Lesson 7: Hemoglobin - Allosteric Modifiers Introduction to Enzymes Flashcards
positive allosteric effector: O2
positive because binding O2 to one site increases affinity at another site
negative allosteric effectors: H+, BPG, and CO2
heterotropic negative allosteric modifiers — these all decrease a subunit’s affinity for O2
what happens if we mutate 2 His residues in central cavity to Ala
(Hb)
if ala binds, means that 2,3 - BPG binds less (needs a basic, positive environment) … if you bind less of it it’ll shift to R state (graph pushed left)
- still cooperativity from the other oxygen binding
- between stripped Hb and Blood Hb
what happens if we mutate 4 His residues in central canal to Ala?
same as 2 mutations, but even more so to the left
- N+ terminals will bind some BPG still, but the amount overall is reduced, more R dominant reaciton
- between stripped Hb and Blood Hb
what if we kept 4 His residues and added 2 more Lys to the central cavity
Lys is basic –> adding more positive charge denity, more 2,3-BPG stabilization for T-state, in front of the Blood Hb
where can the homodimer in the lecture show cooperative oxygen binding
salt bridges between N and C termini, multiple subunits
will the homodimer exhibit a Bohr effect
yes - HIS 13 will function like HIS 146 in Hb
- deprotonate which increases oxygen bonding
what is a bohr effect
changing O2 binding affinity by changing pH
would the addition of BPG have an effect on O2 bindingt (to the central cavity that does not have a lot of positive charge density)
not likely – requires a LARGE amount of positive charge density in the central cavity
what would be the effect of a mutation which replaced aspartic acid 85 with a lysine
most likely increase O2 binding
——- charge charge respulsion, cause the helix to spread apart and O2 can get into the canal
how does CO2 modulate Hb binding affinity
through the HCO3- buffering and carbonic anhydrase
CO2 buffering
- CO2 diffuses from the tissues to the rbc
- carbonic anhydrase causes rxn to quickly yield HCO3- and H+ (a majority of CO2 is carried through the vascular system in the form of HCO3-)
- since H+ is being released, it stimulates O2 to be released into the tissues
CO2 alone helps shift () and () O2 transfer
R –> T and increase
BPG alone helps shift () and () O2 transfer
R –> T and increase
combined, BPG and CO2+….
most efficiently shift R–> T and increase O2 transfer
Carbonic Anhydrase
CO2 + H2 <-> H2CO3 <-> H+ and HCO-
- increase CO2 means increase H+
- increase H+ shifts
R –> O2 dissociates
CO2 decreases what
Hb affinity for O2 because of carbonic acid buffering sytem
where are a lot of the pathological substitution mutations structured near
oxygen binding sites
Hiroshima
- B146 (HC3)
- His –> Asp
- disrupts salt bridge in deoxy state
Suresnes
- A141 (HC3)
- Arg –> His
- eliinates bond between Arg 141 and Asn 126 in deoxy state
Hiroshima and suresnes both favor
both favor R state because they destabilize the T-state (“deoxy”)
would ppl with Hb suresnes and Hb hiroshima be sigmoidal
yes – we know because they are still alive meaning that they must have some level of cooperactivity
enzymes allow for
- increased reaction rates (10^3 - 10^19 times)
- “mild” reaction conditions (i.e. physiological, temp, pH)
- great specificity: both substrates consumed and products produced
- coordinate control: reactions can be turned on and off by modulating activity of enzyme
