MCAT Biochemistry Review

Chapter 5: Lipid Structure and Function

Conclusion

In this chapter, we examined the myriad biological functions performed by lipids. We first learned the structural functions of lipids, looking at the phospholipids that are the primary component of the phospholipid bilayer and other membrane lipids. Making our way through terpenes, we looked at the structure and function of signaling lipids, examining steroid hormones in particular. We looked into the fat-soluble vitamins and their downstream functions in the body. Finally, we summarized energy storage in the form of triacylglycerols and applied our acid–base chemistry knowledge to the formation of soap. In the next chapter, we turn our attention to the final class of biomolecules: nucleic acids.

Concept Summary

Structural Lipids

·        Lipids are insoluble in water and soluble in nonpolar organic solvents.

·        Phospholipids are amphipathic and form the bilayer of biological membranes.

o   They contain a hydrophilic (polar) head group and hydrophobic (nonpolar) tails.

o   The head group is attached by a phosphodiester linkage and, because it interacts with the environment, determines the function of the phospholipid.

o   The saturation of the fatty acid tails determines the fluidity of the membrane; saturated fatty acids are less fluid than unsaturated ones. Fatty acids form most of the structural thickness of the phospholipid bilayer.

o   Glycerophospholipids are phospholipids that contain a glycerol backbone.

·        Sphingolipids contain a sphingosine backbone.

o   Many (but not all) sphingolipids are also phospholipids, containing a phosphodiester bond; these are termed sphingophospholipids.

o   Sphingomyelins are the major class of sphingophospholipids and contain a phosphatidylcholine or phosphatidylethanolamine head group. They are a major component of the myelin sheath.

o   Glycosphingolipids are attached to sugar moieties instead of a phosphate group. Cerebrosides have one sugar connected to sphingosine; globosides have two or more.

o   Gangliosides contain oligosaccharides with at least one terminal N-acetylneuraminic acid (NANA; also called sialic acid).

·        Waxes contain long-chain fatty acids esterified to long-chain alcohols. They are used as protection against evaporation and parasites in plants and animals.

Signaling Lipids

·        Terpenes are odiferous steroid precursors made from isoprene, a five-carbon molecule.

o   One terpene unit (a monoterpene) contains two isoprene units.

o   Terpenoids are derived from terpenes via oxygenation or backbone rearrangement. They have similar odorous characteristics.

·        Steroids contain three cyclohexane rings and one cyclopentane ring. Their oxidation state and functional groups may vary.

o   Steroid hormones have high-affinity receptors, work at low concentrations, and affect gene expression and metabolism.

o   Cholesterol is a steroid important to membrane fluidity and stability; it serves as a precursor to a host of other molecules.

·        Prostaglandins are autocrine and paracrine hormones that regulate cAMP levels. They have powerful effects on muscle contraction, body temperature, the sleep–wake cycle, and pain.

·        The fat-soluble vitamins include vitamins A, D, E, and K.

o   Vitamin A (carotene) is metabolized to retinal for vision and retinoic acid for gene expression in epithelial development.

o   Vitamin D (cholecalciferol) is metabolized to calcitriol in the kidneys and regulates calcium and phosphorus homeostasis in the intestines (increasing calcium and phosphate absorption), promoting bone formation. A deficiency of vitamin D causes rickets.

o   Vitamin E (tocopherols) act as biological antioxidants. Their aromatic rings destroy free radicals, preventing oxidative damage.

o   Vitamin K (phylloquinone and menaquinones) is important for formation of prothrombin, a clotting factor. It performs posttranslational modifications on a number of proteins, creating calcium-binding sites.

Energy Storage

·        Triacylglycerols (triglycerides) are the preferred method of storing energy for long-term use.

o   They contain one glycerol attached to three fatty acids by ester bonds. The fatty acids usually vary within the same triacylglycerol.

o   The carbon atoms in lipids are more reduced than carbohydrates, giving twice as much energy per gram during oxidation.

o   Triacylglycerols are very hydrophobic, so they are not hydrated by body water and do not carry additional water weight.

·        Animal cells specifically used for storage of large triacylglycerol deposits are called adipocytes.

·        Free fatty acids are unesterified fatty acids that travel in the bloodstream. Salts of free fatty acids are soaps, and can be synthesized in saponification.

o   Saponification is the ester hydrolysis of triacylglycerols using a strong base, like sodium or potassium hydroxide.

o   Soaps act as surfactants, forming micelles. A micelle can dissolve a lipid-soluble molecule in its fatty acid core, and washes away with water because of its shell of carboxylate head groups.

Answers to Concept Checks

·        5.1

1.    Membrane lipids are amphipathic: they have hydrophilic heads and hydrophobic tails, allowing for the formation of bilayers in aqueous solution. The fatty acid tails form the bulk of the phospholipid bilayer, and play a predominantly structural role. On the other hand, the functional differences between membrane lipids are determined by the polar head group, due to its constant exposure to the exterior environment of the phospholipid bilayer (remember, this can be either the inside or outside of the cell). The degree of unsaturation of fatty acid tails can also play a small role in function.

2.    The difference is the bond between the sphingosine backbone and the head group. When this is a phosphodiester bond, it's a phospholipid (note the phospho– prefixes). Nonphospholipid sphingolipids include glycolipids, which contain a glycosidic linkage to a sugar.

3.     

Type

Phospholipid or Glycolipid?

Functional Group(s)

Sphingomyelin

Phospholipid

Phosphatidylethanolamine/phosphatidylcholine

Glycosphingolipid

Glycolipid

Sugars (mono- or polysaccharide)

Ganglioside

Glycolipid

Oligosaccharides and N-acetylneuraminic acid (NANA)

4.    In a nonpolar solvent, we would see the opposite of what happens in a polar solvent like water: The hydrophilic, polar part of the molecules would be sequestered inside, while the nonpolar, hydrophobic part of the molecules would be found on the exterior and exposed to the solvent.

·        5.2

1.    A diterpene has 20 carbon molecules in its backbone. One terpene unit is made from two isoprene units, each of which has five carbons.

2.    A steroid is defined by its structure: it includes three cyclohexane rings and a cyclopentane ring. A steroid hormone is a molecule within this class that also functions as a hormone, meaning that it travels in the bloodstream, is active at low concentrations, has high-affinity receptors, and affects gene expression and metabolism.

3.    Prostaglandins regulate the synthesis of cAMP, which is involved in many pathways, including those that drive pain and inflammation.

4.     

Name

Function

A (carotene)

As retinal: vision; as retinoic acid: epithelial development

D (cholecalciferol)

As calcitriol: calcium and phosphate regulation

E (tocopherols)

Antioxidants, using aromatic ring

K (phylloquinone and menaquinones)

Posttranslational modification of prothrombin, addition of calcium-binding sites on many proteins

·        5.3

1.    The human body stores energy as glycogen and triacylglycerols. Triacylglycerols are preferred because their carbons are more reduced, resulting in a larger amount of energy yield per unit weight. In addition, due to their hydrophobic nature, triacylglycerols do not need to carry extra weight from hydration.

2.    Triacylglycerols, also called triglycerides, are composed of a glycerol backbone esterified to three fatty acids. They are used for energy storage.

3.    The ester bonds of triacylglycerols are broken to form a glycerol molecule and the salts of fatty acids (soap).

4.    Soap appears to dissolve in water because amphipathic free fatty acid salts form micelles, with hydrophobic fatty acid tails toward the center and carboxylate groups facing outward toward the water. Fat-soluble particles can then dissolve inside micelles in the soap–water solution and wash away. Water-soluble compounds can freely dissolve in the water.

Shared Concepts

·        Biochemistry Chapter 2

o   Enzymes

·        Biochemistry Chapter 8

o   Biological Membranes

·        Biochemistry Chapter 11

o   Lipid and Amino Acid Metabolism

·        Biology Chapter 5

o   The Endocrine System

·        Biology Chapter 9

o   The Digestive System

·        Organic Chemistry Chapter 8

o   Carboxylic Acids