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Chapter 8

BIOL2020 chapter 8 notes.docx

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BIOL 2020

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BIOL2020 : CHAPTER 8 – Hemoglobin, an Allosteric Protein  hemoglobin – component of red blood cells; carries oxygen from lungs to tissues and contributes to transport of carbon dioxide and hydrogen ions back to lungs  myoglobin – located in muscles; facilitates diffusion of oxygen to cellular sites that require oxygen and provides a reserve supply of oxygen in times of need 8.1 Hemoglobin Displays Cooperative Behaviour  cooperative effect in hemoglobin – oxygen must be transported in blood from lungs where partial pressure of oxygen is high (100torr) to tissues where partial pressure of oxygen is low (20torr)  hemoglobin – sigmoidal curve  in lungs, hemoglobin becomes nearly saturated with oxygen such that 98% of oxygen binding sites are occupied; when it moves to tissues, saturation levels drop to 32%; therefore 66% of potential oxygen binding sites release oxygen in tissues; under these conditions, myoglobin releases only 7% of its oxygen  allosteric regulators released at tissues can further enhance oxygen release 8.2 Myoglobin and Hemoglobin Bind Oxygen in Heme Groups  Myoglobin – single polypeptide chain; contains mostly -helices that are linked to one another by turns to form globular structure; recurring structure called globin fold and also seen in hemoglobin  Myoglobin – can exist in two forms o Deoxymyoglobin : oxygen free form o Oxymyoglobin : form having bound oxygen molecule  Ability for myoglobin and hemoglobin to bind oxygen depend on presence of a bound prosthetic group called heme  Heme group – gives muscle and blood their red color; consists of organic component called protoporphyrin, made of four pyrrole rings linked by methine bridges to form tetrapyrole ring; four methyl groups, two vinyl groups and two propionate side chains are attached  Iron atom lies in center of protoporphyrin, bonded to 4 pyrrole nitrogen atoms; normal conditions, iron in ferrous (Fe ) oxidation state; iron ion can form 2 additional bonds, one on th th each side of heme plane (binding sites called 5 and 6 coordination sites)  In myoglobin and hemoglobin, 5 coordination site occupied by imidazole ring of histidine residue of protein – histidine residue called proximal histidine th  In deoxyhemoglobin and deoxymyoglobin – 6 coordination site remains unoccupied  Iron ion likes approximately 0.4Å outside porphyrin plane since iron ion in this form is slightly th too big to fit into well defined hole within porphyrin ring; binding of oxygen at 6 coordination site rearranges electrons within iron so ion becomes effectively smaller, allowing it to movie into plane of porphyrin 8.3 Hemoglobin Binds Oxygen Cooperatively  Hemoglobin displays quaternary structure, like all allosteric proteins  Human hemoglobin A, present in adults, consists of four subunits o Two -subunits o Two -subunits  -subunits and -subunits have similar 3d structures and are similar to myoglobin  3d structure of hemoglobin – pair of identical αβ dimers 1 1β and 2 2 ) that associate to form hemoglobin tetramer  Deoxyhemoglobin corresponds to T state of hemoglobin whereas oxyhemoglobin corresponds to R state (T state is less biochemically active than R state); in hemoglobin, T state has lower affinity for oxygen than R state does  When iron ion moves into plane of porphyrin, histidine residue bound in 5 coordination site moves with it; histidine residue is part of α-helix which moves too; the carboxyl terminal end of α-helix lies in interface between the two αβ dimers  α1 1and α 2 2imers rotate 15 with respect to one another  rearrangement of dimer interface provides pathway for communication between subunits: presence of oxygen on one subunit is immediately communicated to other so subunits change from T to R  combined model is required  Hemoglobin behaviour is concerted in that hemoglobin with three sites occupied by oxygen is almost always in quaternary structure associated with R state; remaining open binding site has affinity for oxygen more than 20 fold greater than that of fully deoxygenated hemoglobin binding its first oxygen  Behaviour not fully concerted since hemoglobin with oxygen bound to only one of four sites remains primarily in T state quaternary structure; this molecule binds oxygen three times as strongly as does fully deoxygenated hemoglobin (consistent with sequential model) 8.4 An Allosteric Regulator Determines the Oxygen Affinity of Hemoglobin  Oxygen affinity of purified hemoglobin is much greater than that of hemoglobin within blood cells  Allosteric regulator molecule called 2, 3-biphosphoglycerate (2,3-BPG) is reason there is a difference between purified hemoglobin and hemoglobin in red blood cells; highly anionic compound present in red blood cells at approximately same concentration of hemoglobin; it binds to hemoglobin, reducing its oxygen affinity so 66% of oxygen is released instead of just 8%  Single molecul
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