2 Структура и функция белка (1160071), страница 6
Текст из файла (страница 6)
This is a condensation reaction. The a-amino group of amino acid 2 acts as anucleophile (see Table 3-6) to displace the hydroxylgroup of amino acid 1 (red). Amino groups are goodnucleophiles, but the hydroxyl group is a poor leaving group and is not readily displaced.
At physiological pH the reaction as shown does not occur toany appreciable extent. Peptide bond formation isendergonic, with a free-energy change of about+21 kJ/mol.OHQH3 CH3CH-ICHoOH H HIIH 3 N—CH1I IH CH3I IH CH2I IC—N—C—I IIC—N—C—C—N—C—C—N—C—COCTOJlAminoterminal endH1 O1HI OIHI OIIHCarboxylterminal endFigure 5-16 Structure of the pentapeptide serylglycyltyrosinylalanylleucine, or Ser-Gly-Tyr-AlaLeu. Peptides are named beginning with theamino-terminal residue, which by convention isplaced at the left.
The peptide bonds are shownshaded in gray, the R groups in red.126Part II Structure and CatalysisPeptides Can Be Distinguished by Their Ionization BehaviorAlanylglutamylglycyllysineNH 3AlaCH—CH30=CN—HIGluGlyCH—CH2—CH2—COO"~~lN—HICH 2IIN—HLysPeptides contain only one free a-amino group and one free a-carboxylgroup (Fig. 5-17).
These groups ionize as they do in simple aminoacids, although the ionization constants are different because the oppositely charged group is absent from the a carbon. The a-amino anda-carboxyl groups of all other constituent amino acids are covalentlyjoined in the form of peptide bonds, which do not ionize and thus do notcontribute to the total acid-base behavior of peptides. However, the Rgroups of some amino acids can ionize (Table 5-1), and in a peptidethese contribute to the overall acid-base properties (Fig. 5-17).
Thusthe acid-base behavior of a peptide can be predicted from its singlefree a-amino and a-carboxyl groups and the nature and number of itsionizable R groups. Like free amino acids, peptides have characteristictitration curves and a characteristic isoelectric pH at which they do notmove in an electric field. These properties are exploited in some of thetechniques used to separate peptides and proteins (Chapter 6).CH—CH 2 —CH 2 -CH 2 -CH 2 -NH 3COO"Peptides Undergo Characteristic Chemical Reactions(a)AlanylalanineCH 3H CH3H3N—CH—C—N—CH—COOHOCationic form (below pH 3)HCH,CH 3Like other organic molecules, peptides undergo chemical reactionsthat are characteristic of their functional groups: the free amino andcarboxyl groups and the R groups.Peptide bonds can be hydrolyzed by boiling with either strong acid(typically 6 M HC1) or base to yield the constituent amino acids.HH HII I-C —C—N—C— COOI 1 II2R ORH,0HC-H3N—C—COO-1R2RH3N—CH—C-N—CH—COO"OIsoelectric formH CH3CH 3H 2 N—CH-C-N—CH—COO"IIOAnionic form (above pH 10)(b)Figure 5-17 Ionization and electric charge of peptides.
The groups ionized at pH 7.0 are in red.(a) A tetrapeptide with two ionizable R groups.(b) The cationic, isoelectric, and anionic formsof a dipeptide lacking ionizable R groups.Hydrolysis of peptide bonds in this manner is a necessary step in determining the amino acid composition of proteins. The reagents shown inFigure 5-14 label only free amino groups: those of the amino-terminalresidue and the R groups of any lysines present. If dabsyl chloride,dansyl chloride, or l-fluoro-2,4-dinitrobenzene is used before acid hydrolysis of the peptide, the amino-terminal residue can be separatedand identified (Fig.
5-18).Peptide bonds can also be hydrolyzed by certain enzymes calledproteases. Proteolytic (protein-cleaving) enzymes are found in allcells and tissues, where they degrade unneeded or damaged proteins oraid in the digestion of food.Some Small Polypeptides Have Biological ActivityMuch of the material in the chapters to follow will revolve around theactivities of proteins with molecular weights measured in the tens andeven hundreds of thousands. Not all polypeptides are so large, however.
There are many naturally occurring small polypeptides and oligopeptides, some of which have important biological activities and exerttheir effects at very low concentrations. For example, a number of vertebrate hormones (intercellular chemical messengers) (Chapter 22) aresmall polypeptides. The hormone insulin contains two polypeptidechains, one having 30 amino acid residues and the other 21. Otherpolypeptide hormones include glucagon, a pancreatic hormone of 29residues that opposes the action of insulin, and corticotropin, a 39-127CH 3 CH 3CH 3 CH 3NDabsylchlorideDabsylaminoacidFigure 5-18 The amino-terminal residue of a tetrapeptide can be identified by labeling it with dabsyl chloride, then hydrolyzing the peptide bonds instrong acid.
The result is a mixture of amino acidsof which only the amino-terminal amino acid (andlysine) is labeled.Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg(a)6 M HC1, 110 °C, 24 hS SI~1Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH22R -CH(b)CO/=NTetrapeptideDabsylC=O PePtidea-AminoacidsH2CNH0"COCT-CH2COO"CH2—CH2HOHc=oIPyroglutamateresidue hormone of the anterior pituitary gland that stimulates theadrenal cortex.Some biologically important peptides have only a few amino acidresidues. That small peptides can have large biological effects is readily illustrated by the activity of the commercially synthesized dipeptide, L-aspartylphenylalanyl methyl ester. This compound is an artificial sweetener better known as aspartame or NutraSweet®:His(c)NH2ProlinamideTyr-Gly-Gly-Phe-MetTyr-Gly-Gly-Phe-Leu(d)D-Phe —* L-Leu —> L-Orn —» L-Val —* L-ProtiL - P r o <— L-Val <— L - O r n <— L - L e u <— D - P h eH3N-CH-C-N-CH-C-OCH3HL-Aspartyl-L-phenylalanyl methyl ester(aspartame)Among naturally occurring small peptides are hormones such asoxytocin (nine amino acid residues), which is secreted by the posteriorpituitary and stimulates uterine contractions; bradykinin (nine residues), which inhibits inflammation of tissues; and thyrotropin-releasing factor (three residues), which is formed in the hypothalamus andstimulates the release of another hormone, thyrotropin, from the anterior pituitary gland (Fig.
5-19). Also noteworthy among short peptidesare the enkephalins, compounds formed in the central nervous system(e)Figure 5-19 Some naturally occurring peptideswith intense biological activity. The amino-terminalresidues are at the left end. (a) Bradykinin, a hormonelike peptide that inhibits inflammatory reactions, (b) Oxytocin, formed by the posterior pituitary gland.
The shaded portion is a residue ofglycinamide (H2N—CH2—CONH2). (c) Thyrotropinreleasing factor, formed by the hypothalamus.(d) Two enkephalins, brain peptides that affect theperception of pain, (e) Gramicidin S, an antibioticproduced by the bacterium Bacillus brevis. The arrows indicate the direction from the amino towardthe carboxyl end of each residue. The peptide hasno termini because it is circular. Orn is the symbolfor ornithine, an amino acid that generally does notoccur in proteins.
Note that gramicidin S containstwo residues of a D-amino acid (D-phenylalanine).128Part II Structure and CatalysisBOX 5-2Chemical Synthesis of Peptides and Small ProteinsMany peptides are potentially useful as pharmacological reagents, and their synthesis is of considerable commercial importance. There are three waysto obtain a peptide: (1) purification from tissue, atask often made difficult by the vanishingly lowconcentrations of some peptides; (2) genetic engineering; or (3) direct chemical synthesis.
Powerfultechniques now make direct chemical synthesis anattractive option in many cases. In addition to commercial applications, the synthesis of specific peptide portions of larger proteins is an increasinglyimportant tool for the study of protein structureand function.The complexity of proteins makes the traditional synthetic approaches of organic chemistryimpractical for peptides with more than four or fiveamino acids. One problem is the difficulty of purifying the product after each step, because thechemical properties of the peptide change eachtime a new amino acid is added.The major breakthrough in this technology wasprovided by R. Bruce Merrifield.
His innovationinvolved synthesizing a peptide while keeping itattached at one end to a solid support. The supportis an insoluble polymer (resin) contained within acolumn, similar to that used for chromatographicprocedures. The peptide is built up on this supportone amino acid at a time using a standard set ofreactions in a repeating cycle (Fig.
1).The technology for chemical peptide synthesishas been automated, and several commercial instruments are now available. The most importantlimitation of the process involves the efficiency ofeach amino acid addition, as can be seen by calculating the overall yields of peptides of variouslengths when the yield for addition of each newamino acid is 96.0 versus 99.8% (Table 1).
Thechemistry has been optimized to permit the synthesis of proteins 100 amino acids long in about 4days in reasonable yield. A very similar approachis used to synthesize nucleic acids (Fig. 12-38). Itis worth noting that this technology, impressive asit is, still pales when compared with biological processes. The same 100 amino-acid protein would besynthesized with exquisite fidelity in about 5 seconds in a bacterial cell.Table 1 Effect of stepwise yield on overallyields in peptide synthesisOverall yieldsof final peptide (%)when the yieldof each step is:Number ofresiduesin the finalpolypeptide96.0%99.8%11213151100664429131.79896949082Figure 1 Chemical synthesis of a peptide on asolid support.
Reactions (2) through (4) are necessary for the formation of each peptide bond.that bind to receptors in certain cells of the brain and induce analgesia(deadening of pain sensations). Enkephalins represent one of thebody's own mechanisms for control of pain. The enkephalin receptorsalso bind morphine, heroin, and other addicting opiate drugs (althoughthese are not peptides).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
