* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 18,5 Primory structure of proteins 18.6 Secondory stractare of proteins
Ancestral sequence reconstruction wikipedia , lookup
Amino acid synthesis wikipedia , lookup
Gene expression wikipedia , lookup
Point mutation wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Expression vector wikipedia , lookup
Magnesium transporter wikipedia , lookup
Biosynthesis wikipedia , lookup
Genetic code wikipedia , lookup
Interactome wikipedia , lookup
Metalloprotein wikipedia , lookup
Protein purification wikipedia , lookup
Peptide synthesis wikipedia , lookup
Western blot wikipedia , lookup
Two-hybrid screening wikipedia , lookup
Protein–protein interaction wikipedia , lookup
Biochemistry wikipedia , lookup
Ribosomally synthesized and post-translationally modified peptides wikipedia , lookup
I8.6 Secondary Structureof Proteins 565 Fouow-upro rHECnsrln Polnr: Runner'shish Karla sometimes experiences runner's high during her runs. \Alhat is causing this effect? Many sports physiologists believe that runner's high comes from increases in the levels of enkephalins in the brain. These increases, which remarkabie feel"* "urrr" ings of well-belng, appear to be stimulated by endurance exercises such as distance running. The effects of enkephalins appear to last beyond the period of exercise to increase all-around mental alertness and a sense of comfort. Exercise is a good thiag, but like most good things, it can be overdone, since the body may suffer from exhaustion and other breakdornmswhen overtaxed without sufficient time to recover. Enkephalins may be responsrble in part for the potentially harmful addiction of some people IO eXCeSStVe exerclse_ 18,5Primorystructureof proteins AIM: To chorocterizethe primory structureof o protein Focus The primary structure is the first level of organization of a protein. Protein proteios(Greek): offirst importance INhen the number of amino acid residuesbecomesgreater than about 40, a naturally occurringpeptide is called aprotein. On average,a peptide molecule containing 100amino acid residueshas a molar mass of about 10,000 g. Proteins are so vitai to living organisms that many of the remaining topics in this book concernthem. The structuresof proteins areusuallystudied at four levels of organization: primary structlffe, secondarystructure, tertiary structure, and quaternary structure. The prirnary structure of a peptide or protein is the order in which the amino acid residuesof a peptide or protein molecule are linked by peptide bonds. The primary structure of a protein molecule alsoincludesany disulfidebridgesthat the molecule contains.Iust aswe sawfor peptidesin the precedingsection,differencesin the chemicaland biologicalpropertiesof proteins result from differencesin the order of amino acidsin the polypeptide chain (primary structure).The secondary tertiary and quaternary structuresof proteins are discussedin the following three sections. 18.6Secondory stractareof proteins AIMS: To describethe bonding ond structurein the following secondarystructuresof proteins: alpho helix, beto-pleoted sheet,ond collogenhelix. Todistinguishomong the following fibrous proteins: olpho keratin, beto kerotin, ond collagen. The alpha heli5 beta-pleated sheet, and collagen helix are three types of secondary structures found in proteins. The polypeptide chains of many proteins contain specific,repeating patterns of folding of the peptide backbone. Thesespecific,repeating patterns of folding of the peptide backboneconstitute the protein'ssecondary structure. Three commonly found patterns are the alpha helix, t]r.ebeta-pleated sheet,andthe collagen helix. 564 l8 Amino Acids,Peptides,and Proteins CHAPTER Alpha helix In someproteins, regionsof the backboneof the peptide chain are coiled into a spiral shape called an alplaalnelix, similar to a corkscrew.As Figure lB.3 shows, a corkscrew must be turned in a right-handed, or clockwise, direction to penetrate a cork. The alpha helixes of proteins are always righthanded. The helixes are held together by hydrogen bonds, shornmin Figure 18.4,formed between the hydrogen of an N-H of a peptide bond and the carbonyl oxygen of another peptide bond group four residues away in the samepeptidechain. The tightnessof coiling is such that 3.6 amino acid residuesof the peptide backbone make each ftrll turn of the alpha helix. There are no amino acid residue side chains inside the alpha helix; they are located on the outside. The cyclic amino acid proline does not fit well into the peptide backbone of alpha helixes. Alpha helixes in long protein chains often end at a place where proline residues occur in the primary structure. Proline is Figute18.5 A corkscrewmust be turned in a right-handed,or clockwise,direction to penetratea cork. t To N-terminal I Figute18.4 A peptide chain twisted into a right-handedalpha helix constitutes a protein'ssecondarystructure.The N-terminalto C-terminaldirection is from top to bottom. The dotted lines show the hydrogenbonds betweenthe carbonylorygen of one amino acid residueand the N-H hydrogenof another,four amino acid residuesfurther down the chain. t; 18.6Secondary Structure of Proteins (a) Helical peptide chain (b) Alpha keratin Figure18.5 Threehelicalpeptidechains,like the one shown in (a), are twisted or supercoiled to form a rope in alpha keratin(b). 565 sometimes called an alpha helix disrupter for this reason. The inability of proline to fit into the alpha helix is one of many piecesof euidencethai the secondarystructure of a protein is determined by its primary structure. Much of what is knolrrn about protein alpha helixes comes from studies of fibrous proteins. Fibrous proteins tend to be long, rod-shapedmolecules with great mechanical strength. Such proteins are usually insoluble in water, dilute salt solutions, and other solvents. The polypeptide chains of a class of fibrous proteins called alpha keratins consist mainly of alpha helixes.Alpha keratins form the hard tissue of hooves, horns, outer skin layer (epidermis), hair, wool, and nails of mammals. We see in Figure lB.5 complexprotein molecules consisting of long alpha helixes that are twisted together-supercoiled-in ropes of three or seven strands. It takes many supercoiled ropes to make a strong but elastic wool fiber. If you have ever washed a wool sweater, you know that warm, wet wool fibers can be stretched,but they eventually return to their original length. This is because the alphahelixes of the damp fibers are easilypulled into an extended form. The extended form is less stable than the alpha helix. It will, in time, return to the original alpha helix. Disulfide bridges (seeSec.lB.1) between alpha helixes help to make alpha keratins rigid; the alpha keratins of a hard hoof have more disulfide linkages than relatively elastic epidermis. Beta-pleated sheet Figure l8.6 showsthe beta-pleatedsheet,another kind of secondarystructure commonly found in proteins. Thebeta-pleated sheet consistsof peptide chains arranged side by side toform a structure that resemblesa pieceof paper folded into many pleats. The carbonyl groups of one zigzagpeptide backbone are hydrogen bonded to peptide N-H hydrogens of adjacent peptide chains that run in opposite directions in the N-terminal to C-ter- Figure18.6 A beta-pleated sheetis another kindof secondary structure. Hydrogenbonds(shownasdottedlines) holdadjacentstrandsof the sheet together. C-terminal t-= N-terminal ':a:-- -a. @ carbon @ o*yg"n @ uirrog".r $ ;g Hydrogen sidectrain I Hydrogenbond 566 l8 Amino Acids,Peptides,and Proteins CHAPTER Figure18.8 Silkfibroinconsistsof stackedbeta-pleated sheets.The smallR groupsof glycyland alanylresidues permit the stacking to occur. Figure18.7 Spiderwebs are made of fibroin,a protein that existsmainly as betapleatedsheets. . ; .Jl'_' ::: minal sense.Beta-pleatedsheetsare formed by separatestrandsof protein or by a single chain looping back on itself. Beta keratin s are a classof fibrous proteins that consist mainly of betapleated sheets.The long, thin fibers secretedby silkworms and spiders (Fig' 18.7)are composedof fibroin, a well-studied beta keratin. Fibroin consists mainly of layersof beta-pleatedsheets,as shornrnin Figure lB.B.Again, the primary structure of a protein determinesits secondarystructure.Fibroin peptide chains are particularly rich in glycyl and alanyl residues.The small side chains of these residuesare important to the organization of fibroin, since larger side chains would interfere with the packing of the sheetsin layers. Collagen helix Collagenis yet another kind of fibrous protein. Collagen is the most abundant protein in human beingsand many otheranimals with spinal columns (uertebrates). About one-third of the protein in the human body is present as the collagenof bones,teeth, inner skin layer (dermis),tendons,and cartilage.The inner material of the eye lens is almost pure collagen.Collagen occurs in all organs,where it imparts strength and stiffness. Collagenis formed from three peptide chains,each a helix, wound into a rope.Thereare important differencesbetweenalpha helixesand collagen helixes,however.Collagenis rich in proline (40To),which does not fit into regular alpha helixes, and collagen helixes are not as tightly coiled as alpha helixes. Unlike alpha helixes, which are right-handed, collagen helixes are left-handed. Collagen contains large quantities of glycine (35%) and the unusual amino acids4-hydroxlproline (5%)and S-hydroxylysine(1%). OH I H9-9u, tt cHr. .cH-co2H N ?' H2N-CH2-CH-CH2 -C"r-f I -n co2H H 4-Hydroxyproline T"' 5-Hydroxylysine