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3Proteins, Carbohydrates,

and Lipids

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3.1 What Kinds of Molecules Characterize Living Things?

Molecules in living organisms: proteins,carbohydrates, lipids, nucleic acids

Living organisms are carbon-based.

The 4 classes of biological molecules allcontain large numbers of carbon atoms.

Most are polymers of smallermolecules called monomers

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Proteins:are formed from different

combinations of 20 amino acids

Carbohydrates: sugar monomers(monosaccharides) are linked to form

polysaccharides

Nucleic acids: are formed from 4 kindsof nucleotide monomers

Lipids: also form large structures butnoncovalent forces maintain interactionsbetween lipid monomers

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How biological molecules function andinteract with other molecules dependson the properties of certain chemicalgroups called functional groups.

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Functional groups: groups of atomswith specific chemical properties andconsistent behavior

Some functional groups are polar, someare acidic, etc.

A single macromolecule may containmany different functional groups.

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Figure 3.1 Some Functional Groups Important to Living Systems (Part 1)

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Isomers: molecules with the same chemicalformula, but atoms are arranged differently

Structural isomers: differ in how theiratoms are joined together

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Optical isomers occur when a carbon atom

has four different atoms or groups of atomsattached to it.

This allows two different ways of making the

attachments, each the mirror imageof theother

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Such a carbon atom is called an

asymmetrical carbon and the two resultingmolecules are optical isomers

Some biochemical molecules that can

interact with one optical isomer are unable tofit the other.

Fi 3 2 O ti l I

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Figure 3.2 Optical Isomers

Optical isomers result from asymmetrical carbons.

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Biochemical unity:the four kinds ofmacromolecules are present in roughlythe same proportions in all livingorganisms, and have similar functions

This reflects evolution of all life from acommon ancestor

Some organisms can acquire needed rawmaterials by eating other organisms

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Figure 3.3 Substances Found in Living Tissues

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Polymers are formed in condensationreactions.

Monomers are joined by covalent bonds.

A water is removed; so they are alsocalled dehydrationreactions.

Figure 3 4 Condensation and Hydrolysis of Polymers (A)

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Figure 3.4 Condensation and Hydrolysis of Polymers (A)

Covalent bond

Water is removed

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Polymers are broken down intomonomers in hydrolysis reactions.

(hydro, water; lysis, break)

Figure 3 4 Condensation and Hydrolysis of Polymers (B)

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Figure 3.4 Condensation and Hydrolysis of Polymers (B)

Water is added

Covalent bond

between monomers

is broken

3 2 What Are the Chemical Structures and Functions of

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3.2 What Are the Chemical Structures and Functions ofProteins?

Functions of proteins include:

enzymescatalytic proteins

defensive proteins (e.g., antibodies)

hormonal and regulatory proteinscontrol physiological processes

receptor proteinsreceive and respondto molecular signals


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Storage proteinsstore amino acids.

Structural proteinsprovide physicalstability and movement.

Transport proteinscarry substanceswithin the organism (e.g.,hemoglobin).

Genetic regulatory proteinsregulatewhen, how, and to what extent a gene isexpressed.


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Proteins are polymers of 20 different amino

acids the building blocks of proteins

All proteins are composed of one or morepolypeptide chains

Polypeptide chain:single, unbranchedchain of amino acids.

The chains are folded into specific threedimensional shapes defined by thesequence of the amino acids.


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Proteins that have more than onepolypeptide chaininclude the oxygencarrying protein, hemoglobin

Hemoglobin has 4 polypeptide chainsthat are folded separately and cometogether to make the functional protein.


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Amino acids have carboxyl and amino

groupsso they function as both acidand base.


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The carbon atom is asymmetrical.

Amino acids exist in two isomeric forms:

D-amino acids (dextro, right)

L-amino acids (levo, left)this form is

found in organisms


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The side chains or R-groups also havefunctional groups.

Amino acids can be grouped based on

side chains.

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These hydrophylic amino acids attract ions of opposite charges.

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Hydrophylic amino acids with polar but uncharged side chainsform hydrogen bonds.

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Hydrophobic amino acids


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The terminalSH group of cysteinecanreact with another cysteine side chain toform a disulfide bridge, or disulfide

bond(SS).

These are important in protein folding.

Figure 3.5 A Disulfide Bridge

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Figure 3.5 A Disulfide Bridge


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Glycineis small and fits into tight cornersin the interior of proteins.

The prolineside chain has a ringstructure, which limits its hydrogen-bonding ability and ability to rotate aboutthe carbon. It is often found where a

protein bends or loops.


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Amino acids bond together covalently in acondensation reaction by peptidelinkages (peptide bonds).

Figure 3.6 Formation of Peptide Linkages

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Repetition of this reaction links many amino acids together into a polypeptide chain.

3.2 What Are the Chemical Structures and Functions of

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A polypeptide chain is like a sentence:

The capital letter is the amino group of

the first amino acidthe N terminus

The period is the carboxyl group of the

last amino acidthe C terminus

Figure 3.6 Formation of Peptide Linkages

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Repetition of this reaction links many amino acids together into a polypeptide chain.


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There are four levels of protein structure:Primary

Secondary

Tertiary

Quaternary


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The primary structure of a protein is thesequence of amino acids.

The sequence determines secondary and

tertiary structurehow the protein isfolded.

The number of different proteins that can

be made from 20 amino acids isenormous!

Figure 3.7 The Four Levels of Protein Structure (Part 1)

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What Are the Chemical Structures and Functions ofProteins?

Secondary structure: helixright-handed coil resulting fromhydrogen bonding between NH

groups on one amino acid and C=Ogroups on another.

pleated sheettwo or more

polypeptide chains are aligned;hydrogen bonds from between thechains.


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a C a s a sProteins?

Tertiary structure: Bending and foldingresults in a macromolecule with specificthree-dimensional shape.

The outer surfaces present functionalgroups that can interact with othermolecules.


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Proteins?

Tertiary structure is determined byinteractions of R-groups:

Disulfide bridges

Hydrogen bonds

Aggregation of hydrophobic side chains

van der Waals forces

Ionic bonds

Figure 3.8 Three Representations of Lysozyme

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Complete descriptions of tertiary structure have been workedout for many proteins.


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Proteins?

Quaternary structure results from theinteraction of subunitsby hydrophobicinteractions, van der Waals forces, ionic

bonds, and hydrogen bonds.

Each subunit has its own unique tertiary

structure.

Additional molecules may becomeincorporated at the quaternary stage

Figure 3.10 Quaternary Structure of a Protein

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Quaternary structure of hemoglobin


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Proteins?

Under extreme conditions, the foldedstructure is broken down; the protein issaid to be denatured.

When returned to normal conditions, theprotein MAY return to its normal folded

shape or renature.


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Proteins?

Conditions that affect protein folding:

High temperature

pH changes

High concentrations of polar molecules

Nonpolar substances


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Proteins?

Proteins bind noncovalently with specificmolecules. The specificity is determinedby:

Protein shapethere must be a generalfit between the 3-D shapes of theprotein and the other molecule.

ChemistryR groups on the surfaceinteract with molecules via ionic bonds,hydrophobically, or hydrogen bonds.


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Proteins?

Proteins can sometimes bind to the wrongmolecules after denaturation or when theyare newly made and still unfolded.

Chaperones are proteins that help preventthis.

Heat shock proteins (HSPs) are a generalclass of stress-induced chaperoneproteins.

Figure 3.12 Chaperones Protect Proteins from Inappropriate Binding

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3.3 What Are the Chemical Structures and Functions ofC b h d ?

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Carbohydrates?

Carbohydrates have the general formulaCn(H2O)n

Source of stored energy

Transport stored energy

Carbon skeletonsfor many othermolecules


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Carbohydrates?

Monosaccharides: simple sugars

Disaccharides: two simple sugars linked bycovalent bonds

Oligosaccharides: three to 20monosaccharides

Polysaccharides: hundreds or thousands ofmonosaccharidesstarch, glycogen,cellulose


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Carbohydrates?

Cells use glucose (monosaccharide) asan energy source and for manystructural molecules.

Exists as a straight chain or ring form.Ring is more commonit is morestable.

Ring form exists as - or -glucose,which can interconvert.

Figure 3.13 From One Form of Glucose to the Other

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3.3 What Are the Chemical Structures and Functions ofC b h d t ?

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Carbohydrates?

Monosaccharides have different numbersof carbons:

Hexoses: six carbonsstructural

isomers

Pentoses: five carbons

Figure 3.14 Monosaccharides Are Simple Sugars

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3.3 What Are the Chemical Structures and Functions ofC b h d t ?

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Carbohydrates?

Monosaccharides bind together incondensation reactions to formglycosidic linkages.

Glycosidic linkages can be or .

Figure 3.15 Disaccharides Form by Glycosidic Linkages (Part 1)

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Figure 3.15 Disaccharides Form by Glycosidic Linkages (Part 3)

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3.3 What Are the Chemical Structures and Functions ofCarbohydrates?

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Carbohydrates?

Oligosaccharides often attached toother biological molecules (conjugatedmolecules).

Often covalently bonded to proteins andlipids on cell surfaces and act asrecognition signals.

Human blood groups get specificity fromoligosaccharide chains.


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Carbohydrates?

Polysaccharides are giant polymers ofmonosaccharides.

Starch:storage of glucose in plants

Glycogen:storage of glucose in animals

Cellulose: very stable, good for structuralcomponents

Figure 3.16 Representative Polysaccharides (Part 1)

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Carbohydrates?

Carbohydrates can be modified by theaddition of functional groups:

Sugar phosphate

Amino sugars

Chitin

Figure 3.17 Chemically Modified Carbohydrates (Part 1)

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3.4 What Are the Chemical Structures and Functions ofLipids?

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Lipids?

Lipids are nonpolar hydrocarbons.

When sufficiently close together, weak butadditive van der Waals forces hold them

together.

Not polymers in the strict sense, because

they are not covalently bonded.


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Lipids?

Fats and oils store energy

Phospholipidsstructural role in cellmembranes

Carotenoids and chlorophyllscapture lightenergy in plants

Steroids and modified fatty acids

hormones and vitamins


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Lipids?

Animal fatthermal insulation/cushion

Lipid coating around nerves provides

electrical insulation

Oil and wax on skin, fur, and feathers

repels water


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Lipids?

Fats and oils are triglycerides(simplelipids): composed of fatty acids andglycerol

Glycerol: 3 OH groups (an alcohol)

Fatty acid: nonpolar hydrocarbon with a

polar carboxyl group

Carboxyls bond with hydroxyls of glycerolin an ester linkage.

Figure 3.18 Synthesis of a Triglyceride

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Lipids?

Saturated fatty acids: no double bondsbetween carbonsit is saturated with Hatoms.

Unsaturated fatty acids: some doublebonds in carbon chain.

monounsaturated: one double bond

polyunsaturated: more than one

Figure 3.19 Saturated and Unsaturated Fatty Acids (Part 1)

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Figure 3.19 Saturated and Unsaturated Fatty Acids (Part 2)

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Lipids?

Animal fats tend to be saturated: packedtogether tightly; solid at room temperature.

Plant oils tend to be unsaturated: the kinksprevent packing; liquid at roomtemperature.


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Lipids?

Fatty acids are amphipathic: they haveopposing chemical properties.

When the carboxyl group ionizes it forms

COO and is strongly hydrophilic; theother end is hydrophobic.


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Lipids?

Phospholipids: fatty acids bound toglycerol; a phosphate group replacesone fatty acid.

Phosphate group is hydrophilicthe

head

Tails are fatty acid chainshydrophobic

They are amphipathic

Figure 3.20 Phospholipids (Part 1)

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p ds

In water, phospholipids line up with thehydrophobic tails together and the

phosphate heads facing outward, to

form a bilayer.

Biological membranes have this kind ofphospholipid bilayer structure.

Figure 3.20 Phospholipids (Part 2)

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Figure 3.21 -Carotene is the Source of Vitamin A

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Carotenoids: light-absorbing pigments

Figure 3.22 All Steroids Have the Same Ring Structure

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Steroids: multiple rings share carbons


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p

A typical wax structure

Documents

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