Chapter 7: Hemoglobin and Myoglobin pt.2 Flashcards
protein family
group of evolutionary related proteins
-share a common ancestor and have similar 3D structures, functions, have significant sequence similarity
Myoglobin (Mb)
-monomer
-have 8 alpha-helices (A-H)-both
*onlynhave 24/141 AA identical to Hb
*Mb is very similar to 3D structure of beta subunit of hemoglobin
Hemoglobin (Hb)
-tetramer of 4 Mb like subunits
-have 8 alpha-helices (A-H)-both
Yo2 curves for Myoglobin (Mb)
-hyperbola curve: no cooperativity
Yo2 curves for hemoglobin (Hb)
-sigmoidal curve: cooperativity
cooperativity/ allosteric effect
-allostery= the other site
-binding at one site affects binding at others
-allosterism req with protein with >1 subunit
-S curve indicates cooperativity (=allosterism)
Homotropic allosteric interaction
effector and ligand regulated by the effector are the same molecule
*part of equation
Heterotropic allosteric interaction
effector and ligand are different molecule
*effector is not in rxn
Activator
shifts equilibrium towards R-state
=positive allosteric interaction
inhibitor
shifts equilibrium towards T-state
=negative allosteric interaction
Heterotropic inhibitors for hemoglobin
-BPG
-H+
-Co2
-Cl-
homotropic effector for hemoglobin
O2
The Bohr effect Summary
-increasing [CO2], increasing H+, decreasing pH, increasing temp. = releases O2 from Hb
*curve shifts to the right
*low affinity for O2
what does the curve, temp, CO2, and pH look like when there is a high affinity for O2
-pH increases
-CO2 decreases
-temperature decreases
Where on T-state do the Bohr protons bind?
on the N-terminal and His 146-beta (amide)
- H-bonds and ion pairs in the T-state are not present in the R-State
- T to R transition changes the pka’s of these groups causing them to release their coordinated H+
*N-term needs to be protonated to be an ion pair
*surroundings can effect pka of WA/WB of groups
Hb (t-state)+ O2 = Hb(r-state)O2 + H+
*H+ (bohr protons) released when Hb binds O2
N-terminal amino groups are responsible for how much of the bohr effect?
20%-30%
His 146 beta accounts for how much of the Bohr effect (has a NH group)
40%
Bohr effect CO2: the carbonate rxn
-CO2 can react to form carbamates with the N-terminal amino groups of blood proteins
*CO2 reacts with the R-state Hb to create a T-State HB with carbonate
*Carbonate only in T-state
Bohr effect CO2: CO2 transport
H2O+CO2<–>H2CO3<–>HCO3- + H+
-when there is more CO2 then H+ released increases so pH in blood decreases
*means that Hb is dumping off oxygen
*favors deoxy-Hb (t-state) = CAPILLARIES
-when there is less CO2 then H+ in blood decreases which means the pH is blood increases
* means that Hb is binding oxygen
*favors Oxy-Hb (r-state) = LUNGS
D-2,3-bisphosphoglycerate (2,3-BPG) basic summary
-also known as BPG, 2,3-diphosphoglycerate, 2,3-DPG
- MOST ABUNDANT organic phosphate in blood
-occurs in red blood cells as a HETEROTROPHIC INHIBITOR
-inhibits oxygen from binding
-curve goes towards x-axis
2,3-BPG: where does it bind and what it does?
-binds to the central cavity of T-state and stabilizes it and “fine tunes” Hb’s O2 binding capability
*can’t bind the R-state
*causes HB to release O2
-gives Hb a P50 of 26 torr
*without it it only has 12 torr
2,3-BPG and altitude
-body increases BPG production in higher altitudes
-restores 37% release of O2
Allosteric effector: Cl-
-heterotrophic effector
- Cl- floods into RBCs to neutralize the electrostatic potential
-Cl- binds to deoxy Hb
*causes O2 to be released (t State)
why are Hb’s and Mb’s called honorary enzymes
they bind O2 like how a enzyme binds a substrate
Hill equation
YO2= pO2/P50+pO2
Ys=[S]^n/Kd+[S]^n
*S=ligand/substrate
*E=enzyme/protein
*n=number of S molecules bound
HIll plot
-slope of the line shows the degree of cooperativity
-nH is experimentally determined
-nH< n actual usually
-nH= n actual when all ligand binds simultaneously
1.n=1
2.n>1
3.n<1
- non-cooperative
- positive coop
- negative coop
hemoglobin cooperativity
about 3
-when the first O2 binds, the rest bind and when 1 is released the rest are released
Monod, Wyman, Changeux
-MWC MODEL: model for allosteric proteins
- equilibrium btwn T and R states
-ligand can bind to T or R state conformation
-all subunits have to be in the SAME STATE
-in ABSENCE of ligand, equilibrium favors T state
-Ligand binding shifts equilibrium toward R state
-Limitation: only models positive cooperativity
Koshland-Nemethy-Filmer
-found the Sequential (induced fit) model for allosteric protein
-KNF model
-subunits can exists in R or T conformations but can also change conformation independently on binding of a ligand
-PRESENCE of R-site induces increased affinity in adjacent sites
-complete T –> R transition is a sequential process
*accounts for both positive and negative cooperativity
Perutz mech for HB allostery
look at slide 16 for mech
-basically once one oxygen binds it shifts from t-state to r-state and then all the other subunits get bonded to oxygen
Mutation on hemoglobin
(methemoglobin and cyanosis)
- when mutations occur in germ line they are passed to offspring
*inheritable or genetic diseases
-sickle cell anemia
-stabilizes Methemoglobin: eliminates O2 binding
-cyanosis: blood is blue to chocolate brown
Pathologic Mutation in Hb specific (Boston, Black mouth, and Hb Hammersmith)
-Boston: His E7–>Tyr
*type of Methemoglobin
-Black Mouth: HisF8–>Tyr
-Hb Hammersmith: Phe–>Ser
*loses heme so it doesn’t bind oxygen
Pathologic mutation at surface of HB
-HbE: Glu 26 on beta chains –>Lys
*not harmful
-HbS: Glu 6 on beta chains –>Val
*sickle cell anemia
*causes precipitation
*both have same O2 binding ability
*have 4 hemes
*iron in Fe2+ state
*have same active site
sickle cell anemia
-autosomal recessive genetic disorder
-can get pain crises
-10% african americans are heterozygous for this trait
Hbs pathology (valine): causes sickle cell anemia
-E–>V in beta chain decreases surface charge making HbS more hydrophobic
-ONLY in T-state (deoxy Hbs) does Val fit into a hydrophobic pocket of beta chain
-causes long linear polymers (fibers) to form
Hbs pathology in capillaries
HbS loses O2–>deoxy HbS starts to polymerize which causes the blood to sickle
*sickled RBCs block capillaries and blood flow
Benefit of HbS gene
people who have are heterozygous for HbS gene have a higher survival rate from malaria
*25% African Americans
difference btwn adult Hb and Fetal Hb
-normal adult Hb (HbA) = alpha2beta2
-Fetal Hb (HbF)= alpha2gamma2
*gamma is a separate gene product made by the fetus
O2 delivery to fetus
- in presence of 2,3 BPG, HbF has a higher affinity for O2 than HbA
*allows transfer of O2 from the mother (less tightly bound) to the baby (more tightly bonded)
-so HbF has a lower affinity for 2,3 BPG than HbA and thus it is less affected by it
*gamma chains have Ser instead of His143(+) so they have less affinity for (-) phosphates on 2,3 BPG
