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576 CHAPTERl8 Amino Acids,Peptides, and Proteins
denaturation of proteins by disrupting weak attractive forces. Figure lB.t5 shows disulfide bonds cleaved in the presenceof oxidants and reductants. In reversible denaturation, the protein unfolds in the presence of a denaturing agent, such as a concentrated urea solution, but is restored to its native state on removal of the agent. \Mhether a protein undergoes reversible or irreversible denaturation varies with the kind of protein react- ing and the conditions of reaction.
Thiol RSH (reducing agent)
Hydrogen peroxide Hroz (oxidizing agent) Figure18.15 Disulfidebridges are cleaved by oxidationor reduction
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Twenty alpha amino acids commonly occur in Proteins are peptides of greater than about 100 nature. Thesecompounds are grouped accordingto amino acid residues.Structural proteins are often the chemical properties of their side chains. All the fibrous, water-insoluble molecules; other proteins amino acids but glycine belong to the I family of are globular and water-soluble.Conjugated proteins stereoisomers.At biological pH, most amino acids have nonprotein portions called prosthetic groups, exist as electrically neutral zwitterions, in which the which are often relatively small organic molecules. positive charge of the protonated alpha amino group Protein molecules fold into unique shapes.The is compensated by the negatively charged alpha car- chemical nature of the amino acid side chains and boxylate ion. the order of their occurrence within the peptide The formation of an amide bond (peptidebond) chain (primary structure) govern the type of folding. between the alpha amino group of one amino acid Two patterns of chain folding (secondary structure) and the alpha carboxylic acid group of another pro- are found in many proteins; these are the alpha helix ducesa peptide. Peptideshave a common backbone and the beta-pleated sheet. Collagen forms a sec- and N-terminal and C-terminal ends. The order in ondary structure of collagen helixes. Each molecule which amino acids are linked in a peptide is the pep- of a particular kind of protein has the same overall tide's amino acid sequence.The peptide's amino acid three-dimensional folding (tertiary structure). Forces sequencedetermines its biologicalrole. that hold proteins in their respectivefoldings include Exercises 577 hydrophobic aggregation, hydrogen bonds, ionic structure-two alpha chains and two beta chains. bonds, and disulfide bridges. Sickle cell anemia is an example of how a minor Myoglobin is a conjugated globular protein that modification of the primary structure of a protein stores oxygen in muscle. The folding of its single- affects the protein's biological function. peptide chain is mostly alpha helix. The heme pros- Peptide chains of proteins are unfolded (dena- thetic group is tucked into folds in the chain. Hemo- tured) by chemical agents, heat, or extremes of pH. globin, the oxygen carrier of blood, consistsof four Denaturation may be reversible or irreversible. single-chain subunits comprising its quaternary
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Alpha carbon (18.1) Collagen(18.6) Glycoprotein(18.11) Polypeptide(18.4) Alpha helix (18.6) Conjugatedprotein (18.7) Hemoglobin (18.8) Primary structure (18.5) Alpha keratin (18.6) Cyanohemoglobin(18.9) IsoelectricpH (18.3) Prostheticgroup (18.7) Amino acid (18.1) Denaturation (18.12) Isoelectricpoint (18.3) Protein (18.5) Amino acid residue(18.4) Deoxyhemoglobin(18.8) Methemoglobin (18.8) Quaternarystructure (18.8) Amino acid side chain Disulfidebridge (18.4) Methemoglobinemia(18.8) Saltbridge (18.7) (18.1) Electrophoresis(18. 10) Myoglobin (18.7) Secondarystructure (18.6) Antidiureric (18.4) Enkephalin(18.4) Nativestate (18.12) Subunit (18.8) Betakeratin (18.6) Fibrousprotein (18.6) Oxyhemoglobin(18.8) Tertiarystructure (18.7) Beta-pleatedsheet (18.6) Globin (18.8) Peptide(r8.4) Zwitterion (18.3) Carboxyhemoglobin (I 8.9) Globularprotein (18.7) Peptidebond (18.4) ffiKHR€ESgS Amino Acids (Sections 18.1, 18,2, 18.5) 18.7 Write the structure and identify the alpha carbon 18.13 Define isoelectricpH. and R group for the following amino acids: (a) alanine, 18.14 At which pH will alaninebe leastsoluble, (a) pH2, (b) serine, and (c) glutamic acid. (b) pH 7, or (c) pH 10?\iVhy? 18.8 Categorizethe following amino acids according to their side-chaingroups. (a) valine (b) glutamine (c) cysteine Peptides(Section 18.4) (d) phenylalanine 18.15 In peptide chemistry what is the meaning of the 18.9 Draw a stereorepresentationof (a)o-serine and word residue? (b) r-alanine. 18.16 \Mhat is the name given to the amide bond in a 18.10 Identifu the following compounds representedby peptide chain? their Fischer projection formulas. l8.l7 Consider the tripeptide seryl-glycyl-phenylala- (a) (b) (c) nine. (a) Draw the complete structural formula, and (b) fo,t fo,t fo," write the three-letter abbreviation. (c) How many pep- HTNH, HTNH, H,NTH tide bonds does this molecule have? lB.lB Translatethe following three-letter abbreviations: cH2oH H CH2SH (a) Ala-Ser-Glyand (b) GIy-Ser-Ala.Are the structures 18.11 Define the term zwitterion.Draw the amino acid of these tripeptides the same?Explain your answer. leucine as a zwitterion. fB.f 9 Explain the biological functions of the peptide 18.12 Consider the amino acids valine, glutamic acid, hormones (a) oxytocin and (b) vasopressin. and lysine. Draw structural formulas for the species 18.20 Name two pentapeptides that act as the body's that predominate in solution at (a) pH 2 and (b) pH 10. own opiates. 578 CHAPTER18 Amino Acids,Peptides, and Proteins
Primaryand SecondaryStructures of Proteins Glycoproteinsand Denaturation (Sections18.5, 18.6) (Sectionsl8.l l, lS.l2) l8.2l \Alhatis meant by the primary structure of a lB.3B Name and describethe biologicalfunctions of at protein? least two glycoproteins. 18.22 Define the secondarystructureof aprotein. 18.39 \Mhat happens when a protein is denatured? 18.23 Describethree common repeatingpatterns that 18.40 Describeone way a protein can be denatured are found in the secondary structure ofproteins. irreversibly and one way a protein can be denatured 18.24 Consider the structure of the alpha helix. Are the reversibly. amino acid residue side chains all inside the helix, part of the helix, or all outside the helix? 1B.25 \A/hatis the function of collagen in the body? 1B.26 Comparethe molecularstructures of collagenand alpha keratin with regard to secondary structure and amino acid composition. AdditionalExercises 18.41 Match the following. Tertiaryand QuaternaryStructures of Proteins (a) lysine (l) fibrous protein (Sections18,7, 18.8) (b) vasopressin (2) antidiuretichormone (c) denaturation (3) basic amino acid 18.27 Define what is meant by a conjugatedprotein. (d) peptide link (4) protein of tendon 1B.2B (a) In what ways are the molecular structures of (e) globularprotein (5) unfolding myoglobin and hemoglobin similar? (b) How are they (f) betakeratin (6) net chargeequals zero different? (g) zwitterion (7) sphericalshape (h) (B) 18.29 With the aid of diagrams, describe the factors that collagen amide bond contribute to the folding of peptide chainsin the ter- 18.42 (a)Write the three-letter abbreviation and draw tiary structures of proteins. the structural formula of the tripeptide glutamyl-cys- (b) 18.30 Describethe heme group and explainits function. teinyl-glycine. Draw the different ionic speciesof this tripeptide at low (pH:2), neutral (pH : 7), and lB.3l \Mhat is the oxidation state of iron in (a) oxyhemo- high pH (pH : 11).(c) A disulfidebridge is formed globin and (b) methemoglobin? when this tripeptide is treated with a mild oxidizing agent. Draw the structural formula of the resulting HemoglobinFunction (Section 18.9) hexapeptide. 1B.43 Write the three-letter abbreviations of all the dif- 18.32 Discussthe role of hemoglobin in the transport of ferent tripeptides that can be made if the peptides oxygen in the body. contain one amino acid residueeach of histidine.me- 1B.33 Why are carbon monoxide and cyanide ions thionine, and glutamic acid. poisons? 18.44 For each of the following fibrous proteins, match all the items at the right that apply. Some answersmay be used more than once,some not at all. SickleCellAnemia (Section 18.t0) (a) betakeratin (1) spiderwebs (b) collagen (2) cartilage 18.34 \.A/hatis the basic structural difference between (c) alpha keratin (3) predominantly alpha normal hemoglobin (HbA) and sickle cell hemoglobin helixes (HbS)? (4) rich in proteins 18.35 \lVhyis exposureto low oxygen levels potentially (5) a glycoprotein dangerousfor a person who has sickle cell trait? (6) found in hair 18.36 \A'/hattechnique is used for screening individuals (7) rich in glycine and for sickle cell trait or sickle cell anemia? alanine 1B.37 The symptoms of sickle cell anemia can be treated 18.45 Draw the structural formulas of the expectedprod- by hyperbaric oxygenation.\A4ry does this procedure ucts of acid hydrolysis of (a) Cys-Gly-Ala, offer relief for the patient? (b) Glu-Leu-Val- Pro, and (c) Ser-His -Phe-T1'r- Trp. Self-Test(Review) 579
18.46 \Mhat unique structural features do each ofthe 18.49 Match the following.Each answer will be used following amino acids have that help to distinguish once only. them from all the others? (a) fibrous protein (1.) water-soluble pro- (a) serine (b) native state tein, easily denatured (b) phenylalanine (c) disulfide bridge (2) canbe reversibleor (c) glycine (d) peptide bond irreversible (d) cysteine (e) globular protein (3) the metal ion of a (e) tyrosine (f) denaturation conjugated protein (f) proline (g) prosthetic group (4) a protein in its normal, 18.47 Deflne (a) salt bridge, (b) prosthetic group, (h) salt bridge biologically active (c) tertiarystructure, (d) globularprotein, (e) beta- (i) conjugatedprotein form (5) pleated sheet, (f) secondarystructure, (g) alpha-helix, ionic bond between ion and and (h) primary structure. carboxylate protonated amine Distinguish among alpha keratin, beta keratin, and 18.48 group collagen by biological function and molecular struc- (6) important structural ture. protein (7) covalent bond that can hold two peptide chains together (B) protein that has a prosthetic group (9) covalentbond that holds two amino acids together
sHE"S-?ES?{REV;EEjS} True/False MultipleChoice l. Amino acids found in nature are generally the r ll. All the following amino acids have aliphatic side forms. charns except (a) (b) alanine. (c) serine. 2. At the isoelectric pH of an amino acid, its solubility in valine. (d) water is maximum. leucine. bridges can be broken by 3. A basic amino acid such as asparaginewould have a 12. Disulfide (a) dissolving the protein in water. net negative charge at high PH. (b) cooling the protein. 4. The amino acid glycine is the C-terminal amino acid (c) oxidizingagents. of the pentapeptide Gln-Asp-Pro-Val-Gly. (d) dehydrating agents. 5. In aqueous solutions, the hydrocarbon side chains of 13. Proteins that tend to be water soluble and generally a protein tend to point inward, avoiding interaction have spherical shapesare with thewater. (a) conjugatedproteins. (b) globularproteins. 6. Protein denaturation is always irreversible. (c) beta keratins. (d) flbrous proteins. 7. The amino acid proline disrupts the alpha-helical 14. Which ofthe following contributes most importantly secondary structure of a Protein. to the secondary structure of a protein? B. Myoglobin is used for oxygen storagein the muscle. (a) hydrogen bonds (b) disttlfide bridges 9. Proteins are composed of amino acids. (c) saltbridges (d) noneoftheabove 10. All protein molecules have a quaternary structure. 580 CHAPTERl8 Amino Acidl Peptides,and proteins
15. A disruption of the secondary and tertiary structures 24. A molecule of the amino acid valine in a strongly acid of a protein solution would be a (a) is never reversible. (a) zwitterion. (b) is causedonlybyheat. (b) negatively charged species. (c) results in a conjugated form. (c) positively charged species. (d) is called deneturation. (d) neutral molecule. 16. The peptide Phe-GIu-Ala-Val 25. \iVhich of these is r?otusuallydisrupted in the denatu- (a) has valine at the N-terminal end. ration of a protein? (b) containsfour peptide bonds. (a) ionicbonds (b) hydrogenbonds (c) is the same as the peptideVal-Ala-Glu-Phe. (c) peptidebonds (d) hydrophobicaggregation (d) has four amino acid residues. 26. The major type of hemoglobin of arterialblood is 17. TWocommonly occurring peptide secondary struc- (a) cyanohemoglobin. (b) oxyhemoglobin. tures in proteins are (c) methemoglobin. (d) deoxyhemoglobin. (a) alpha helix and beta-pleated sheet. 27. The correct general formula for a dipeptide at neutral (b) protoflbril and globular. pH is (c) globular and beta-pleatedsheet. (d) protofibril and alphahelix. (a)oHR lB. The most abundant protein in higher vertebrates is ll ,, I I (a) collagen. (b) oxytocin. (c) (d) ,rC.'.'|'r,N.r rrC alpha keratins. hemoglobin. H,N--|il C C OH 19. Amino acids found in biological systemsare usually (a) in the left-handed, or r, form. RO (b) in the o form. (b)RH (c) found in an uncombined state. (d) beta amino acids. ,'+'.,JJ .?r'( .o 20. The heme group in a hemoglobin molecule ,.*''I'f; (a) is absent in sickle cell anemia. (b) is called a prostheticgroup. OR (c) is held jointly by the four peptide subunits. (c) RR (d) can never contain Fe(III) ions. 21. The complexpresentin carbon monoxide poisoning ,r9... ,r9... ,-*n. is H,N-rifilf (a) cyanohemoglobin. (b) deoxyhemoglobin. (c) carboxyhemoglobin. (d) methemoglobin. 22. Myoglobin, a globular, conjugated protein, (d)HR (a) has most rrl I of its aliphatic side chains on the exte- HOrC\'il,Nr rior surface of the molecule. ,r9., (b) would be expectedto be water soluble. C CJNH, I l'n (c) carries oxygen from the lungs to the tissues. ROH (d) has no prosthetic group. 'tVhich 23. A structural protein found in mammalian hair is type of bonding holds proteins in the beta- (a) alpha keratin. (b) collagen. pleated sheet? (c) beta keratin. (d) fibroin. (a) ionicbonds (b) hydrogenbonds (c) hydrophobicaggregation (d) covalentbonds