ligand binding and hemoglobin Flashcards
remember what theta, Kd, and p50 are?
theta: fraction of occupied binding sites
Kd: dissociation constant, determined to be [L] at 1/2 theta
p50: Kd but for gasses, measured in partial pressure
what are globins?
oxygen binding proteins. oxygen molecules are captured with heme that is protein bound .
myoglobin: main storage protein
hemoglobin: circulating oxygen binding protein (transports O2)
how are the structures of myoglobin and hemoglobin related?
myoglobin is a monomer. hemoglobin is a tetramer with each subunit resembling myoglobin
how does CO bind to myoglobin and hemoglobin?
CO has similar size and shape as O2 and fits into the same binding site, but binds over 20000 times better than O2 because CO has a filled lone electron pair that can be donated to vacant d-orbitals on the Fe. Protein pocket decreases CO affinity to 250 times better than O2. CO is highly toxic because it blocks function of myoglobin and hemoglobin
what His is important for affecting CO affinity?
the distal His decreases CO affinity by supporting the bent conformation of O2 with H bonding
why can’t myoglobin transport O2?
myoglobin has a high affinity for O2 in both the lungs and tissues. it does not release O2 when in tissues. see slide 13 for graph explanation
affinity must vary for effective O2 transport. how is this achieved?
the protein (hemoglobin) must have multiple binding sites and the binding sites interact with each other in positive cooperativity where the first binding event increases affinity at remaining sites
how to interpret hill coefficient?
I don’t think I need to know the equation, but the coefficient n will indicate cooperativity. if n=1 no cooperativity. if n>1 positive cooperativity
what are the states of hemoglobin?
R: relaxed state. fewer interactions, more flexible. higher O2 affinity
T: tense state, more interactions, more stable. lower O2 affinity
what triggers R/T states of hemoglobin and why?
O2 binding triggers a T to R conformational change, increasing O2 affinity when bound
what is the effect of pH on O2 binding to hemoglobin?
H+ binds to hemoglobin and stabilizes T state (decreasing O2 affinity) and releases oxygen. This makes sense because actively respiring tissues generate H+ and need more oxygen. The pH difference between the lungs and metabolic tissues increases efficiency of O2 transport, known as Bohr effect
basically: lower pH makes O2 release more
how is CO2 exported?
CO2 is produced by metabolism and 15-20% is exported as carbamate on the amino terminal residues of each hemoglobin subunit. the rest is exported as dissolved bicarbonate
how does CO2 contribute to the Bohr effect?
formation of carbamate yields a proton, contributing to Bohr effect. carbamate also forms salt bridges, stabilizing T state (releasing O2). Bicarbonate formation by carbonic anhydrase also produces a proton
what does 2,3 BPG do to Hb?
2,3 bisphosphoglycerate regulate O2 binding. it is a negative heterotrophic regulator of Hb function (?). it’s a small negatively charged molecule that binds to the positively charged central cavity of Hb and stabilizes the T state (O2 releasing). 2,3 BPG allows for O2 release in tissues and adaptation to altitude
how does 2,3 BPG affect O2 release at different altitudes?
it promotes the T state and releases O2 in tissues. So at higher altitutes BPG concentration is higher, which lowers the Kd and promotes oxygen release. at lower altitudes BPG is at lower concentrations and doesn’t promote oxygen release as much. slide 27 for graphic
