The Structure and Function of Large Biological Molecules

1. The Structure and Function of Large Biological Molecules Chapter 5

2. Macromolecules • Macromolecules (or polymers) are long, chain-like molecules • Consists of many similar or identical building blocks (monomers) linked by covalent bonds • Includes carbohydrates, nucleic acids, and proteins • Lipids are not a true macromolecule • Built via condensation or dehydration reaction (or dehydration synthesis) • Take away water molecule • Helped by enzymes to speed reaction • Breakdown via hydrolysis • Add water molecule • Occurs in digestion

3. Carbohydrates • Monomers: monosaccharides or simple sugars • Simplified formula is CH2O • Structure is used to classify sugars • General structure includes a carbonyl group and multiple hydroxyl groups • Location of carbonyl will determine if it is aldose or ketose (aldehyde or ketone sugars) • Sugars are made up of 3-7 carbons in skeleton which may be linear or ringed • Spatial arrangement around asymmetric carbons is important • Examples: glucose, fructose, galactose • Important in cellular respiration and synthesis of materials

4. Carbohydrates • Disaccharides: 2 sugars joined by a covalent bond • The covalent bond is known as a glycosidic linkage when it is between 2 monosaccharides • The bond is formed by dehydration reaction • Examples: Maltose, sucrose, lactose

5. Carbohydrates • Polymers: polysaccharides; these are macromolecules also formed via glycosidic linkages • Storage polysaccharides • Starch – polymer of glucose monomers found in plants; starch allows plants to stockpile glucose • α configuration of glucose • Humans consume these in potatoes and grains • Glycogen – a branched polymer of glucose found in most vertebrates; largely stored in liver and muscle cells and is released when the body needs sugar

6. Carbohydrates • Structural Polysaccharides • Cellulose – major component in cell walls • β configuration of glucose (every other glucose monomer in upside down) • Important in digestion – humans do not have the appropriate enzymes to digest β linkages, but promotes healthy digestion • Most abundant organic compound on Earth • Chitin – used by arthropods in exoskeletons • Similar structure to cellulose, but contains nitrogen

7. Lipids • Lipids do not include true polymers and are not generally considered macromolecules • They are grouped together because they are hydrophobic • Largely composed of hydrocarbons • Includes: fats, phospholipids, steroids, waxes and pigments

8. Lipids • Fats (triacylglycerol or triglyceride) – composed of glycerol attached to 3 fatty acids bonded via an ester linkage • Ester linkage occurs between hydroxyl and carboxyl groups • Glycerol – alcohol with 3 carbons each with its own hydroxyl group • Fatty acid – long carbon skeleton (16-18 common) with one carbon end associated with a carboxyl group. The rest is a long hydrocarbon chain. • Important in energy storage and protection

9. Lipids • Fats • Saturated fat or fatty acid • No double bonds which allows the greatest number of hydrogens to be attached to the carbon skeleton • Includes most animal fats • Solid at room temp • Unsaturated fat or fatty acid • Has 1 or more double bonds and thus fewer hydrogen atoms • A kink in the chain will occur whenever a cis double bond occurs (as opposed to trans double bonds – ie trans fats found in hydrogenated veg. oil) • Includes plant and fish oils • Liquid at room temp

10. Lipids • Phospholipids – essential for cell membrane composition • Similar to fat molecule, but only have 2 fatty acids attached to glycerol • The 3rd hydroxyl group is attached to a phosphate group (these can in turn bond to other molecules) • Hydrocarbon tail is hydrophobic (inside the bilayer), phosphate group is hydrophilic (face outward)

11. Lipids • Steroids – carbon skeleton composed of 4 fused rings with different chemical groups attached • Includes many hormones and cholesterol • Fat can affect cholesterol levels

12. Proteins • Proteins account for ~50% of cell’s dry mass and extremely important in functions

13. Proteins • Monomers are amino acids • 20 different amino acids that are composed of an asymmetric carbon surrounded by an amino group, carboxyl group, hydrogen and an R group or side chain which varies • Polymers are polypeptides • Different combinations of A.A. allows for the variety of proteins • A.A. are attached with a covalent bond between the carboxyl group of one to the amino group of another called a peptide bond

14. Proteins • Protein structure and function are intimately linked • The specific folds of a protein are determined by the ordering of A.A. in the polypeptide chain. This folding in turn determines shape. • Shape will then determine function.

15. Proteins • Primary Structure – the unique sequence of amino acids • Secondary Structure – coils and folds in the polypeptide chain caused by hydrogen bonds between repeating constituents • α helix – a coil held together by hydrogen bonds at every 4th A.A. • β pleated sheet – folding creating pleats at particular intervals

16. Proteins • Tertiary Structure – Overall shape of a polypeptide due to interactions of R groups • Shape may be reinforced by disulfide bridges • Covalent bond between sulfhydryl groups • Quaternary Structure – overall protein structure (potentially several polypeptide chains interacting)

17. Proteins • Changes in primary structure lead to changes in further structures, potentially leads to a misfunctioning or nonfunctioning protein • Example: Sickle Cell • Protein shape and function can also be changed via denaturation • pH, temperature, salt concentration, etc.

18. Proteins • Chaperonins or chaperone structure are specialized proteins that assist in the proper folding of proteins • Are not specific, but keep the protein away from potentially bad influences • Folding is spontaneous

19. Nucleic Acids • Main function is to store and transmit genetic information • 2 kinds: RNA and DNA • These are both polymers/macromolecules • The monomers are nucleotides • Composed of a nitrogenous base, a 5-carbon sugar, and a phosphate group • Nucleosides are this unit minus the phosphate group

20. Nucleic Acids • Two groups of nitrogenous bases • Pyrimidines: single 6-C ring • Cytosine, thymine, uracil • Purines: double fused rings (1 5-C, 1 6-C) • Adenine, guanine • Two kinds of sugars • RNA – ribose • DNA – deoxyribose

21. Nucleic Acids • Nucleotides are linked together by phosphodiester linkages • Covalent bond between a phosphate group and a sugar • This creates the sugar-phosphate backbone • One end will have a phosphate attached to a 5’ carbon; the other will have a hydroxyl group on a 3’ carbon (these are the ends of DNA and this plays a role in replication) • The opposing sides of DNA are linked via hydrogen bonds and twist about an imaginary axis creating the double helix