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

• 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

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 – 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