Hemoglobina e Mioglobina - Estrutura Funcao e Transporte de Oxigenio

Hemoglobina The observation of crystalline hemoglobin was first reported by Friedrich Hünefeld in 1840 and by 1909 Edward Reichert and Amos Brown had published a photographic atlas of hemoglobin crystals from several hundred species In contrast it was not until 1926 that crystals of an enzyme those of jack bean urease were first reported Hemoglobin was one of the first proteins to have its molecular mass accurately determined The first protein to be characterized by ultracentrifugation The first to be associated with a specific physiological function that of oxygen transport and in sicklecell anemia the first in which a point mutation was demonstrated to cause a single amino acid change The first protein Xray structures to be elucidated were those of hemoglobin and myoglobin Hemoglobina Proteína Honorária Não é um tanque de oxigênio mas uma sofisticada estrutura funcional que transporta e entrega oxigênio de forma modulada com o pH da sangue e outras condições de regulação Mioglobina Reserva O2 mamíferos aquáticos Fornece oxigênio rapidamente para o músculo em atividade difusão lenta a partir dos capilares Metabolismo NO 65kD heterotetrâmero α₂β₂ Figure 103 Oxygendissociation curves of Mb and of Hb in whole blood The normal sea level values of human arterial and venous pO₂ values are indicated The dashed line is a hyperbolic O₂dissociation curve with the same p₅₀ as Hb 26 torr See the Animated Figures Figure 104 Hill plots for Mb and purified stripped Hb Note that this is a loglog plot Hence the horizontal axis logYO₂1 YO₂ 0 occurs where YO₂1 YO₂ 1 and pO₂ p₅₀ logYO₂1 YO₂ n log pO₂ n log p₅₀ O Papel da Globina Microambiente Previne autooxidação do oxiheme Possibilita modulação da afinidade pelo oxigênio molecular pelo pH do sangue Efeito Bohr pCO2 e Cl e 23 difosfoglicerato Figure 106 Effect of pH on the O2dissociation curve of Hb the Bohr effect The vertical dashed line indicates the pO2 in actively respiring muscle tissue After Benesch RE and Benesch R Adv Prot Chem 28 212 1974 See the Animated Figures Figure 108 The effects of BPG and CO2 both separately and combined on hemoglobins O2dissociation curve compared with that of whole blood red curve In the Hb solutions which were 01M KCl and pH 722 pCO2 40 torr and the BPG concentration was 12 times that of Hb The blood had pCO2 40 torr and a plasma pH of 740 which corresponds to a pH of 722 inside the erythrocyte After Kilmartin JV and RossiBernardi L Physiol Rev 53 884 1973 See the Animated Figures Figure 1011 Structure of sperm whale myoglobin Its 153 C positions are numbered from the Nterminus and its eight helices are sequentially labeled A through H The last half of the EF corner is now regarded as a turn of helix and is therefore designated the F helix The heme group is shown in red Also see Fig 839 Illustration Irving Geis Image from the Irving Geis Collection Howard Hughes Medical Institute Reprinted with permission Based on an Xray structure by John Kendrew MRC Laboratory of Molecular Biology Cambridge UK PDBid 1MBN See Kinemage Exercise 61 Figure 1013 The Xray structures of a deoxyHb and b oxyHb as viewed down their exact 2fold axes The C atoms numbered from each Nterminus and the heme groups are shown The Hb tetramer contains a solventfilled central channel paralleling its 2fold axis whose flanking β chains draw closer together on oxygenation compare the lengths of the doubleheaded arrows In the deoxy state His FG497b small singleheaded arrows fits between Thr 0641 xb and Pro CD244b lower right and upper left The relative movements of the two αP protomers on oxygenation large gray arrows of Fig 7 shift His FG497b to a new position between Thr C3 38x and Thr C641x See Fig 864 for a similarly viewed spacefilling model of deoxyHb Illustration Irving Geis Image from the Irving Geis Collection Howard Hughes Medical Institute Reprinted with permission Based on Xray structures by Max Perutz MRC Laboratory of Molecular Biology Cambridge UK PDBids a 2HHB and b 2MIIR See Kinamage Exercises 62 and 63 Figure 1014 The major structural differences between the quaternary conformations of a deoxyHb and b oxyHb On oxygenation the α1β1 shaded and α2β2 outline dimers move as indicated on the right as rigid units such that there is an 15 offcenter rotation of one protomer relative to the other that preserves the molecules exact 2fold symmetry Note how the position of His FG4β pentagovern changes with respect to Thr C3b Thr C6b and Pro CD2xb yellow dots at the α1β1 and α2β2 interfaces The view is from the right side relative to that in Fig 1013 Illustration Irving Geis Image from the Irving Geis Collection Howard Hughes Medical Institute Reprinted with permission Figure 1015 The heme group and its environment in the unliganded α chain of human Hb Only selected side chains are shown and the heme D propionate group is omitted for clarity The F helix runs along the left side of the drawing The close contact between the proximal His and the heme group that inhibits oxygenation of tstate hemes is indicated by a dashed red line After Gelin BR Lee AWN and Karplus M J Mol Biol 171 542 1983 PDBid 2HHB See Kinemage Exercise 64 Leu H19 136 Leu F4 83 Leu F7 86 Close contact Lys E10 61 His F8 87 Val E11 62 Leu FG3 91 His E7 58 Leu G8 101 Val FG5 93 Phe CD4 46 Tyr C7 42 Figure 1016 Triggering mechanism for the T R transition in Hb In the T form blue the Fe is 06 Å above the mean plane of the domed porphyrin ring On assuming the R form red the Fe moves into the plane of the now undomed porphyrin where it can readily bind O2 and in doing so pulls the proximal His F8 and its attached F helix with it The FeO2 bond is thereby strengthened because of the relaxation of the steric interference between the O2 and the heme See Kinamage Exercise 64 and the Animated Figures Figure 1017 The α1Cβ2FG interface of Hb in a the T state and b the R state The upper drawings show the C helix in ribbon form purple and its contacting portion of the FG region in ballandstick form colored according to atom type C green N blue and O red The dots outline the contacting van der Waals surfaces and are also colored according to atom type The lower drawings are the corresponding schematic diagrams of the α1Cβ2FG contact On a T R transformation this contact snaps from one position to the other with no stable intermediate note how in both conformations the knobs formed by the side chains of His 97β and Asp 99β fit between the grooves on the C helix formed by the side chains of Thr 38α Thr 41α and Pro 44α The subunits are joined by different hydrogen bonds in the two quaternary states Figures 1013 and 1014 provide additional structural views of these interactions Based on Xray structures by Giulio Fermi Max Perutz and Boaz Shaanan MRC Laboratory of Molecular Biology Cambridge UK PDBids a 2HHB and b 1HHO See Kinemage Exercise 65 a HbS Fibers Are Stabilized by Intermolecular Contacts Involving Val β6 and Other Residues Figure 1023 Electron micrograph of deoxyHbS fibers spilling out of a ruptured erythrocyte Courtesy of Robert Josephs University of Chicago Perhaps the most elegant model for describing cooperative ligand binding to a protein is the symmetry model of allosterism which was formulated in 1965 by Jacques Monod Jeffries Wyman and JeanPierre Changeux This model alternatively termed the MWC mode