Lecture 22 Function and Regulation of Hemoglobin

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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 H2+its red colour  has Fe coordinated at centre of protoporphyrin ring 2+ +  2+e 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 O22+nding  O 2inds bw Fe and His E-7 (64) = oxygenation 2+  heme 2+ buried in a crevice in each subunit to prevent Fe oxidation  Fe is rapidly oxidized if heme is by itself in H O2  oxygenated Hb = oxyHb  Hb w no O =2deoxyHb 3+  Hb w Fe = methemoglobin (Met-Hb)  Met-Hb can’t bind O , 2nly H O 2  CO binds tightly to heme = CO toxicity MYOGLOBIN  myoglobin is an O ca2rier inside muscle  monomeric protein, no quaternary structure  supplies O 2o tissues in reptiles, birds, and mammals  Mb stays inside the muscle cell (myocyte)  unless you get a heart attack—death of muscle cells  myoglobin released into blood  myoglobin & hemoglobin reversibly bind O 2  Hb is the travelling protein, binds O v2,y efficient  single subunit of Hb or Mb is largely α-helical  Mb goes from 1 to 100% very quickly (percentage of protein molecules with O ) 2  very efficient, very hyperbolic curve  increase in pressure of O ,2more oxygenation of Hb and Mb  more O binding as O conc. goes up 2 2  at lungs, 100 torr—Hb and Mb are saturated in lungs  Hb has a sigmoidal curve  as you move to lower pressure (not lungs), Hb loses O quick2y  shouldn’t use Mb in red cells  too efficient in binding O 2n red cells  okay in muscle cells because of a lot lower pressure BCH210H © Lisa | Page 22  poor release of O a2 ~20-40 torr  sigmoidal curve reveals cooperativity bw subunits in Hb (effect of having a quaternary structure with multiple subunits)  starts slow, but as O b2nds, rate speeds up  subunits affect each other—binding of one subunit to O increas2s affinity for O on other2 subunits  progressive increase  has an allosteric effect  Fe in heme ring changes its geometry as O binds2rises (was originally below plane of ring) 
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