Sinh học - Chapter 4 (part 3): 3 - D Structure/ Function

Chapter 4 (part 3)3-D Structure / FunctionAnimalScrapie: sheep TME (transmissible mink encephalopathy): mink CWD (chronic wasting disease): muledeer, elk BSE (bovine spongiform encephalopathy): cowsHuman CJD: Creutzfeld-Jacob Disease FFI: Fatal familial Insomnia Kuru   PrPC PrPSc solubility solublenon solublestructure predominantly alpha-helicalpredominantly beta-sheetedmultimerisationstate monomericmultimeric(aggregates)infectivity non infectiousinfectiousSusan W. Liebman, and James A. Mastrianni (2005) Trends in Molecular Medicine 11: 439-441 Myoglobin/HemoglobinFirst protein structures determinedOxygen carriersHemoglobin transport O2 from lungs to tissuesMyoglobin O2 storage proteinMb and Hb subunits structurally similar8 alpha-helicesContain heme groupMb monomeric proteinHb heterotetramer (a2b2)myoglobinhemoglobinHeme groupHeme = Fe++ bound to tertapyrrole ring (protoporphyrin IX complex)Heme non-covalently bound to globin proteins through His residueO2 binds non-covalently to heme Fe++, stabilized through H-bonding with another His residueHeme group in hydrophobic crevice of globin proteinOxygen Binding CurvesMb has hyberbolic O2 binding curveMb binds O2 tightly. Releases at very low pO2Hb has sigmoidal O2 binding curveHb high affinity for O2 at high pO2 (lungs)Hb low affinity for O2 at low pO2 (tissues)Oxygen Binding CurveOxygen Binding CurveO2 Binding to Hb shows positive cooperativityHb binds four O2 moleculesO2 affinity increases as each O2 molecule bindsIncreased affinity due to conformation changeDeoxygenated form = T (tense) form = low affinityOxygenated form = R (relaxed) form = high affinity O2 Binding to Hb shows positive cooperativityO2 Binding induces conformation changeT-conformationR-conformationHeme moves 0.34 nmExposing crystal of deoxy-form to air cause crystal to crackShow MovieAllosteric InteractionsAllosteric interaction occur when specific molecules bind a protein and modulates activityAllosteric modulators or allosteric effectorsBind reversibly to site separate from functional binding or active siteModulation of activity occurs through change in protein conformation2,3 bisphosphoglycerate (BPG), CO2 and protons are allosteric effectors of Hb binding of O2Bohr EffectIncreased CO2 leads to decreased pHCO2 + H2O HCO3- + H+At decreased pH several key AA’s protonated, causes Hb to take on T-conformation (low affinty)In R-form same AA’s deprotonated, form charge charge interactions with positive groups, stabilize R-conformation (High affinity)HCO3- combines with N-terminal alpha-amino group to form carbamate group. --N3H+ + HCO3-  --NHCOO-Carbamation stabilizes T-conformationBisphosphoglycerate (BPG)BPG involved acclimation to high altitudeBinding of BPG to Hb causes low O2 affinityBPG binds in the cavity between beta-Hb subunitsStabilizes T-conformationFeta Hb (a2g2) has low affinity for BPG, allows fetus to compete for O2 with mother’s Hb (a2b2) in placenta.Mutations in a- or b-globin genes can cause disease stateSickle cell anemia – E6 to V6Causes V6 to bind to hydrophobic pocket in deoxy-HbPolymerizes to form long filamentsCause sickling of cellsSickle cell trait offers advantage against malariaFragile sickle cells can not support parasiteFourteen-strand fibers of Hb S and the interactions involving beta 6 Val RETURN TO RESEARCH ACTIVITIESFor additional details see:Dykes, G., Crepeau, R.H. and Edelstein, S.J. (1978). Nature 272, 506-510. [Abstract]Rodgers, D.W., Crepeau, R.H. and Edelstein, S.J. (1987). [Abstract]Cretegny, I. and Edelstein, S.J. (1993). [Abstract]