Chapter 16 Biomolecules: Lipids and Nucleic Acids Suggested Problems: 21

About Lipids Lipids - natural materials that preferentially extract into nonpolar organic solvents Includes fats, oils, waxes, some vitamins and hormones, some of the components of cell membranes General types: esters (“saponifiable”) and those that can’t be hydrolyzed

16.1 Waxes, Fats, and Oils Waxes - contain esters formed from long-chain (C16-C36) carboxylic acids and long-chain alcohols (C24-C36) Triacontyl hexadecanoate is in beeswax

Triacylglycerol Triester of glycerol with three long-chain carboxylic acids (called fatty acids). Fatty acids that occur in nature typically have an even # of carbons (12-20) and if double bonds are present they exist in the cis orientation.

Fatty Acids (from Fats and Oils) Straight-chain (C12 - C20) carboxylic acids Double bonds are cis-substituted but trans-fatty acids also occur A fat or oil in nature occurs as a mixture of many different triacylglycerols The most abundant saturated fatty acids are palmitic (C16) and stearic (C18)

Unsaturated and Polyunsaturated Fatty Acids Oleic (C18 with one C=C) and linolenic (C18 with 3 C=C) are the most abundant unsaturated fatty acids

Structures of Common Fatty Acids

16.2 Soaps A mixture of sodium or potassium salts of long-chain fatty acids produced by alkaline hydrolysis (saponification) of animal fat with alkali

Cleansing Action of Soap The carboxylate end of the long-chain molecule is ionic and therefore is preferentially dissolved in water The hydrocarbon tail is nonpolar and dissolves in grease and oil Soaps enable grease to be dissolved into water Micelles spherical clusters with hydrocarbon tails on the interior

Detergents “Hard” water contains Mg+2 and Ca+2 that form insoluble salts with soaps Synthetic detergents are alkylbenzene sulfonates that dissolve dirt like soaps but do not form scums with Mg+2 and Ca+2 .

16.3 Phospholipids Phospholipids are diesters of H3PO4, phosphoric acid Phosphoric acid can form monoesters, diesters and triesters In general these are known as “phosphates” Two general classes: glycerophospholipids and sphingomyelins

Glycerophospholipids Contain a glycerol backbone linked by ester bonds to two fatty acids and one phosphoric acid residue or phosphate Fatty acid residues with C12–C20 The phosphate group at C3 has an ester link to an amino alcohol

Sphingolipids The other major group of phospholipids Sphingosine or a dihydroxyamine backbone Constituents of plant and animal cell membranes Abundant in brain and nerve tissue, as coating around nerve fibers.

Phosphoglyceride Membranes Phospholipids comprise the major lipid component of cell membranes Nonpolar tails aggregate in the center of a lipid bilayer Ionic head is exposed to solvent

16.4 Steroids Steroids are another class of nonsaponifiable lipids, defined by structure Are composed of four fused rings A, B, C, and D, beginning at the lower left Carbon atoms are numbered beginning in the A ring The six-membered rings are in fixed chair conformations

Functions of Steroids Present in humans as hormones Steroids are chemical messengers secreted by glands and carried through the bloodstream to target tissues Also widely distributed as cholesterol

Male Sex Hormones Testosterone and androsterone are the two most important male sex hormones, or androgens Androstanedione is a precursor

Female Sex Hormones Estrone and estradiol are the two most important female sex hormones, or estrogens Progesterone is the most important progestin, steroids that function during pregnancy

Adrenocortical Hormones Adrenocortical steroids: secreted by the adrenal glands near the upper end of each kidney Mineralocorticoids: control tissue swelling by regulating cellular salt balance Glucocorticoids: regulation of glucose metabolism and in the control of inflammation

16.5 Nucleotides and Nucleic Acids Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are the chemical carriers of genetic information Nucleic acids are biopolymers made of nucleotides, aldopentoses linked to a purine or pyrimidine and a phosphate RNA is derived from ribose DNA is from 2-deoxyribose (the ' is used to refer to positions on the sugar portion of a nucleotide)

Heterocycles in DNA and RNA Adenine, guanine, cytosine and thymine are in DNA RNA contains uracil rather than thymine

Nucleotides Nucleotides join together to form nucleic acids Nucleotides are composed of nucleosides bonded to a phosphate group Each nucleoside is composed of a aldopentose sugar linked via the anomeric carbon to the nitrogen of an aromatic heterocyclic amine

Nucleotides In DNA and RNA the heterocycle is bonded to C1 of the sugar and the phosphate is bonded to C5 (and connected to 3 of the next unit)

Nucleotides (Continued) Nucleotides join together in DNA and RNA by phosphodiester bonds between the phosphate on the 5′ hydroxyl group on one nucleotide and 3 hydroxyl group on another nucleotide One end of the nucleic acid polymer has a free hydroxyl at C3 (the 3 end), and the other end has a phosphate at C5 (the 5 end).

Nucleotides (Continued)

16.6 Base Pairing in DNA: The Watson–Crick Model In 1953 Watson and Crick noted that DNA consists of two polynucleotide strands, running in opposite directions and coiled around each other in a double helix Strands are held together by hydrogen bonds between specific pairs of bases Adenine (A) and thymine (T) form strong hydrogen bonds to each other but not to C or G Guanine (G) and cytosine (C) form strong hydrogen bonds to each other but not to A or T

Hydrogen Bonds in DNA The G-C base pair involves three H-bonds The A-T base pair involves two H-bonds

AZT Effect on DNA Replication