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 (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

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

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

Groove Groove

• sequence of aa determines a 3D structure • structure determines function

Antibody protein Protein from flu virus

Four Levels of Protein Structure

• Primary structure =unique sequence of amino acids

25

20

15

10

5 1

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

β pleated sheet

α helix

Example: spider silk Strong as steel Stretchy

• Tertiary structure determined by interactions between amino acids

Tertiary structure

• Quaternary structure two or more polypeptide chains may form one macromolecule

• ex. hemoglobin

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)

(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%