Aminoácidos: Estrutura, Classificação e Propriedades - Bioquímica

Aminoácidos Figure 41 General structural formula for αamino acids There are 20 different R groups in the commonly occurring amino acids Table 41 Figure 42 Zwitterionic form of the αamino acids that occurs at physiological pH values Table 41 Covalent Structures and Abbreviations of the Standard Amino Acids of Proteins Their Occurrence and the pK Values of Their Ionizable Groups Name ThreeLetter Symbol and OneLetter Symbol Structural Formulaa Residue Mass Db Average Occurrence in Proteins c pK1 αCOOHd pK2 αNH3d pKR Side Chaine Amino acids with nonpolar side chains Glycine Gly G COO HCH NH3 570 71 235 978 Alanine Ala A COO HCCH3 NH3 711 83 235 987 Valine Val V COO CH3 HCCH NH3 CH3 991 69 229 974 Leucine Leu L COO CH3 HCCH2CH NH3 CH3 1132 97 233 974 Isoleucine Ile I COO CH3 HC CCH2CH3 NH3 H 1132 60 232 976 Methionine Met M COO HCCH2CH2SCH3 NH3 1312 24 213 928 Proline Pro P COO C3H2 N CH2 H2 971 47 195 1064 Phenylalanine Phe F COO HCCH2 NH3 1472 39 220 931 Tryptophan Trp W COO HCCH2 NH3 1862 11 246 941 continued Figure 44 Structures of the αamino acids alanine glutamine and phenylalanine The amino acids are shown as ballandstick models embedded in their transparent spacefilling models The atoms are colored according to type with C green H white N blue and O red pI 12 pKi pKj 41 pH 12 10 8 6 4 2 0 0 05 10 15 20 H ions dissociatedmolecule Figure 46 Titration curve of glycine Other monoamino monocarboxylic acids ionize in a similar fashion After Meister A Biochemistry of the Amino Acids 2nd ed Vol1 p30 Academic Press 1965 See the Animated Figures Amino acids with uncharged polar side chains Serine COO 871 65 219 921 Ser HCCH2OH S NH3 Threonine COO 1011 53 209 910 Thr HCCCH3 T NH3 OH Asparaginef COO 1141 40 214 872 Asn HCCH2C N N NH3 NH2 Glutaminef COO 1281 39 217 913 Gln HCCH2CH2C NH3 NH2 Tyrosine COO 1632 29 220 921 1046 phenol Tyr HCCH2 OH NH3 Y Cysteine COO 1031 14 192 1070 837 sulfhydryl Cys HCCH2SH NH3 C Amino acids with charged polar side chains Lysine COO 1282 59 216 906 1054 εNH3 Lys HCCH2CH2CH2CH2NH3 NH3 K Arginine COO 1562 55 182 899 1248 guanidino Arg HCCH2CH2CH2NHC NH3 NH2 NH2 R Histidinef COO 1371 23 180 933 604 imidazole His HCCH2 NH NH3 1 2 3 4 5 Aspartic acidf COO 1151 54 199 990 390 βCOOH Asp HCCH2C NH3 O D Glutamic acidf COO 1291 68 210 947 407 γCOOH Glu HCCH2CH2C NH3 O E Glu Lys Figure 49 Greek lettering scheme used to identify the atoms in the glutamyl and lysyl R groups Figure 411 Schematic diagram of a polarimeter This device is used to measure optical rotation Figure 412 Fischer convention configurations for naming the enantiomers of glyceraldehyde Glyceraldehyde enantiomers are represented by geometric formulas top and their corresponding Fischer projection formulas bottom Note that in Fischer projections all horizontal bonds point above the page and all vertical bonds point below the page The mirror planes relating the enantiomers are represented by a vertical dashed line Fischer projection formulas as traditionally presented omit the central C symbolizing the chiral carbon atom The Fischer projection formulas in this text however will generally have a central C Figure 414 CORN crib mnemonic for the hand of Lamino acids Looking at the Cα atom from its H atom substituent its other substituents should read CORN in the clockwise direction as shown Here CO R and N respectively represent the carbonyl group side chain and main chain nitrogen atom After Richardson JS Adv Protein Chem 34 171 1981 4Hydroxyproline eNNNTrimethyllysine 3Methylhistidine 5Hydroxylysine OPhosphoserine γCarboxyglutamate eNAcetyllysine ωNMethylarginine NAcetylserine NNNTrimethylalanine NFormylmethionine Figure 424 Some uncommon amino acid residues that are components of certain proteins All of these residues are modified from one of the 20 standard amino acids after polypeptide chain biosynthesis Those amino acid residues that are derivatized at their Nα position occur at the Ntermini of proteins γAminobutyric acid GABA Histamine Dopamine Thyroxine Citrulline Ornithine βCyanoalanine Homocysteine Azaserine SAdenosylmethionine Figure 425 Some biologically produced derivatives of standard amino acids and amino acids that are not components of proteins Fluorescence Native Protein Apoprotein Denatured Protein Tryptophan in pH2 protein absorption Wavelength nm Multiphoton Excitation Fluorescence Microscopy Laser Pulses Microscope Objective Front Lens Coverslip Cells in Culture Focal Point Microscope Slide Laser Pulses Figure 2 Tryptophan Multiphoton Absorption λnm hveV Tryptophan 200 60 250 50 300 40 400 30 600 20 800 10 Single Photon 45 eV Transition Two Photon Three Photon Figure 3 CH2CH COO NH3 N H Emission 348 nm 3P 2P 1P Excitation Tyrosinase COOH NH2 O2 COOH NH2 Tyrosine Dopaquinone DQ Cysteine Tyrosinase O2 HO HO COOH NH2 HO COOH NH2 Cyclodopa Tyrosinase O2 HO HO COOH NH2 HO COOH NH2 Dopa Dopachrome 5SCysteinyldopa 5SCD 2SCysteinyldopa 2SCD DO DO Dopa CDquinones Dopachrome tautomerase Tyrp2 HO CO2 HO COOH DHICA 56Dihydroxyindole 56Dihydroxyindole2Carboxylic acid DHI 14Benzothiazine Intermediates O Pheomelanin DQ O2 Tyrp1 DHICA oxidase Or Tyrosinase Eumelanin O OH H COOH N H HO COOH HO COOH O N H O OH O N H S HOOC H2N OH N COOH HOOC S HOOC N N OH S H2N COOH