Lipids and Proteins
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Chapters 17 and 18 in General, Organic, and Biological Chemistry, 7th ed., Denniston

Chapters 17 and 18 in General, Organic, and Biological Chemistry, 7th ed., Denniston

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  • Look at the structures for cis and trans unsaturated fatty acids on slide 22. The trans form is much more linear. Therefore, the molecules can pack together more closely, mimicking the behavior of saturated fatty acids, which generally have higher melting points than unsaturated fatty acids.

Lipids and Proteins Lipids and Proteins Presentation Transcript

  • Chapter 17Lipids and Their Functions in Biological Systems 1
  • Lipids Lipids are  biomolecules that contain fatty acids or a steroid nucleus.  soluble in organic solvents, but not in water.  named for the Greek word lipos, which means “fat.”  extracted from cells using organic solvents. 2
  • Lipids—one classification The types of lipids containing fatty acids are  waxes  fats and oils (triacylglycerols)  glycerophospholipids  prostaglandins The types of lipids that do not contain fatty acids are  steroids 3
  • Lipids—one classification
  • Lipids—another classification Four main groups  Fatty acids  Saturated  Unsaturated  Glycerides: glycerol-containing lipids  Nonglyceride lipids  Sphingolipids  Steroids  Waxes  Complex lipids: lipoproteins 5
  • Lipids—another classification
  • Fatty acids Fatty acids are  long-chain carboxylic acids.  typically 12-18 carbon atoms.  insoluble in water.  saturated or unsaturated. Olive oil contains 84% unsaturated fatty acids and 16% saturated fatty acids. 7
  • Fatty acids Fatty acids are  saturated with all single C–C bonds. palmitic acid, a saturated acid  unsaturated with one or more double C=C bonds. palmitoleic acid, an unsaturated acid 8
  • Fatty acids Saturated fatty acids  contain only single C–C bonds.  are closely packed.  have strong attractions between chains.  have high melting points.  are solids at room temperature. COOH COOH COOH 9
  • Fatty acids Unsaturated fatty acids HOOC  contain one or more cis H double C=C bonds. COOH C H  have “kinks” in the fatty acid C chains.  do not pack closely. “kinks” in  have few attractions H chain C between chains. C  have low melting points. H  are liquids at room temperature. 10
  • Fatty acids Melting points of fatty acids 11
  • Waxes, fats, and oils Waxes are:  esters of saturated fatty acids and long-chain alcohols.  coatings that prevent loss of water by leaves of plants. 12
  • Waxes, fats, and oils Fats and oils are  also called triglycerides.  esters of glycerol.*  produced by esterification.  formed when the hydroxyl groups of glycerol react with the carboxyl groups of fatty acids. *The IUPAC name for glycerol is 1,2,3-propanetriol. 13
  • Waxes, fats, and oils In a triglyceride, glycerol forms ester bonds with three fatty acids. 14
  • Waxes, fats, and oils glycerol + 3 fatty acids  triglyceride + 3 waters OCH2 OH HO C (C H 2 ) 1 4 C H 3 OCH OH HO C (C H 2 ) 1 4 C H 3 O OCH2 OH HO C (C H 2 ) 1 4 C H 3 CH2 O C (C H 2 ) 1 4 C H 3 O CH O C (C H 2 ) 1 4 C H 3 + 3H2O O CH2 O C (C H 2 ) 1 4 C H 3 15
  • Waxes, fats, and oils Example of a triglyceride: O CH2 Stearic acid O C (C H 2 ) 16 C H 3 O Oleic acid CH O C (C H 2 ) 7 C H C H (C H 2 ) 7 C H 3 O Myristic acid CH2 O C (C H 2 ) 12 C H 3 16
  • Waxes, fats, and oilsA fat • is solid at room temperature. • is prevalent in meats, whole milk, butter, and cheese.An oil • is liquid at room temperature. • is prevalent in plants such as olive and safflower. 17
  • Waxes, fats, and oils Oils  have more unsaturated fats.  have cis double bonds that cause “kinks” in the fatty acid chains.  with “kinks” in the chains do not allow the triglyceride molecules to pack closely.  have lower melting points than saturated fatty acids.  are liquids at room temperature. 18
  • Waxes, fats, and oils Unsaturated fatty acid chains with kinks cannot pack closely. olive oil 19
  • Waxes, fats, and oils Percent saturated and unsaturated fatty acids 20
  • Chemical reactions of fatty acids In hydrogenation1, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst. O OCH2 O C (C H 2 ) 5 C H C H (C H 2 ) 7 C H 3 CH2 O C (C H 2 ) 1 4 C H 3 O Ni OCH O C (C H 2 ) 5 C H C H (C H 2 ) 7 C H 3 + 3 H2 CH O C (C H 2 ) 1 4 C H 3 O OCH2 O C (C H 2 ) 5 C H C H (C H 2 ) 7 C H 3 CH2 O C (C H 2 ) 1 4 C H 3 glyceryl tripalmitoleate glyceryl tripalmitate 1See Chapter 11, pages 363-365, Power point slides 35-39 21
  • Chemical reactions of fatty acids Unsaturated fatty acids can be  cis with bulky groups on same side of C=C.  trans with bulky groups on opposite sides of C=C. 22
  • Chemical reactions of fatty acids Most naturally occurring fatty acids have cis double bonds.  During hydrogenation, some cis double bonds are converted to trans double bonds.  In the body, trans fatty acids behave like saturated fatty acids.  Why would trans fatty acids behave like saturated fatty acids, while cis fatty acids do not? 23
  • Chemical reactions of fatty acids In hydrolysis2, ester bonds are split by water in the presence of an acid, a base, or an enzyme. O CH2 O C (C H 2 ) 1 4 C H 3 O H+ CH O C (C H 2 ) 1 4 C H 3 + H 2O O CH2 O C (C H 2 ) 1 4 C H 3 CH2 OH O CH OH + 3 HO C (C H 2 ) 1 4 C H 3 CH2 OH 2See Chapter 14, pages 472-473, Power point slides 31-33 24
  • Chemical reactions of fatty acids In saponification, a fatty acid or O triglyceride reacts with a strong baseCH2 O C (C H 2 ) 1 4 C H 3 to form an alcohol or glycerol, and the salt(s) of fatty acid(s). OCH O C (C H 2 ) 1 4 C H 3 + 3NaOH O CH2 OHCH2 O C (C H 2 ) 1 4 C H 3 CH OH O + 3 N a + -O C (C H 2 ) 1 4 C H 3 “soap” CH2 OH
  • Triglycerides Glycerides are lipid esters. A triglyceride places fatty acid chains at each alcohol group of a molecule of glycerol. glycerol portion fatty acid chains 26
  • Triglycerides Triglycerides undergo three fundamental reactions, identical to those studied in carboxylic acids. Triglyceride H2O, H+ H2, Ni NaOH Glycerol More saturated Fatty Acids triglyceride Glycerol Fatty Acid Salts 27
  • Glycerophospholipids Glycerophospholipids are  the most abundant lipids in cell membranes.  composed of glycerol, two fatty acids, phosphate, and an amino alcohol. Fatty acid example Fatty acid Glycerol Amino PO4 alcohol 28
  • Cholesterol and steroid hormones A steroid nucleus consists of  three cyclohexane rings, and  one cyclopentane ring. The rings are fused (joined along one side). 29
  • Cholesterol and steroid hormones Cholesterol  is the most abundant steroid in the body.  has methyl (-CH3) groups, an alkyl chain, and an -OH attached to the steroid nucleus. 30
  • Cholesterol and steroid hormones A steroid is any natural or synthetic compound based on the four-fused-ring structure in cholesterol. cortisone testosterone progesterone 31
  • Protein Structure and Function Chapter 18 32
  • 2. The α-amino acids Amino acids  are the building blocks of proteins.  contain a carboxylic acid group 33
  • 2. The α-amino acids Amino acids  are the building blocks of proteins.  contain a carboxylic acid group and an amino group 34
  • 2. The α-amino acids Amino acids  are the building blocks of proteins.  contain a carboxylic acid group and an amino group on the alpha () carbon. 35
  • 2. The α-amino acids Amino acids  are the building blocks of proteins.  contain a carboxylic acid group and an amino group on the alpha () carbon.  are ionized in solution. H2O ionized form 36
  • 2. The α-amino acids Amino acids  are the building blocks of proteins.  contain a carboxylic acid group and an amino group on the alpha () carbon.  are ionized in solution.  all have a different side chain. H2O ionized form 37
  • 2. The α-amino acids Examples of amino acids  glycine  alanine 38
  • 2. The α-amino acids All but one of the amino acids have a chiral carbon. The α carbon is attached to  a carboxylate group (COO-),  a protonated amino group (-NH3+),  a hydrogen atom, and  a side chain (R group). Glycine has R = H, which gives it two hydrogens attached to the α carbon and no chiral carbon. 39
  • 2. The α-amino acids Each amino acid (except glycine) can exist as one of two stereoisomers. Only L- isomers of amino acids are found in proteins.  Recall that mainly D- isomers of monosaccharides exist in nature. As is true for monosaccharides, Fischer projections for amino acids have the most oxidized group at the top. L-alanine D-alanine L-cysteine D-cysteine 40
  • 2. The α-amino acids Amino acids are classified based on the nature of their side chains.  Nonpolar amino acids are hydrophobic and have hydrocarbon side chains.  Polar amino acids are hydrophilic and have polar or ionic side chains.  Acidic amino acids are hydrophilic and have acidic (carboxylic acid) side chains.  Basic amino acids are hydrophilic and have amino side chains. 41
  • 2. The α-amino acids Nonpolar: The R group is H, alkyl, or aromatic. glycine alanine valine leucine isoleucine phenylalanine tryptophan methionine proline 42
  • 2. The α-amino acids Polar: the R group is an alcohol, thiol, or amide serine threonine tyrosine glutamine asparagin cysteine e 43
  • 2. The α-amino acids Acidic: The R group is a carboxylic acid. aspartate glutamate 44
  • 2. The α-amino acids Basic: The R group is an amine. histidine lysine arginine 45
  • 3. The peptide bond A peptide bond is an amide bond that forms between the carboxyl group of one amino acid and the amino group of the next amino acid. glycine alanine + H2O 46
  • 3. The peptide bond Naming peptides  For small peptides, the general name “peptide” is preceded by a prefix indicating how many amino acids were condensed to form the peptide.  The peptide formed on the previous slide is a dipeptide.  The end of the peptide with the free NH3+ group is called the N- terminal amino acid.  The end of the peptide with the free –COO- group is called the C- terminal amino acid. 47
  • 3. The peptide bond Naming peptides (cont.)  The root name of a peptide is the name of the C-terminal amino acid, which uses its entire name.  For all other amino acids in the peptide, the ending –ine is changed to –yl.  The amino acids are named in order starting with the N-terminal amino acid. A peptide composed of aspartine, glutamine, and serine (in that order) would be named: aspartyl-glutamyl-serine 48
  • 3. The peptide bond The structures of small peptides are based on a repeating backbone: N—C—C—N—C—C—N—C—C  α-amino group (N)  α-carbon (always attached to H and R)  α-carboxyl group (C) 49
  • 3. The peptide bond Draw the structure of aspartyl-glutamyl-serine.  This is a tripeptide, so the backbone will have three repeats.  The left end is the N-terminal amino acid and the right end is the C- terminal amino acid.  The carboxyl carbons all have a carbonyl and the α carbons all have a hydrogen. 50
  • 3. The peptide bond Draw the structure of aspartyl-glutamyl-serine. (cont.)  Each α carbon has the R group characteristic of the particular amino acid.  aspartame  glutamate  serine 51
  • 4. The primary structure of proteins Primary structure is the amino acid sequence of the polypeptide chain.  It is the result of covalent bonding (peptide bonds) between amino acids. Each protein has a different primary structure with different amino acids in different places along the chain. CH3 CH3 S CH CH3 SH CH2 CH3 O CH O CH2 O CH2 O - H 3N CH C N CH C N CH C N CH C O H H H Ala─Leu─Cys─Met 52
  • 4. The primary structure of proteins Insulin was the first protein to have its primary structure determined. It has a primary structure of two polypeptide chains linked by disulfide bonds. One chain (A) has 21 amino acids and the other (B) has 30 amino acids. 53
  • 5. The secondary structure of proteins When the primary sequence of the polypeptide folds into regularly repeating structures, secondary structure is formed.  Secondary structure results from hydrogen bonding between the amide hydrogens and carbonyl oxygens of the peptide bonds. 54
  • 5. The secondary structure of proteins The α-helix is the most common type of secondary structure. Features:  a three-dimensional spatial arrangement of amino acids in a polypeptide chain.  held by H bonds between the H of –N-H group and the O of C=O of the fourth amino acid down the chain.  a corkscrew shape that looks like a coiled “telephone cord”. 55
  • 5. The secondary structure of proteins Top view of the α-helix, looking down into the “barrel.”  The side chains (-R) point out. 56
  • 5. The secondary structure of proteins The second most common secondary structure is the β- pleated sheet.  The β-pleated sheet consists of polypeptide chains arranged side by side with hydrogen bonds between chains.  Side chains (-R) are above and below the sheet. This structure is typical of fibrous materials like silk. 57
  • 6. The tertiary structure of proteins Soluble proteins are usually globular proteins. A third level of structure, tertiary structure, is added to the primary and secondary structures. Areas of α-helix and β-pleated secondary structure are folded in on themselves and held in place by the forces responsible for tertiary structure. 58
  • 6. The tertiary structure of proteins Crosslinks in tertiary structures involve attractions and repulsions between the side chains (-R) of the amino acids in the polypeptide chain.  Hydrophobic interactions: attractions between nonpolar groups.  Hydrophilic interactions: attractions between polar groups and water.  Salt bridges: ionic interactions between acidic and basic amino acids.  Hydrogen bonds: between H and oxygen or nitrogen.  Disulfide bonds: covalent links between sulfur atoms of two cysteine amino acids. 59
  • 6. The tertiary structure of proteins salt bridge hydrophobic interactions disulfide bonds hydrogen bonds 60
  • 7. The quaternary structure of proteins Quaternary structure is the arrangement of subunits or peptides that form a larger protein.  A subunit is a polypeptide chain having primary, secondary, and tertiary structural features that is a part of a larger protein.  Quaternary structure is maintained by the same forces which are active in maintaining tertiary structure. Hemoglobin consists of four polypeptide chains as subunits. 61
  • 8. Overview of protein structure Primary structure:  Amino acid sequence  Results from formation of covalent peptide bonds between amino acids Secondary structure:  Includes α-helix and β-sheet  Hydrogen bonding between amide hydrogens and carbonyl oxygens of the peptide bonds 62
  • 8. Overview of protein structure Tertiary structure:  Overall folding of the entire polypeptide chain  Interactions between different amino acid side chains Quaternary structure:  Concerned with topological, spatial arrangement of two or more polypeptide chains  Involves both disulfide bridges and noncovalent interactions 63
  • 10. Denaturation of proteins Denaturation involves the disruption of bonds in the secondary, tertiary and quaternary protein structures.  Denaturation is the loss of organized structure of a globular protein.  Denaturation does not alter primary structure. Causes of denaturation:  heat and organic compounds that break apart H bonds and disrupt hydrophobic interactions.  acids and bases that break H bonds between polar R groups and disrupt ionic bonds.  heavy metal ions that react with S-S bonds to form solids.  agitation such as whipping that stretches peptide chains until bonds break. 64
  • 10. Denaturation of proteins Heat: As the temperature rises, molecules move and vibrate more. The weaker hydrogen bonds are the first to break. pH: Amino acids include basic (amino) and acidic (carboxylate) groups. An excess of H+ or OH- changes ionic interactions involving these groups. Organic solvents: Alcohols disrupt hydrogen bonding because they take part in it themselves. The nonpolar portions of alcohols disrupt nonpolar interactions. 65
  • 10. Denaturation of proteins Detergents: The hydrophobic region of detergents disrupts hydrophobic interactions in proteins. Heavy metals: Metal cations such as mercury or lead can bond with negative side chains and disrupt their interactions. They can also bind to sulfur and disrupt disulfide bonds. Mechanical stress: Shaking or whipping can disrupt the intermolecular forces that maintain the conformation of the protein. 66