CH 4 (L10) Flashcards
Myoglobin
(Mb), Myo refers to muscle, exists as a monomer in muscle of reptiles, birds, and mammals, found in muscle
Hemoglobin
(Hb), hemo, heme refers to blood, exists as a tetrameric complex
- heme consists of a tetrapyrrole ring system connected by methene bridges (-CH=)
red blood
red color associated with blood due to the heme prosthetic group: protein-bound organic molecule essential for the activity of the protein
–> ferrous iron is bound and forms a complex with 6 ligands, 4 being nitrogen atoms of the protoporphyrin IX
sperm whale oxymyoglobin
myoglobin is part of a family proteins called globins
–> myoglobin consists of 8 alpha helices in a bundle –> all-alpha helical bundle –> they are angled to allow the R-groups to interdigitate
Myoglobin (Mb)
interior is almost all hydrophobic amino acid residues
- -> the surface is made of both hydrophilic and hydrophobic residues
- –> the heme prosthetic group occupies a hydrophobic cleft stabilized by weak interactions
- –> accessibility to oxygen depends on nearby amino acid side chains
Neuroglobin (Ngb)
another globin, recently discovered
- -> preferentially expressed in nervous system tissue
- -> monomer, 150 aa in length, 16kDA, typical globin fold, binds O2, CO, or NO
- provides neuroprotection
Cytoglobin (Cygb)
another recently discovered Globin
–> expressed widely in tissues, can bind O2, may be involved in protection during oxidative stress, 1900aa in length (human Cygb)
Erythrocytes
erythro-red, cytes- cells
oxygen and hemoglobin carrying the cells of the blood in vertebrates,
each 6-8 um in diameter cell carries ~ 3x10^8 molecules of Hb
Hemoglobin (Hb)
- an (α2β2) alpha2beta2 tetramer made of α-globin (7 α helices) and β-globin (8 α helices)
- each globin is similar to myoglobin in structure
- each of the four contains a heme prosthetic group
- 1 α chain interacts extensively with a β chain so sometimes Hb is referred to as an αβ dimer
oxygenation
refers to the reversible binding of the oxygen to the heme group
- -> without oxygen , Hb is called deoxyhemoglobin
- -> with oxygen, it is called oxyhemoglobin
tissue distribution of vertebrate globins
blood - hemoglobin
muscle - myoglobin
neurons - neuroglobin
all organs - cytoglobin
Hb
hydrophobic residues allow heme prosthetic group to be buried inside the hydrophobic cleft
- -> 4 nitrogen help coordinate the iron Fe II
- -> the fifth ligand is an imidazole ring (His-93) called the proximal Histidine
- -> the 6th ligand is molecular oxygen (coordinated to the Fe II and imidazole side-chain of His-64
Holding in Place
free heme (in solution) does not reversibly bind oxygen in aqueous solution
- -> Fe2+ is partially oxidized to Fe3+ when oxygne is bound
- -> an electron is temporarily transferred toward the oxygen atom attached to the iron (dioxygen is partially reduced)
Reversible binding of oxygen
binding of oxygen depends on
- -> concentration of protein
- -> concentration of oxygen
Fractional saturation (Y) of myo/Hb
measured as the amount of which has molecular oxygen bound divided by total (bound + unbound)
Formula:
Y = [MbO2] / [MbO2] + [Mb]
Oxygen-binding curves
the curve for myoglobin is hyperbolic (so just one equilibrium constant for binding)
- the curve for hemoglobin is sigmoidal (S-shaped) indicating more than one molecule is binding –> 4 molecular oxygen binds –> these represent environmental pO2 levels
- -> oxygen binds in a positive-cooperative fashion (1 bound will help the 2nd bind which will help the 3rd)
- -> physiologically at 3 this stops as only 1 slot is open per Hb molecule in vivo
- -> notice the pressure at half- saturation (P50)
Partial pressure
- is a measurement of the percentage of a give constituent in the gas according to the total pressure
1 atm - 760 mmHg - the partial pressure of oxygen (~21% of the atmospheric gases) at 1 atm = 760 x 0.21 = ~160 mm Hg
The cooperative binding wiggle
- when the heme iron (red) of hemoglobin is oxygenated, the proximal histidine residue is pulled towards the porphyrin plane
- this disrupts ion pairs that cross-link deoxyHb helping cooperative binding to occur
conformation changes
- explain the sigmoidal curve of Hb
- T conformation (tense) resists binding oxygen (low linear line)
- R conformation (relaxed) facilitates binding of oxygen (high curve line)
- -> hemoglobin observed in the middle
- R and T states are in dynamic equilibrium
allosteric action
binding of oxygen to Hb is regulated by allosteric interactions
allosteric modulator or allosteric effector
binds to a protein and modulates its activity
allosteric protein
protein regulated by allosteric effectors
–> substrate/ligand binds when the protein is in R state
Allosteric inhibitors
bind their own regulatory sites and bind most avidly to the T state
Allosteric modulation
carried out by conformational changes
Allosteric proteins
- are in equilibrium when their R states and T states are rapidly interconverting
2,3-biphosphate-D-glycerate
- 2,3BPG is an allosteric effector of Hb in mammals
- When 2,3BPG is present, the P50 for binding of O2 in Hb goes from 12 to 26 torr
- 2,3BPG binds to deoxyHB and stabilizes the T state
- What does that mean for binding to oxygen? Lower affinity
- In the R state, the binding pocket is too small for 2,3BPG
- O2 increases affinity
- 2,3BPG decreases affinity
- -> Without 2,3BPG, Hb would not release oxygen at the low levels found in tissues
- -> 2,3BPG allows oxygen to follow its concentration gradient into tissues where it is needed
Enter CO2
- -> OxyHb binding is further regulated by the buffering system in the blood
- -> CO2 decreases the affinity of Hb for O2 by lower pH inside red blood cells
- -> CO2 is hydrated in RBCs to produce carbonic acid which dissociates to form bicarbonate and a hydronium ion
Formula: CO2 +H20 H2CO3 H+ + HCO3-
Changes in pH
- Lower pH in the RBCs leads to a change in the environment
- -> There are amino acids with ionizable side chains… so what do you think can happen?
- -> changes in protonation to several groups in Hb (alpha-amino groups with pKa values around 7-8)
Carbamate Adducts
- most CO2 is carried in the blood as bicarbonate ions
- -> some is carried by Hb when CO2 reacts with the N-terminus of Hb to form carbamate adducts
The Bohr Effect
These stabilize the deoxyHb conformation.
- -> P50 of Hb rises: this is called the Bohr effect
- -> What happens to the oxygen that was on the Hb? it gets unloaded more easily
Oxygen-hemoglobin Dissociation: Exercise
you exercise heavily –> your mitochondria churn in your tissues –> your muscles use of a lot of oxygen –> you produce more CO2 which needs to be unloaded in lungs –> CO2 causes decrease in pH in RBCs –> Hb-O2 affinity decreases
Net effect: more CO2 is unloaded, O2 more easily gets into tissues
Bohr effect
low pH (more acidic), high temp, high P of CO2, high 2,3-BPG
Immunoglobulins
antibodies (Ig) are part of the vertebrate immune system
- -> highly specialized glycoproteins
- -> they are highly adapted to binding to portions of foreign molecules to help eliminate them (antigens)
- -> they are produced by B-lymphocytes (B cells)
–> IgG: most abundant form of immunoglobulin in your blood
IgG
most abundant form of immunoglobulin in your blood
- 2 heavy chains
- 2 light chains
immuglobulin fold
- most common motif for the antibody is the Ig Fold domain (110 residues)
- 2 antiparallel beta sheet sandwich
- the variable region of an antibody is so specific, no water molecules are thought to exist between the antibody and the antigen it binds
binding antigen
- hydrogen bonds and electrostatic charges stabilize the binding
