelles. In the past archea were thought to belong to the prokaryotes. Yet they differ significantly in genetic structure and in their metabolic pathways.

C. Lipid and Protein Metabolism

Figure: Structures of common phospholipids

The diagram below indicates the mechanisms of uptake and utilization of amino acids for protein synthesis, glucose for lactose synthesis, fatty acids and glycerol for milk fat synthesis, immunoglobulins for transport across the cells, and the paracellular pathway.

The simple classification of lipids based on their reactivity towards bases belies the complexity of possible lipid structures as over 1000 different lipids are found in eukaryotic cells. This complexity has led to the development of a comprehensive classification system for lipids. In this system, lipids are given a very detailed as well as all-encompassing definition: "hydrophobic or amphipathic small molecules that may originate entirely or in part by carbanion-based condensations of thioesters (fatty acyl, glycerolipids, glycerophospholipids, sphingolipds, saccharolipds and polyketides) and/or by carbocation-based condensations of isoprene units (prenol lipids and sterol lipids)."

Lipid metabolism is the synthesis and degradation of lipids in cells

Fats(or lipids), like triglycerides, are also metabolized to produce energy.

The generic structures of glycerophospholipid is show below, along with the most common glycerophospholipids. Learn the structures of phosphatic acid (PA), phosphatidyethanolamine (PE), phosphatidylcholine (PC) which is often called lechitin, phosphatidylserine (PS) which is often called cephalin, and sphingomylein (shown in an earlier figure).

Archea often have very unique chemistries. Members of this domain can use not only carbohydrates and fats as sources of energy, but they can also use inorganic species such as ammonium, hydrogen, and metal ions as well as organic molecules such as methane. Some (methanogens) actually make methane. Archea were once thought to be found only in extreme environments (hence they were also called extremophiles), but in actuality they inhabit many environmental niches, including the oceans and soil. Since many do live in extreme environments, you would expect them to have evolved to synthesize stable, structural molecules. Archea use phospholipids in the membrane bilayers, but the lipids differ in three very important ways. Instead of fatty acid chains, they use isoprenoid chains as the nonpolar chains. Instead of using an ester link, the isoprenoids are covalently attached to the glycerol backbone with an ether link, which is obviously more stable than an ester bond used in the phospholipids discussed above. Finally, the stereochemistry of the phospholipids is based on sn-glycerol-1-phosphate and not sn-glycerol-3-phosphate.

i. Lipid Metabolism
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carbs, lipids, proteins, nucleic acids diagrams

Lipids are small biological molecules which are soluble in organic solvents, such as chloroform/methanol, and are sparingly soluble in aqueous solutions. They can be classified in a variety of ways. In one categorization, they can be divided into two majors classes, saponifiable and nonsaponifiable lipids, based on their reactivity with strong bases. Saponifiable lipids contain long chain carboxylic (of fatty) acids, that are linked to an alcoholic functional group through an ester linkage. These fatty acids are released on based catalyzed ester hydrolysis. The nonsaponifiable classes include the "fat-soluble" vitamins (A, E) and cholesterol. Lipids are often distinguished from another commonly used word, fats. Some define fats as lipids that contain fatty acids that are esterified to glycerol. I will use the lipid and fat synonymously.

The structure of lipids determines their function

The biosynthesis of serine and glycine constitute a major metabolic pathway that plays a central role in the formation of other amino acids, nucleic acids and phospholipids. When is grown on glucose, fully 15% of carbon assimilated passes through the serine pathway. Synthesis of serine and glycine starts with oxidation of 3-phosphoglycerate forming 3-phosphohydroxy pyruvate and NADH. A transamination reaction with glutamate forms 3-phosphoserine and removal of the phosphate yields serine. Glycine is generated by removal of the methyl group from serine. Energy is not required for this pathway, in fact it yields energy in the form of reduced NADH.

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