Carbohydrates, proteins and lipids

Transcription

1 Carbohydrates, proteins and lipids Chapter 3 MACROMOLECULES Macromolecules: polymers with molecular weights >1,000 Functional groups THE FOUR MACROMOLECULES IN LIFE Molecules in living organisms: proteins, carbohydrates, lipids, nucleic acids Most are polymers of smaller molecules called monomers SUBUNITS OF MACROMOLECULES Proteins: combinations of 20 Carbohydrates: sugar monomers ( monosaccharides ) are linked to form polysaccharides Nucleic acids: 4 kinds of nucleotide monomers Lipids: noncovalent forces maintain interactions between lipid monomers FUNCTIONAL GROUPS Groups of atoms with specific chemical properties and consistent behavior A single macromolecule may contain many different functional groups. ISOMERES Molecules with the same chemical formula, but atoms are arranged differently Structural isomers: differ in how their atoms are joined together Optical isomers occur when a carbon atom has four different atoms or groups of atoms attached to it. Some biochemical molecules that can interact with one optical isomer are unable to fit the other. Biochemical unity: the four kinds of macromolecules are present in roughly the same proportions in all living organisms, and have similar functions Organisms can obtain required macromolecules by eating other organisms CONDENSATION REACTIONS Polymers are formed in condensation reactions. Monomers are joined by covalent bonds. Water is removed; so they are also called dehydration reactions.

2 HYDROLYSIS Polymers are broken down into monomers in hydrolysis reactions. (hydro, water ; lysis, break ) FUNCTIONS OF PROTEINS enzymes catalytic proteins defensive proteins (e.g. antibodies) hormonal and regulatory proteins control physiological processes receptor proteins receive and respond to molecular TYPES OF PROTEINS Storage proteins store amino acids. Structural proteins provide physical stability and movement. Transport proteins carry substances within the organism (e.g., hemoglobin ) Genetic regulatory proteins regulate when, how, and to what extent a gene is expressed. AMINO ACIDS Amino acids have carboxyl and amino groups so they function as both acid and base. The α carbon atom is asymmetrical. Amino acids exist in two isomeric forms: D-amino acids (dextro, right ) L-amino acids (levo, left ) this form is found in organisms The terminal SH group of cysteine can react with another cysteine side chain to form a disulfide bridge, or disulfide bond ( S S ). These are important in protein folding. Figure 3.5 A Disulfide Bridge PEPTIDE BONDS Amino acids bond together covalently in a condensation reaction by peptide linkages (peptide bonds). A polypeptide chain is like a sentence: The capital letter is the amino group of the first amino acid the N terminus The period is the carboxyl group of the last amino acid the C terminus

3 PROTEINS SHAPE Proteins can consist of more than one type of polypeptide chain. Chains are folded into specific three dimensional shapes defined by the sequence of the amino acids. Hemoglobin has 4 chains that are folded separately and come together to make the functional protein. The primary structure of a protein is the sequence of amino acids. The sequence determines secondary and tertiary structure how the protein is folded. The number of different proteins that can be made from 20 amino acids is enormous! Figure 3.7 The Four Levels of Protein Structure (Part 1) 3.2 What Are the Chemical Structures and Functions Secondary structure: α helix right-handed coil resulting from hydrogen bonding between N H groups on one amino acid and C=O groups on another. β pleated sheet two or more polypeptide chains are aligned; hydrogen bonds from between the chains. Tertiary structure: Bending and folding results in a macromolecule with specific three-dimensional shape. The outer surfaces present functional groups that can interact with other molecules. Tertiary structure is determined by interactions of R-groups: Disulfide bridges Hydrogen bonds Aggregation of hydrophobic side chains van der Waals forces Ionic bonds DENATURATION If a protein is heated, the secondary and tertiary structure is broken down; the protein is said to be denatured. Quaternary structure results from the interaction of subunits by hydrophobic interactions, van der Waals forces, ionic bonds, and hydrogen bonds. Each subunit has its own unique tertiary structure.

4 Conditions that affect secondary and tertiary structure: High temperature ph changes High concentrations of polar molecules Nonpolar substances Chaperones: Heat shock proteins (HSPs) are the general class of stress-induced chaperone proteins. They are made by most eukaryotic cells, and many also enhance protein folding. Figure 3.12 Chaperones Protect Proteins from Inappropriate Binding CARBOHYDRATES Carbohydrates have the general formula C n (H 2 O) n Source of stored energy Transport stored energy Carbon skeletons for many other molecules Monosaccharides: simple sugars Disaccharides: two simple sugars linked by covalent bonds Oligosaccharides: three to 20 monosaccharides Polysaccharides: hundreds or thousands of monosaccharides starch, glycogen, cellulose GLUCOSE All cells use glucose (monosaccharide) as an energy source. Exists as a straight chain or ring form. Ring form exists as α- or β-glucose Hexoses: six carbons structural isomers Pentoses: five carbons Monosaccharides bind together in condensation reactions to form glycosidic linkages. Glycosidic linkages can be α or β. Oligosaccharides may include other functional groups. Often covalently bonded to proteins and lipids on cell surfaces and act as signals. Human blood groups get specificity from oligosaccharide chains. Polysaccharides Are giant polymers of monosaccharides. Starch:

5 Glycogen: Cellulose: Carbohydrates can be modified by the addition of functional groups: Sugar phosphate Amino sugars Chitin LIPIDS Nonpolar hydrocarbons. When sufficiently close together, weak but additive van der Waals forces hold them together. Not polymers in the strict sense, because they are not covalently bonded. Fats and oils store energy Phospholipids structural role in cell membranes Carotenoids and chlorophylls capture light energy in plants Steroids and modified fatty acids hormones and vitamins Animal fat thermal insulation Lipid coating around nerves provides electrical insulation Oil and wax on skin, fur, and feathers repels water Triglycerides (simple lipids): composed of fatty acids and glycerol Glycerol: 3 OH groups (an alcohol) Fatty acid: nonpolar hydrocarbon with a polar carboxyl group Carboxyls bond with hydroxyls of glycerol in an ester linkage. SATURATED VS UNSATURATED FATS Saturated fatty acids: no double bonds between carbons it is saturated with H atoms. Unsaturated fatty acids: some double bonds in carbon chain. monounsaturated: one double bond polyunsaturated: more than one WHAT IS BETTER? Animal fats or Plant oils? Fatty acids are amphipathic: they have opposing chemical properties. When the carboxyl group ionizes it forms COO and is strongly hydrophilic; the other end is hydrophobic. 3.4 What Are the Chemical Structures and Functions of Lipids?

6 Phospholipids: fatty acids bound to glycerol; a phosphate group replaces one fatty acid. Phosphate group is hydrophilic the head Tails are fatty acid chains hydrophobic They are amphipathic In water, phospholipids line up with the hydrophobic tails together and the phosphate heads facing outward, to form a bilayer. Biological membranes have this kind of phospholipid bilayer structure. Vitamins small molecules not synthesized by the body and must be acquired in the diet. Waxes highly nonpolar and impermeable to water.