Biology Chapter on Lipids and Proteins

Lecture Presentations

• Chapter 5: The Structure and Function of Large Biological Molecules
• Lectures by Erin Barley and Kathleen Fitzpatrick

Overview: The Molecules of Life

• All living things are made up of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids
• Macromolecules are large molecules composed of thousands of covalently connected atoms
• Molecular structure and function are inseparable

Concept 5.1: Macromolecules are Polymers, Built from Monomers

• A polymer is a long molecule consisting of many similar building blocks
• These small building block molecules are called monomers
• Three of the four classes of life's organic molecules are polymers: carbohydrates, proteins, and nucleic acids

The Synthesis and Breakdown of Polymers

• A dehydration reaction occurs when two monomers bond together through the loss of a water molecule
• Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction

The Diversity of Polymers

• Each cell has thousands of different macromolecules
• Macromolecules vary among cells of an organism, vary more within a species, and vary even more between species
• An immense variety of polymers can be built from a small set of monomers

Concept 5.2: Carbohydrates Serve as Fuel and Building Material

• Carbohydrates include sugars and the polymers of sugars
• The simplest carbohydrates are monosaccharides, or single sugars
• Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks

Sugars

• Monosaccharides have molecular formulas that are usually multiples of CH₂O
• Glucose (C₆H₁₂O₆) is the most common monosaccharide
• Monosaccharides are classified by the location of the carbonyl group (as aldose or ketose) and the number of carbons in the carbon skeleton

Figure 5.3

• Aldoses (aldehyde sugars) and ketoses (ketone sugars) are shown
• Trioses (3-carbon sugars), pentoses (5-carbon sugars), and hexoses (6-carbon sugars) are shown
• Structures of various sugars (e.g., glucose, galactose, fructose, ribose, ribulose) are included

Figure 5.4

• Linear and ring forms of monosaccharides are shown
• Abbreviated ring structure of monosaccharides are shown

Disaccharides

• A disaccharide is formed when a dehydration reaction joins two monosaccharides
• This covalent bond is called a glycosidic linkage

Polysaccharides

• Polysaccharides, the polymers of sugars, have storage and structural roles
• The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages

Storage Polysaccharides

• Starch, a storage polysaccharide of plants, consists entirely of glucose monomers
• Plants store surplus starch as granules within chloroplasts and other plastids
• The simplest form of starch is amylose
• Glycogen, a storage polysaccharide in animals, is mainly stored in liver and muscle cells

Structural Polysaccharides

• The polysaccharide cellulose is a major component of the tough wall of plant cells
• Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ
• The difference is based on two ring forms for glucose: alpha (α) and beta (β)
• Polymers with α glucose are helical, and polymers with β glucose are straight
• Parallel cellulose molecules held together this way are grouped into microfibrils, which form strong building materials for plants

Chitin

• Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods
• Chitin also provides structural support for the cell walls of many fungi

Concept 5.3: Lipids are a Diverse Group of Hydrophobic Molecules

• Lipids are the one class of large biological molecules that do not form polymers
• The unifying feature of lipids is having little or no affinity for water
• Lipids are hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bonds
• The most biologically important lipids are fats, phospholipids, and steroids

Fats

• Fats are constructed from two types of smaller molecules: glycerol and fatty acids
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon
• A fatty acid consists of a carboxyl group attached to a long carbon skeleton

Figure 5.10

• One of three dehydration reactions in the synthesis of a fat is shown
• A fat molecule (triacylglycerol) is shown

Fats (continued)

• Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fats
• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride

Fatty Acids

• Fatty acids vary in length (number of carbons) and in the number and locations of double bonds
• Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds
• Unsaturated fatty acids have one or more double bonds
• Fats made from saturated fatty acids are called saturated fats, and are solid at room temperature
• Most animal fats are saturated
• Fats made from unsaturated fatty acids are called unsaturated fats or oils, and are liquid at room temperature
• Plant fats and fish fats are usually unsaturated

Hydrogenation

• A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits
• Hydrogenation is the process of converting unsaturated fats to saturated fats by adding hydrogen
• Hydrogenating vegetable oils also creates unsaturated fats with trans double bonds
• These trans fats may contribute more than saturated fats to cardiovascular disease

Essential Fatty Acids

• Certain unsaturated fatty acids are not synthesized in the human body
• These must be supplied in the diet

Functions of Fats

• The major function of fats is energy storage
• Humans and other mammals store their fat in adipose cells
• Adipose tissue also cushions vital organs and insulates the body

Phospholipids

• In a phospholipid, two fatty acids and a phosphate group are attached to glycerol
• The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head

Figure 5.12

• Structural formula of a phospholipid is shown
• Space-filling model of a phospholipid is shown
• Phospholipid symbol is shown
• When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior
• The structure of phospholipids results in a bilayer arrangement found in cell membranes
• Phospholipids are the major component of all cell membranes

Steroids

• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings
• Cholesterol, an important steroid, is a component in animal cell membranes

Figure 5.14

• Structure of cholesterol is shown

Concept 5.4: Proteins Include a Diversity of Structures, Resulting in a Wide Range of Functions

• Proteins account for more than 50% of the dry mass of most cells
• Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances

Figure 5.15a

• Enzymatic, defensive, storage, and transport proteins are described
• Examples for each class are also given

Figure 5.15b

• Hormonal, receptor, contractile and motor, and structural proteins are described
• Examples for each class are also given

Enzymes

• Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions
• Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life

Polypeptides

• Polypeptides are unbranched polymers built from the same set of 20 amino acids
• A protein is a biologically functional molecule that consists of one or more polypeptides

Amino Acids

• Amino acids are organic molecules with carboxyl and amino groups
• Amino acids differ in their properties due to differing side chains, called R groups

Figure 5.16

• Nonpolar, polar, and electrically charged side chains (R groups) of amino acids are shown
• The structure of some of the 20 amino acids and their properties are shown

Amino Acid Polymers

• Amino acids are linked by peptide bonds
• A polypeptide is a polymer of amino acids
• Polypeptides range in length from a few to more than a thousand monomers

Figure 5.17

• Peptide bond formation between two amino acids, with the loss of a water molecule, is illustrated
• The polypeptide backbone is shown with an amino end (N-terminus) and a carboxyl end (C-terminus).

Protein Structure and Function

• A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape

Figure 5.18

• Ribbon and space-filling models of a protein are shown

Protein Structure (Key Points)

• The sequence of amino acids determines a protein's three-dimensional structure
• A protein's structure determines its function

Figure 5.19

• Ribbon models of an antibody protein and a protein from a flu virus are shown

Four Levels of Protein Structure

• The primary structure of a protein is its unique sequence of amino acids
• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain
• Tertiary structure is determined by interactions among various side chains (R groups)
• Quaternary structure results when a protein consists of multiple polypeptide chains

Sickle-Cell Disease

• A slight change in primary structure can affect a protein's structure and ability to function
• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin

Figure 5.21

• Primary, secondary and tertiary structure of normal and sickle-cell hemoglobin are compared
• The effect on red blood cell shape is shown

What Determines Protein Structure?

• In addition to primary structure, physical and chemical conditions can affect structure
• Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel
• This loss of a protein's native structure is called denaturation

Figure 5.22

• Normal and denatured protein shapes are shown schematically

Protein Folding in the Cell

• It is hard to predict a protein's structure from its primary structure
• Most proteins probably go through several stages on their way to a stable structure
• Chaperonins are protein molecules that assist the proper folding of other proteins
• Diseases such as Alzheimer's, Parkinson's, and mad cow disease are associated with misfolded proteins

Figure 5.23

• Steps of chaperonin action are illustrated

Determining Protein Structure

• Scientists use X-ray crystallography to determine a protein's structure
• Another method is nuclear magnetic resonance (NMR) spectroscopy, and does not require protein crystallization
• Bioinformatics uses computer programs to predict protein structure from amino acid sequences

Figure 5.24

• X-ray crystallography experiment outline is given
• Results showing a protein structure based on X-ray crystallography is shown

Concept 5.5: Nucleic Acids Store, Transmit, and Help Express Hereditary Information

• The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene
• Genes are made of DNA, a nucleic acid made of monomers called nucleotides

The Roles of Nucleic Acids

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two types of nucleic acids
• DNA provides directions for its own replication
• DNA directs synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis
• Protein synthesis occurs on ribosomes

Figure 5.25-1, Figure 5.25-2, Figure 5.25-3

• A schematic diagram of the flow of information from DNA to RNA to protein synthesis is shown.

The Components of Nucleic Acids

• Nucleic acids are polymers called polynucleotides
• Each polynucleotide is made of monomers called nucleotides
• Each nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups
• The portion of a nucleotide without the phosphate group is called a nucleoside

Figure 5.26

• Sugar-phosphate backbone of a polynucleotide is illustrated
• Nitrogenous bases and sugars are shown

Nucleotide (continued)

• Nucleoside = nitrogenous base + sugar
• There are two families of nitrogenous bases: pyrimidines and purines
• In DNA, the sugar is deoxyribose; in RNA, the sugar is ribose
• Nucleotide = nucleoside + phosphate group

Nucleotide Polymers

• Nucleotide polymers are linked together to build a polynucleotide
• Adjacent nucleotides are joined by covalent bonds that form between the -OH group on the 3' carbon of one nucleotide and the phosphate on the 5' carbon on the next
• These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages
• The sequence of bases along a DNA or mRNA polymer is unique for each gene

DNA and RNA Structures

• RNA molecules usually exist as single polypeptide chains
• DNA molecules have two polynucleotides spiraling around an imaginary axis, forming a double helix
• In the DNA double helix, the two backbones run in opposite 5'→ 3' directions from each other, an arrangement referred to as antiparallel
• One DNA molecule includes many genes

Complementary Base Pairing

• The nitrogenous bases in DNA pair up and form hydrogen bonds: adenine (A) always with thymine (T), and guanine (G) always with cytosine (C)
• Complementary pairing can also occur between two RNA molecules or between parts of the same molecule
• In RNA, thymine is replaced by uracil so A and U pair

Figure 5.27

• DNA and transferencia RNA (tRNA) structures showing complementary base pairing are illustrated

DNA and Proteins as Tape Measures of Evolution

• The linear sequences of nucleotides in DNA molecules are passed from parents to offspring
• Two closely related species are more similar in DNA than are more distantly related species
• Molecular biology can be used to assess evolutionary kinship

The Theme of Emergent Properties in the Chemistry of Life: A Review

• Higher levels of organization result in the emergence of new properties
• Organization is the key to the chemistry of life

Figure 5.UN02, Figure 5.UN02a, Figure 5.UN02b

• Summary table of large biological molecules, their components, examples, and functions

Figures 5. UN03, 5. UN04, 5. UN05, 5. UN06, 5. UN07, 5. UN08, 5. UN09, 5. UN10, 5. UN11, 5.UN12, 5. UN13

• Various diagrams of molecular structures and processes.