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. Carbohydrates2. Lipids3. Proteins4. 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 (C6H12O6)• 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

GlycogenAmylose

Chloroplast

(a) Starch: a plant polysaccharide

Amylopectin

MitochondriaGlycogen granules

0.5 µm

1 µm

Amylose - unbranchedAmylopectin - 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 structureof the chitinmonomer.

(a) (b) (c)Chitin forms theexoskeleton ofarthropods.

Chitin is used to makea strong and flexiblesurgical 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

Polypeptides

• 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

GrooveGroove

• 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

25

20

15

10

51

• Secondary structure = coils and folds– helix and pleated sheet– H-bonds

β pleated sheet

α helix

Example: spider silk

Strong as steelStretchy

• 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 ChainsHemoglobin

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 divisioncontains 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 basesPyrimidines

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