Lecture 14 Flashcards
Hemoglobins and physiology
o HbF, less cooperative form that has high o2 affinity
o 700-800 known variants of adult Hb
Detected by change in electrophoretic mobility or DNA sequence; most have no effect; others alter O2 binding; some are inactivated
o Genes clustered on chr16
HbA and O2 transport
Bonds w/ oxygen in lungs and transports it to peripheral tissues
Hb’s cooperative o2 binding behavior is indicated by sigmoidal curve
• no quaternary structure-> no cooperativity
O2 Binding Model
• Hb does not bind by a sequential model b/c O2 doesn’t bind cooperatively in CO experiment
• O2 binds in a concerted manner to Hb b/c the “increase of available subunits in the R0 state”. There’s a disproportionate increase of available R0 states when 1 O2 binds and forms R1
o DeltaG=0 at equilibrium, have same concentration of substrates in both forms
• Mutations can change the shape of oxygen binding by altering T0 state stability relative of R0 state
HbS
- HbS beta-chain is less negative than wt beta-chain
- Glu-6-Val mutations
Sickle Cell Disease
- high metabolic activity lowers peripheral tissue pH, favors deoxygenation and polymerization of (alpha)2(betaS)2.
- Fibers form & distort RBCs, making them sticky and leaky
- Sickled RBCs have poor flow properties and block capillary beds-> sickle cell crisis (necrosis, infection, loss of capillary bed)
Clinical Indications for SCD
- Gel electrophoresis of a Hb sample
- HPLC of alpha and beta chains
- Treat RBCs w/ dithionite to rapidly deoxygenate sample & then observe rate of RBC sickling under a microscope
HbS Polymerization
HbS form when deoxygenated has two valine bulges and EF pockets. Allows to ‘zipper’ into strands and polymerize. HbAS has only one Valine bulge and thus a lower tendency to sickle.
