Chapter 5. The Structure and Function of Macromolecule s

Chapter 5

The Structure and Function of Macromole cule s

Most Macromolecules are polymers:

Polymer

:

Monomer: Subunits or building block molecule of polymers.

Macromolecules: Large organic polymers There are four classes in living organisms:

1.Carbohydrates 2.Lipids

3.Proteins 4.Nucleic acids

Condensation Reaction: (Polymerization): A reaction during which monomers are linked together by covalent bonds to form a polymer (macromolecule).

•1 one monomer loses a hydroxyl (-OH), and the other monomer loses a hydrogen (-H); a water molecule is removed.

•2 Process requires energy

•3 Process requires enzymes

Hydrolysis: (hydro= water; lysis= break). A reaction process that breaks covalent bonds between monomers by the addition of water molecules.

Macromolecules are formed by the condensation reaction (dehydration) and are broken into monomers by the hydrolysis reaction.

Organisms use carbohydrates for fuel and building material:

Carbohydrates:

Organic molecules made of sugars and their polymers.

•4 Monomers of carbohydrates are simple sugars called Monosaccharides.

•5 Polymers are formed by condensation reaction

•6 Are classified based on the number of simple sugars.

Monosaccharides: (Mono=single;sacchar=sugar): Simple sugars in which C, H and O are occur in the ratio of CH2O.

•7 Are major nutrients for the cell

•8 Can be produced by photosynthesis organisms from CO2, H2O and sunlight.

•9 Store energy in their chemical bonds which are harvested by cellular respiration.

•10Can be incorporated as monomers into disaccharides and polysaccharides.

Disaccharides: (Di=two; sacchar=sugar): A double sugar that consists of two monosaccharides joined by glycosidic linkage.

Glycosidic linkage: Covalent bond formed by a condensation reaction between two sugar monomers.

Examples: Maltose = glucose+ glucose Lactose = glucose+ galactose Sucrose = glucose+ fructose

Polysachharides: Macromolecules that are polymers of a few hundred or thousand of monosachharides.

•11Formed by linking monomers in condensation reaction

•12Have two important biological functions:

i. energy storage (starch and glycogen) ii. structural support (cellulose and chitin).

Storage Polysaccharides:

1.Starch: Glucose polymer that is a storage polysaccharide in plants.

•13Helical glucose polymer with alpha 1-4 glycosidic linkages.

•14Stored as granules within plant organelles called plastids.

•15Amylose the simplest form is an unbranched polymer.

•16Amylopectin is branched polymer

•17Most animals have digestive enzymes to hydrolyse starch.

2.Glycogen: Glucose polymer that is a storage polysaccharide in animals.

•18Large glucose polymer that is more highly branched than amylopectin.

•19Stored in the muscles and liver of humans and other vertebrates.

Structural Polysaccharides:

1. Cellulose: Linear unbranched polymer of D-glucose in beta 1-4 linkages.

•20major structural component of plant cell walls.

•21Differ from starch in its glycosidic linkages.

•22Cellulose and starch have different three-dimensional shapes and Properties as a result of differences in glycosidic linkages.

2. Chitin: A structural polysaccharide that is a polymer of an amino sugar.

* forms exoskeletons of arthropods (تايلصفلا))).

•23monomer ia an amino sugar: a sugar with nitrogen containing group.

Lipids

Lipids are diverse ةعونم

Important lipids are:

•24Fats

•25Phospholipids

•26Steroids

1. Fats: macromolecules constructed from:

i. Glycerol, a three carbon alcohol ii. Fatty acid (carboxylic acid):

•27composed of a carboxyl group at one end and an attached hydrocarbon chain (tail).

•28carboxyl group (head) has properties of an acid.

•29Hydrocarbon chain has a long carbon skeleton.

•30Nonpolar C-H bonds make the chain hydrophobic (not water soluble).

Fats are formed by a condensation reaction which links glycerol to fatty acids by an Ester linkage.

Triacylglycerol: A fat composed of three fatty acids bonded to one glycerol by ester linkages.

Saturated fatty acids Unsaturated fatty acids -No double bonds between carbons of

fatty acid tail.

-Carbon skeleton of fatty acid is bonded to maximum number of hydrogens -Usually a solid at room temperature -Most animal fats

-One or more double bonds between carbons of fatty acid tail

-Tail kinks at each C=C, so molecules do not pack enough to solidify at room temperature.

-Usually a liquid at room temperature -Most plant fats

Phospholipids: Phospholipids are compounds with molecular building blocks of glycerol, phosphate group and a cholin (the head), and two fatty acids (the tail).

•31show aphipathic behavior towards water, hydrocarbon tails are hydrophobic and the polar head is hydrophilic.

•32Cluster in water as their hydrophobic portions turn away from water such as a micelle.

•33A major constituent of cell membranes. At the cell surface, pjospholipids form a bilayer (two layers) held together by hydrophobic interactions among the hydrocarbon tails.

Steroids: Steroids are lipids which have four fused carbon rings with various functional groups attached.

Cholesterol, an important steroid:

•34is the precursor to many other steroids including sex hormones and bile salts.

•35Is a common component of animal cell membrane

•36Can contribute to atherosclerosis (ةطلج ) .

Proteins

Polypeptide chains: Polymers of amino acids that are arranged in a specific linear sequence (يطخ عباتت ) and are linked by peptide bonds.

Protein: A macromolecule that consists of one or more polypeptide chains folded and coiled into specific conformation.

•37Are abundant, forming about 50% of cellular dry weight.

•38Have important functions in the cell:

1.structural support 2.storage (of amino acids)

3.transport (e.g. hemoglobin) 4.signaling (chemical messengers) 5.cellular response (receptor proteins) 6.movement (contractile proteins)

7.defense (antibodies) 8.catalysts of biochemical reactions (enzymes).

Amino acid: Building block molecule of a protein. It consist of:

1. hydrogen atom 2. carboxyl group 3. amino group

4. R group (side chain). Properties of the amino acids is determined by its side chain.

There are 20 common amino acids that make all types of proteins.

Polypeptide chains are polymers that are formed when amino acid monomers are linked by peptide bonds.

Peptide bond Covalent bond formed by a condensation reaction that links carboxyl group of one amino acid to the amino group of another.

Polypeptide chain:

•40range in length from a few monomers to more than a thousand.

•41have unique linear sequence of amino acids.

A proteins function depends on its conformation

Protein conformation: Three dimensional shape of a protein.

Protein Structure

Four levels of protein structure:

1.Primary structure 2.Secondary Structure

3.Tertiary structure 4.quaternary structure

1. Primary Structure:

•42unique sequence of amino acids

•43determined by genes

•44slight change can effect the protein conformation and function (e.g. sickle- cell hemoglobin).

•45Can be sequenced in the laboratory.

2. Secondary Structure:

•46Regular, repeated coiling and folding of a protein polypeptide backbone.

•47Contributes to a protein’s conformation.

•49The major types of secondary structure are alpha helix and beta pleated sheets.

i. alpha helix:

•50secondary structure of a poly peptide that is a helical coil

stabilized by hydrogen bonding between every fourth peptide bond.

•51found in fibrous proteins and in some globular proteins.

ii. beta pleated sheets

•52secondary protein structure which is a sheet of antiparallel chains folded into accordion pleats.

•53parallel regions are held together by hydrogen bond .

•54found in globular and fibrous proteins.

3. Tertiary Structure:

•55irregular contortions (change of shape) of a protein due to bonding between side chains.

Type of bonds:

1. Weak interactions: include:

i. hydrogen bonding between polar side chains.

ii. Ionic bonds between charged side chains.

iii. Hydrophobic interactions between nonpolar side chains.

2. Covalent linkage :

Disulfide bridges form between two cysteine monomers brought together by folding of the protein. This is a strong bond.

4. Quaternary Structure:

•56Structure that results from the interaction between several polypeptides in a single protein.

•57Examples: Collagen: a fibrous protein with three helical polypeptides.

Hemoglobin: a globular protein with four polypeptides.

What determines protein conformation:

•58A proteins three-dimensional shape is a consequence of the interactions responsible for the secondary and tertiary structures.

•59This conformation is influenced by physical & chemical environmental conditions.

•60If a protein ‘s environment is changed, it may become denatured and lose its conformation.

Denaturation: A process that changes a protein’s conformation.

A protein can be denatured by:

i. transfer to organic solution.

ii. Chemical agent that dirupt hydrogen bonds.

iii. Excessive heat that disrupt weak interactions.

Nucleic Acids:

There are two types of nucleic acids:

1. Deoxyribonucleic acid (DNA):

•61contains genes that program all cell activity.

•62Contain directions for its own replication ((فعاضت.

•63Is copied and passed from one generation to another.

•64In eukaryotic cells (ةاونلا ةيقيقح ايلخ), is found in the nucleus.

•65Makes up genes that contain instructions for protein synthesis.

2. Ribonucleic acid (RNA):

•66function in the actual synthesis of proteins.

•67Sites of protein synthesis are on ribosomes in the cytoplasm.

•68Messenger RNA (mRNA) carries encoded ( (ةرفشم message from the nucleus to the cytoplasm.

•69The flow of genetic information goes from DNA  RNA protein.

A DNA strand is a polymer with an information-rich sequence of nucleotides

Nucleic acid: Polymer of nucleotides linked together by condensation reaction.

Nucleotide: Building block molecule of a nucleic acid. It is made of:

1. five carbon sugar, covalently bonded to:

2. phosphate group 3. nitrogenous base.

Pentose: (5-carbon sugar): two types:

i. ribose (in RNA)

ii. deoxyribose (in DNA).

Phosphate: the phosphate group is attaché to the number 5 carbon of the sugar.

Nitrogenous base: Two types:

i. Pyrimidine: Characterized by a six-membered ring made up of carbon and nitrogen atoms. Examples:

- Cytosine (C).

ii. Purine: Characterized by a five-membered ring fused to a six- membered ring. Examples:

- Adenine (A) - Guanine (G)

•70DNA is a polymer of nucleotides joined by phosphodiester linkages between the phosphate of one nucleotide and the sugar of the next.

•71Result in a backbone with a repeating pattern of sugar-phosphate.

•72Variable nitrogenous bases are attached to the sugar-phosphate backbone.

•73Each gene contains a unique linear sequence of nitrogenous bases.

(more details on DNA structure are found in Chapter 16).