Proteins include a diversity of structures, resulting in a wide range of functions

• Protein functions include structural support, storage, transport, enzymes, cellular communications, movement, and defense against foreign substances

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Figure 5.15-a

Enzymatic proteins Defensive proteins

Storage proteins Transport proteins

Enzyme Virus

Antibodies

Bacterium

Ovalbumin Amino acidsfor embryo

Transportprotein

Cell membrane

Function: Selective acceleration of chemical reactionsExample: Digestive enzymes catalyze the hydrolysisof bonds in food molecules.

Function: Protection against diseaseExample: Antibodies inactivate and help destroyviruses and bacteria.

Function: Storage of amino acids Function: Transport of substances

Examples: Casein, the protein of milk, is the majorsource of amino acids for baby mammals. Plants havestorage proteins in their seeds. Ovalbumin is theprotein of egg white, used as an amino acid sourcefor the developing embryo.

Examples: Hemoglobin, the iron-containing protein ofvertebrate blood, transports oxygen from the lungs toother parts of the body. Other proteins transportmolecules across cell membranes.

Figure 5.15-b

Hormonal proteins

Function: Coordination of an organism’s activitiesExample: Insulin, a hormone secreted by thepancreas, causes other tissues to take up glucose,thus regulating blood sugar concentration

Highblood sugar

Normalblood sugar

Insulinsecreted

Signalingmolecules

Receptorprotein

Muscle tissue

Actin Myosin

100 m 60 m

Collagen

Connectivetissue

Receptor proteins

Function: Response of cell to chemical stimuliExample: Receptors built into the membrane of anerve cell detect signaling molecules released byother nerve cells.

Contractile and motor proteins

Function: MovementExamples: Motor proteins are responsible for theundulations of cilia and flagella. Actin and myosinproteins are responsible for the contraction ofmuscles.

Structural proteins

Function: SupportExamples: Keratin is the protein of hair, horns,feathers, and other skin appendages. Insects andspiders use silk fibers to make their cocoons and webs,respectively. Collagen and elastin proteins provide afibrous framework in animal connective tissues.

Polypeptides

• Polypeptides are un-branched polymers built from the same set of 20 amino acids

• A protein is a biologically functional molecule that consists of one or more polypeptides

• Amino acids are organic molecules with carboxyl and amino groups which are monomers of proteins

• Amino acids differ in their properties due to differing side chains, called R groups

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Figure 5.UN01

Side chain (R group)

Aminogroup

Carboxylgroup

carbon

Figure 5.16Nonpolar side chains; hydrophobic

Side chain(R group)

Glycine(Gly or G)

Alanine(Ala or A)

Valine(Val or V)

Leucine(Leu or L)

Isoleucine (Ile or I)

Methionine(Met or M)

Phenylalanine(Phe or F)

Tryptophan(Trp or W)

Proline(Pro or P)

Polar side chains; hydrophilic

Serine(Ser or S)

Threonine(Thr or T)

Cysteine(Cys or C)

Tyrosine(Tyr or Y)

Asparagine(Asn or N)

Glutamine(Gln or Q)

Electrically charged side chains; hydrophilic

Acidic (negatively charged)

Basic (positively charged)

Aspartic acid(Asp or D)

Glutamic acid(Glu or E)

Lysine(Lys or K)

Arginine(Arg or R)

Histidine(His or H)

Amino Acid Polymers

• Amino acids are linked by peptide bonds• A polypeptide is a polymer of amino acids• Polypeptides range in length from a few to more

than a thousand monomers • Each polypeptide has a unique linear sequence of

amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus)

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Figure 5.17

Peptide bond

New peptidebond forming

Sidechains

Back-bone

Amino end(N-terminus)

Peptidebond

Carboxyl end(C-terminus)

Protein Structure and Function

• A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape

• The sequence of amino acids determines a protein’s three-dimensional structure

• A protein’s structure determines its function

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Figure 5.19

Antibody protein Protein from flu virus

Four Levels of Protein Structure

• The primary structure of a protein is its unique sequence of amino acids determined by inheritance.

• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain caused by H-bonds between polypeptide backbones

• Tertiary structure is determined by interactions among various side chains (R groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions, sometimes covalent bonds called disulfide bridges may reinforce the protein’s structure

• Quaternary structure results when a protein consists of multiple polypeptide chains, ex. Hemoglobin, collagen

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Figure 5.20aPrimary structure

Aminoacids

Amino end

Carboxyl end

Primary structure of transthyretin

Figure 5.20b

Secondarystructure

Tertiarystructure

Quaternarystructure

Hydrogen bond

helix

pleated sheet strand

Hydrogenbond

Transthyretinpolypeptide

Transthyretinprotein

Figure 5.20h

Collagen

Hemoglobin

Heme

Iron

subunit

subunit

subunit

subunit

Figure 5.20i

Figure 5.21

PrimaryStructure

Secondaryand TertiaryStructures

QuaternaryStructure Function Red Blood

Cell Shape

subunit

subunit

Exposedhydrophobicregion

Molecules do notassociate with oneanother; each carriesoxygen.

Molecules crystallizeinto a fiber; capacityto carry oxygen isreduced.

Sickle-cellhemoglobin

Normalhemoglobin

10 m

10 m

Sick

le-c

ell h

emog

lobi

nN

orm

al h

emog

lobi

n

1234567

1234567

• Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions

• Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life

• Never used up, shape is altered during the enzymatic reaction but returns to normal unless other factors affect the enzyme

• Substance to be acted on is a substrate (S) and binds to an enzymes active site (E) and produces products (P)

E + S- ES E + P

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Enzyme Catalyzed Reaction