Chapter 15 Flashcards
What is allosteric regulation of an enzyme?
non-covalent interaction with an enzyme/protein and a small molecule (not a substrate)
At what site does allosteric regulation occur in an enzyme?
At some place other than the active site
What structural feature is common to many allosteric enzymes?
quaternary structure
Describe the MWC (symmetry) model of allosteric regulation
equilibrium between T and R state, ligand binding shifts the equilibrium toward R but does not change conformation
Describe the KNF (sequential) model of allosteric regulation
ligand binding triggers a conformational change, different conformations have different affinities for ligand
What is the structure of a heme?
Fe ion surrounded by a porphyrin ring
what are the 6 preferred ligands of the Fe ion?
4 nitrogens from porphyrin ring, 1 nitrogen from HisF8, 1 oxygen molecule
What is the function of myoglobin?
oxygen-binding protein, stores oxygen in muscles
what is the structure of myoglobin?
a globular protein made of a single polypeptide chain with 8 helices
what type of O2 binding does it exhibit?
classic saturation binding, similar to an enzyme that obeys Michaelis menten
what is the function of hemoglobin?
oxygen binding protein, binds O2 in lungs and transports it to muscles
what is the structure of hemoglobin?
globular protein, tetramer (4 polypeptide chains) 2 alpha chains, 2 beta chains
what type of O2 binding does it exhibit, why is this important to its biological role?
sigmoidal binding of O2, important because at low O2 content, has a weaker affinity than Mb allowing for transfer
How does the Fe ion in the heme change upon O2 binding? What conformational changes does this trigger?
Fe moves from being above the plane to almost within the plane, this pulls HIsF8 thus pulling the alpha helix which is transmitted to the interface causing a 15 degree rotation and a transition from the T state to the R state
WHat is the difference in the T and R states in Hb?
T state favored in deoxyHb, R state is favored in OxyHb
how does Hb show characteristics of the MWC model?
has T & R states in equilibrium, R state has a higher O2 affinity, binding O2 shifts equilibrium from T to R state
How does Hb show characteristics of the KNF model?
O2 binding triggers conformational change in the subunit it is bound to triggering change in the adjacent subunits, increasing O2 binding affinity
How does pH affect O2 binding in Hb? What s=is the name of this phenomenon?
as pH decreases, O2 disassociation from Hb increases, Bohr effect
What is the chemical/structural explanation for the pH effect in Hb?
at decreased pH, HisB148 is in its protonated form which participates in an ionic bond with AspB94 stabilizing Hb in the T state, lowering O2 affinity
Why is this pH effect in Hb important from a metabolic perspective?
metabolically active tissues (muscles) have a slightly lower pH, by Hb being sensitive to minor pH changes, it easily releases its O2 to Mb when Hb reaches the muscles, allowing Hb to perform its biological role
How does CO2 affect O2 binding in Hb?
CO2 can directly lower blood pH through the bicarbonate equilibrium, thus decreasing Hb’s affinity for O2
WHat is the chemical/structural explanation for the CO2 effect on Hb?
binding of CO2 to Hb turns the N-terminal from a positive/neutral charge to a negatively charged carbamate, which can then form ionic bonds that stabilize the T state and lower Hb’s affinity for O2
How does 2,3-bisphosphoglycerate affect O2 binding in Hb?
binds to tetrameric Hb in the cavity created by the 4 subunits, decreasing Hbs affinity for O2
What is the chemical/ structural explanation for the effect of 2,3-BPG on Hb?
BPG being a highly negative molecule is attracted to the binding pocket in Hb which is lined with positive AA. BPG crosslinks the B-chain stabilizing the T state, lowering Hbs affinity for O2
