18.4 Peptides 559 18.4Peptides AIMS: Tonome ond describethe bond thot linksomino ocids together.To drow completestructirol formulos for simple peptides.To controstthe biologicol functionsof some peptide hormones. A peptide is any combination of amino acids in which the alpha amino Focus group (-NH) of one acid is united with the alpha carboxylic group Amino acids are linked to form (-CO2H) of another through an amide bond. peptides. RO RO til ril H2N-C-C-OH + H-N-C-C-OH ------- I H HH Amino acid Amino acid RO RO til ttl H2N-C-C-N-C-C-OH + H2O ttt H HH Peptide The amide bonds formed in peptides always involve the alpha amino and alpha carboxylic acid groups and never those of side chains. More amino acids maybe added in the same fashion to form chains such as those in Figure l9.l. The amide bond between the carbonyl group of one amino acid and the nitrogen of the next amino acid in the peptide chain is called a peptide bond, or peptide link. Amino acids that haue been incorporated into peptides are called amino acid residues. As more amino acid residues are added, a backbone common to all peptide molecules is formed. The amino acid residue with a free amino group at one end of the chain is the N-termi- nal residue:the residue with a free carboxvlic acid at the other end of the chain is the C-terminal residue.The number of amino acid residuesin a peptide is often indicated by a set of preflxes for peptides of up to l0 C-terminal residue Figutel8.l Partsof a peptide-in this case,a tetrapeptide.The peptide bonds of the zigzagbackbone are shown in color.Note that the C-terminalis at the right and the N-terminqlis at the left. 560 CHAPTERl8 Amino Acids,Peptides, and Proteins Tablel8.l Prefixesfor Short residues, as sho',m in Table l9.l. We call any peptide with more than 20 Peptides amino acid residuesa polypeptide. In theory the process of adding amino acids to a peptide chain may be continued indefinitely. Names of peptides Residues Prefix are derived from names of amino acid residues. By convention, names of peptides written z di- are always from Ieft to right starting with the N-terminal end; a peptide that contains N-terminal glycine, followed by a histidine, fol- J tri- - phenylalanine - - 4 tetra- lowed by C terminal is named glycyl histidyl phenylalanine. q penta- The sequenceis extremelyimportant; glycyl-histidyl-phenylalanine is a dif- 6 hexa- ferent molecule from phenylalanyl-histidyl-glycine. The methyl ester of the (see 7 hepta- dipeptide aspartyl phenylalanine is an artiflcial sweetener A Closer B octa- Look Aspartame). 9 nona- Structural formulas or even full word names for large peptides become 10 deca- very unwieldy and time-consuming to write. To simplify matters, chemists nrite peptide and protein structures by using three-letter abbreviations for the amino acid residueswith dashesto showpeptide bonds.Table 18.2 lists the abbreviations. Peptide structures written with these abbreviations Table| 8.2 Three-letter Abbreviationsfor always startwith the N-terminal group to the left and endwith the C-termi- AminoAcids nal group to the right. For example, we can u'rite glycyl-histidyl-phenylala- nine as Gly-His-Phe and phenylalanyl-histidyl- glycine as Phe-His-Gly. Amino acid Abbreviation EXAMPTElS.5 Naminga peptide alanine Ala arginine Arg Write the (a) the three-letter amino acid name and (b) the name using full asparagine Asn i" abbreviationsfor the peptide in Figure lB.1 assumingthat R1is methyl, R2 asparticacid Asp .. is isopropyl, R3is methyl, and Rnis hydrogen. cysteine cys -,.,.j glutamine Gln "\ soruiloN l.,t\ glutamic acid Glu 1.,,N' (a) The full name, starting at the N-terminal end, is alanyl-valyl-alanyl- glycine Glv glycine. histidine His (b) Using abbreviations: Ala-Val-Ala-Gly. isoleucine Ile leucine Leu Iysine Lys methionine Met PRACTICEEXERCISE I8.4 phenylalanine Phe Glutathione, a tripeptide that is widely distributed in all living tissues,is proline Pro named glutamyl-cysteinyl-glycine.(a) Draw the complete structural serine Ser formula for this peptide. (b) Write the amino acid sequenceof the pep- threonine Thr tide using the three-letter abbreviations. tryptophan Trp tyrosine T1'r valine Val PRACTICEEXERCISE I8.5 Write three-letter amino acid sequencesfor all possible tripeptide structures that contain one residue each of glutamic acid, cysteine,and glycine. Chemists have isolated over 200 peptides important to the smooth functioning of the human body. The peptide hormones oxltocin and vaso- pressin are two examples showing that apparently minor differences in the order of amino acid residues can result in profoundly different biological actions. 18.4 Peptides 561 Aspartame,an artificial sweetenerwith the brand name NutraSweet@,is ihe methyl ester of the dipeptide asparrylphenylalanine. o cH2-cH-NH-C-CH-NH2 Aspartameis a substltutefor co2cH3 cH2co2H sucroseand Aspartylphenylalanine methyl ester (aspartame) ,,, saccharin. , Asparl.ameis more than 50 times sweeter than sweetness.Methanol is a toxic alcohol,and there is sucrose.Now approved for use in more than 30 disagreement among scientists whether the countries,this product of amino acid chemistry breakdornmof the small amounts of aspartame has found wide acceptancein the food industry as used for sweeteningproduces sufficient amounts a substitute for both sucroseand saccharin (see of methanol to causeharm. Food scientistshave figure). The main advantage of aspartame over found that aspartame'sstability is improved by saccharinis its taste,whicfr is very similar to that using it in combination with saccharin.This com- of cane sugar.Itschief disadvantageis its instabil- bination is now used in a number of soft drinks. ity.Aspartame is not recommendedfor the prepa- Products that contain aspartame,however, must ration of foods in which cooking iemperatures carry a warning label for people who suffer from exceed150'C. High temperaturesand extremesof the hereditary diseasephenylketonuria. People pH can causeaspartame in solution to hydrolyze who have phenylketonuria are unable to break to the unesterified dipeptide, aspartylphenylala- dovrn phenylalanine and must therefore limit its nine and methanol,with a simultaneousloss of inLake,as we will seein Section26.8. Oxytocin and vasopressin are formed in the hypothalamus (pituitary gland) and enter the bloodstream.Each hormone is a nonapeptide (con- tains nine amino acid residues)with six of the amino acid residuesdrar,rm into a loop by a disulfide bond. The disulfide bond is formed by the cou- pling of cysteineresidues in the first and sixth positions of their peptide chains, as shown in Figure I8.2. In peptides and proteins, disulfide bonds formed between two cysteine-SH groups that draw a single peptide chain into a loop or hold two peptide chains togetherare called disrtlfide bridges. 2I 2l T1'r-Cys T1'r-Cys tl Figure18.2 .', Ile S 3lhe S Oxytocinand vasopressin.These ll ll 4Gln S +Gln S peptidehormones differ by two aminoacid residues (shown in Asn-Cys-Pro -Leu-GlyNH, Asn-Cys -Pro -Arg-GlyNH, color).The C-terminal residues 56789 56789 haveamide functional groups Oxytocin Vasopressin ratherthan carboxylicacid groups. 562 CHAPTERl8 Amino Acids,Peptides, and Proteins Although the amino acid composition differs at only the third and eighth positions of their peptide chains (counting from the N-terminal end), the biological roles of these two peptides are different. Oxltocin stim- ulates milk ejection in females and contraction of the smooth muscle of the uterus in labor. Oxytocin has been called the "cuddle drug," becausein females it stimulates sensations during lovemaking and produces feelings of relaxed satisfaction and attachment. Vasopressinis an antidiuretic-it helps to maintain a proper water balance in both sexesby helping to retain water Defective production of vasopressinresults in diabetes insipidus, characterizedby the production of massivevolumes of urine. Injections of the hormone control the volume of urine produced. Another example of how the sequence of amino acid residues affects biological function can be seen when we compare the blood pressure- controlling activities of two peptides, bradykinin and boguskinin. Bradykinin is a nonapeptide formed directly in the bloodstream when a fragment is chopped from a large protein, u-2-globulin. Boguskinin is a synthetic octapeptide that lacks only the proline residue at position 7 of bradykinin. Arg-Pro -Pro -Gly-Phe - Ser- Pro- Phe-Arg I23456789 Bradykinin Arg-Pro -Pro - Gly-Phe - Ser-Phe -Arg 12345678 Bogusklnin Bradykinin is partially responsible for triggering pain, welt formation (asin scratches),movement of smooth muscle,and lowering of blood pres- sure. Blood pressureis lowered when, in responseto a signal,bradykinin and related peptides relax muscles of blood vesselwalls. Blood vessels dilate, or expand, and blood flows into the expanded volume, lowering blood pressure.Less than I g,gof bradykinin lowers blood pressurein an average-sizedadult. Boguskinin, on the other hand, is completely inac- tive-hence the name bogus,meaning "false." Parts of the brain contain enkephalins-peptides inuolued withfeelings of emotion and sensation of pain. TWo major enkephalins are methionine enkephalin and leucine enkephalin, which differ in structure by only one amino acid residue: Tyr-Gly-Gly-Phe-Met T1'r-Gly-Gly-Phe-Leu Methionine enkephalin Leucine enkephalin These two pentapeptides are messengersin brain processesassociated with emotional euphoria and relief of pain-the sameprocesses affected by morphine, heroin, and other opiate drugs.Researchers hope that adminis- tration of these peptides, or similar slmthetics,will bring relief to people with chronic pain without the danger of addiction. Karla, the jogger in the Casein Point earlierin this chapter,experiences the effectsof thesepeptide hormones during her workouts..