The Structure and Function of Large Biological Molecules Chapter 5

The Molecules of Life Living things made up of 4 classes of large biological molecules (macromolecules) : 1. Carbohydrates 2. Lipids 3. Proteins 4. Nucleic acids Molecular structure and function are linked Unique, emergent properties

Macromolecules are polymers, built from monomers polymer -long molecule of many building blocks monomers - single unit Sucrose

The Diversity of Polymers Each cell has thousands of different macromolecules – built from monomers Macromolecules vary among cells, among species, and between species

Carbohydrates serve as fuel and building material Carbohydrates = sugars and sugar polymers – Monosaccharides = single sugars Ex. glucose (C 6 H 12 O 6 ) major fuel for cells raw material for building molecules

Sugars often form rings (in aqueous solution) (a) Linear and ring forms(b) Abbreviated ring structure

Disaccharide = two sugars Ex. lactose, sucrose, maltose

Polysaccharides Polysaccharides - polymers of sugars = starch – 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 – plants store starch – Glucose polymer Glycogen – Animals store glycogen (glucose polymer) – Humans in liver and muscle cells

(b) Glycogen: an animal polysaccharide Starch Glycogen Amylose Chloroplast (a) Starch: a plant polysaccharide Amylopectin Mitochondria Glycogen granules 0.5 µm 1 µm Amylose - unbranched Amylopectin - branched Glycogen is more branched than starch

Structural Polysaccharides Cellulose =component of tough wall of plant cells polymer of glucose (glycosidic linkages differ from starch) The difference is based on two ring forms for glucose:

Enzymes digest cellulose in some animals – Cows, termites, have symbiotic relationships with microbes that digest cellulose In humans, cellulose is indigestible fiber Mastigophoran, anaerobic, methane

Chitin in the exoskeleton of arthropods and in fungi The structure of the chitin monomer. (a) (b) (c) Chitin forms the exoskeleton of arthropods. Chitin is used to make a strong and flexible surgical thread. Cicada exoskeleton

Lipids are hydrophobic Lipids - fats, phospholipids, steroids Triglyceride = 3 fatty acids joined glycerol

Saturated fats maximum number of H possible (no double bonds) Solid at room T (animal fats) Unsaturated fats one or more double bonds Liquid at room T (plant, fish oils) (a)

Coronary artery disease associated with diet rich in saturated fats

Hydrogenation – process of converting unsaturated fats to saturated fats by adding hydrogen – Extends shelf life, prevents oil separation – Ex. margarine, peanut butter

The good news: Fats store energy (adipose cells) Cell membranes need lipid Lipid cushions and insulates

Steroids Steroids – – Ex. estrogen, testosterone Cholesterol – Steroid in animal cell membranes – Synthesized in the liver

Proteins Proteins = more than 50% of dry mass of cells Protein functions – structural support – collagen – pigment - melanin – transport - hemoglobin – cellular communications – movement – defense against foreign substances -antibodies

Enzymes – All are proteins – catalyst speeds up chemical reactions – reusable – specific to each reaction – essential to life – Heat or chemicals may denature – animation animation

Polypeptides – polymers built from set of 20 amino acid building blocks – may be a few or thousands long protein – one or more polypeptides – has a function

Peptide Protein

Protein Structure and Function proteins consists of one or more polypeptides twisted, folded, and coiled into unique shape A ribbon model of lysozyme (a)(b) A space-filling model of lysozyme Groove

sequence of aa determines a 3D structure structure determines function Antibody proteinProtein from flu virus

Four Levels of Protein Structure Primary structure =unique sequence of amino acids

Secondary structure = coils and folds –  helix and  pleated sheet – H-bonds β pleated sheet α helix Example: spider silk Strong as steel Stretchy

Tertiary structure determined by interactions between amino acids – hydrogen bonds – ionic bonds – hydrophobic interactions – disulfide bridges (covalent bonds)

Tertiary structure

Quaternary structure two or more polypeptide chains may form one macromolecule ex. hemoglobin activity α Chains β Chains Hemoglobin

A patient with sickle cell disease

Denaturation of proteins Denaturation – Loss of protein structure  biologically inactive – pH, heat, chemicals

The Roles of Nucleic Acids Deoxyribonucleic acid (DNA) replicates prior to cell division contains codes for proteins (genes)

Nucleic acids hold a code Gene – unit of inheritance – code for protein primary structure – composed of DNA

The Structure of Nucleic Acids Nucleotides G,A,T,C building blocks (monomers) – Pyrimidines (cytosine, thymine, and uracil) – Purines (adenine and guanine) (c) Nucleoside components: nitrogenous bases Purines Guanine (G) Adenine (A) Cytosine (C) Thymine (T, in DNA) Uracil (U, in RNA) Nitrogenous bases Pyrimidines

Ribose (in RNA)Deoxyribose (in DNA) Sugars (c) Nucleoside components: sugars Nucleotides contain sugar DNA deoxyribose RNA ribose (ribonucleic acid)

DNA Polymers Sugar phosphate backbone

The DNA Double Helix A DNA molecule has 2 strands that form double helix hydrogen bonds between: – adenine (A) thymine (T) – guanine (G) cytosine (C) DNA replication – Before a cell divides

DNA, Proteins and Evolution DNA is inherited – Cell to cell – Parent to offspring Closely related species more similar in DNA sequence than more distantly related species – Human/human 99.1 % – Human/chimp 98.5% Molecular biology used to assess evolutionary relatedness