Bài giảng Biochemistry 2/e - Chapter 4: Amino Acids

CHAPTER 4Amino Acidsto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline4.1: Amino Acids: Building Blocks of Proteins4.2: Acid-Base Chemistry of Amino Acids4.3: Reactions of Amino Acids4.4: Optical Activity and Stereochemistry of Amino Acids4.5: Spectroscopic Properties of Amino Acids4.6: Separation and Analysis of a.a. MixturesAmino AcidsBuilding Blocks of ProteinsAmino Acids Can Join Via Peptide Bonds20 Common Amino AcidsYou should know names, structures, pKa values, 3-letter and 1-letter codesNon-polar amino acidsPolar, uncharged amino acidsAcidic amino acidsBasic amino acidsUncommon Amino AcidsWe'll see some of these in later chaptersHydroxylysine, hydroxyproline - collagenCarboxyglutamate - blood-clotting proteinsPyroglutamate - bacteriorhodopsinPhosphorylated amino acids - signaling device4.2 Acid-Base ChemistryAmino Acids are Weak Polyprotic AcidsH2A+ + H2O  HA0 + H3O+Ka1 = [ HA0 ] [ H3O+ ] __________________________ [H2A+ ]4.2 Acid-Base ChemistryThe second dissociation (the amino group in the case of glycine):HA0 + H2O  A¯ + H3O+Ka2 = [ A¯ ] [ H3O+ ] _______________________ [ HA0 ]pKa Values of the Amino AcidsYou should know these numbers and know what they mean!Alpha carboxyl group - pKa = 2Alpha amino group - pKa = 9These numbers are approximate, but entirely suitable for our purposes.pKa Values of the Amino AcidsYou should know these numbers and know what they meanArginine, Arg, R: pKa(guanidino group) = 12.5Aspartic Acid, Asp, D: pKa = 3.9Cysteine, Cys, C: pKa = 8.3Glutamic Acid, Glu, E: pKa = 4.3Histidine, His, H: pKa = 6.0pKa Values of the Amino AcidsYou should know these numbers and know what they meanLysine, Lys, K: pKa = 10.5Serine, Ser, S: pKa = 13Threonine, Thr, T: pKa = 13Tyrosine, Tyr, Y: pKa = 10.1Titration of GlycineTitration of Glutamic AcidA Sample CalculationWhat is the pH of a glutamic acid solution if the alpha carboxyl is 1/4 dissociated?pH = 2 + log10 [1] ¯¯¯¯¯¯¯ [3]pH = 2 + (-0.477)pH = 1.523Titration of LysineAnother Sample CalculationWhat is the pH of a lysine solution if the side chain amino group is 3/4 dissociated?pH = 10.5 + log10 [3] ¯¯ ¯¯¯¯¯ [1]pH = 10.5 + (0.477)pH = 10.977 = 11.0Reactions of Amino AcidsCarboxyl groups form amides & estersAmino groups form Schiff bases and amidesSide chains show unique reactivitiesCys residues can form disulfides and can be easily alkylatedFew reactions are specific to a single kind of side chainStereochemistry of Amino AcidsAll but glycine are chiralL-amino acids predominate in natureD,L-nomenclature is based on D- and L-glyceraldehydeR,S-nomenclature system is superior, since amino acids like isoleucine and threonine (with two chiral centers) can be named unambiguouslySpectroscopic PropertiesAll amino acids absorb in infrared regionOnly Phe, Tyr, and Trp absorb UVAbsorbance at 280 nm is a good diagnostic device for amino acidsNMR spectra are characteristic of each residue in a protein, and high resolution NMR measurements can be used to elucidate three-dimensional structures of proteinsSeparation of Amino AcidsMikhail Tswett, a Russian botanist, first separated colorful plant pigments by ‘chromatography’Many chromatographic methods exist for separation of amino acid mixturesIon exchange chromatographyHigh-performance liquid chromatography