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Lecture 22 Function and Regulation of Hemoglobin.docx

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University of Toronto St. George
Michael Baker

BCH210H © Lisa| Page 112 LECTURE 22: FUNCTION AND REGULATION OF HEMOGLOBIN HEMOGLOBIN  hemoglobin is an O -t2ansporting protein  tetrameric protein  carries O 2n blood throughout the body  Hb quaternary structure has 4 polypeptide units, each subunit similar to myoglobin  each Hb subunit has a heme prosthetic group  essential for O 2inding  heme is a disk attached to each Hb subunit  gives Hb its red colour  has Fe coordinated at centre of protoporphyrin ring 2+ +  Fe and Cu have strong O affini2ies  Fe has octahedral geometry  has 4 links to protoporphyrin ring  1 link to His F-8 (His93) in Hb subunit  1 link for O2binding  O binds bw Fe and His E-7 (64) = oxygenation 2 2+  heme is buried in a crevice in each subunit to prevent Fe oxidation  Fe is rapidly oxidized if heme is by itself in H O 3+ 2  Hb w Fe = methemoglobin (Met-Hb)  Met-Hb can’t bind O , 2nly H O 2  oxygenated Hb = oxyHb  Hb w no O =2deoxyHb  CO binds tightly to heme = CO toxicity MYOGLOBIN  myoglobin is an O ca2rier inside muscle  monomeric protein, no quaternary structure  supplies O to tissues in reptiles, birds, and mammals 2  Mb stays inside the muscle cell (myocyte)  unless you get a heart attack—death of muscle cells  myoglobin released into blood HEMOGLOBIN VS. MYOGLOBIN  Hb & Mb reversibly bind O 2  a single subunit of Hb or Mb is largely α-helical  Hb is a very efficient O 2ransport and release protein  Mb is a very efficient O 2inding protein  the oxygen binding curves of Mb and Hb are diff  Mb: hyperbolic curve—efficient O upta2e  Hb: sigmoidal curve—less efficient O upt2ke  increase in pressure of O ,2more oxygenation of Hb and Mb  more O b2nding as O con2. goes up  at lungs (90-100 torr), Hb and Mb are saturated  Hb is well-suited to pick up O a2 lungs AND to release O at t2ssues (20-40 torr)  Mb has poor release of O at 20-40 torr—too efficient in binding O for red2cells BCH210H © Lisa | Page 22 COOPERATIVITY IN HEMOGLOBIN  Hb has a sigmoidal curve  as you move to lower pressure (away from lungs), Hb loses O quickly 2  sigmoidal curve reveals cooperativity bw subunits in Hb  bc of quaternary structure, subunits in Hb influence each other  binding of O at one subunit increases affinity for O at other subunits 2 2  binding of O s2arts slow  after 2 O 2re bound, higher affinities at the 2 remaining heme sites (rate speeds up)  after 3 O 2re bound, the last site has the highest O affi2ity  the progressive increase in affinity is an allosteric effect for Hb (allosteric: other site)  when O b2nds at one subunit, Fe in heme ring changes its conformation—rises (was originally below plane of ring)  Fe shift pulls the His F-8 towards the porphyrin ring  the position of the α-helix w His F-8 also shits  this leads to an o
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