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HEMOGLOBIN AND MYOGLOBIN

By Krynn-Kry Jan 25, 2015 1403 Words
HEMOGLOBIN AND MYOGLOBIN
I.

OXYGEN CARRIERS
A. Why do we need oxygen carriers?
i. Cannot carry enough in blood to meet metabolic demand
ii. Oxygen is very reactive – oxidizes
iii. Oxygen cannot diffuse very easily (we have thick skin)
B. Properties of a good oxygen carrier
i. Binds oxygen at a high [O2]
ii. Doesn’t oxidize cellular components
iii. Gives up oxygen on demand
C. Hemoglobin and Myoglobin
i. Cooperativity
1. Hemoglobin needs to have high affinity to bind O2 in the lungs, but low affinity to unload to myglobin
2. Sigmoidal curve: represents weak-binding state at low P02 and strongbinding state at high P02 ii. Hemes
1. The heme binds O2, not the protein
2. Function of protein: provides crevice – keeps heme from oxidizing a. absence of protein: ferrous atom (Fe2+)  ferric state (Fe3+) b. heme buried in hydrophilic environment of protein: O2 binding does not result in oxidation

3. Heme structure
a. Each polypeptide of protein is made from 8 residues  6 helices – A, B, C, D, E, F
b. Fe2+ has 6 coordinating bonds
i. 4 bonds = nitrogens from tetrapyrrole ring system
ii. 5th bond - Helix F binds to Fe at terminal Histidine
th
* 5 bond = helix F8, residue 93 in Mb,
molecule (His F8)* = proximal histidine
residue 92 in the β-chain of Hb and
iii. 6th bond – deoxygenated: empty, histidine residue from residue 87 in α-chain of Hb
th
helix E** hovers; oxygenated: oxygen bonds here
** 6 bond = helix E7, His 64 for Mb, His
63 in b-chain and 58 in a-chain
c. Oxygen binds to Fe at 120° angle  easily removed
II. MYOGLOBIN
A. Physico-chemical properties
i. 153 amino acids – single polypeptide chain
ii. Very compact: globular structure  little empty space for solution to get in iii. Tertiary structure: 8 alpha helices (A-H), 4 helices terminated by proline residues iv. About 75% is in alpha helical structure

v. Polar side chains on outside of protein  interact with solution vi. Myoglobin = storage protein  mainly in skeletal muscle vii. High O2 affinity – does not change with concentration viii. Monomer  no cooperativity

B. Oxygen binding to Mb
i. θ = (pO2)/(p50 + pO2)
1. θ= fraction of Mb sites bound to O2
2. p50 = O2 partial pressure for half-saturation
ii. θ /(1- θ) = pO2/p50
1. take logs of this equation  linear graph  no cooperativity III. HEMOGLOBIN
A. Structure
i. Primary, secondary and tertiary structures are same as Mb Differences in Hb and Mb
ii. More than 1 subunit  quaternary structure:
- Mb is a storage
protein – binds O2
1. 2  and 2  subunits; cooperativity in binding and release of O2 avidly, dissociates
a.  subunits: 146 residues, identical (same gene)
slowly
b. : 141 residues, differ by 1 or 2 genes
- Mb is not
cooperative
2. In urea, Hb dissolves into dimers of /- - interaction is stronger - Mb is 1 polypeptide
than - or -
3. Tetramer is globular molecule: spherical
iii. Subunits are 2.5nm apart  cooperativity is not due to heme-heme interaction; affinity of O2 varies with concentration (also with pH, CO2, 2,3 biphos. - see c) B. Conformational states
i. Deoxy/Oxy Hemoglobin
1. Deoxy: molecule is very rigid, large cavity in center
2. Oxy: when exposed to O2,molecule loosens, rotates, cavity becomes smaller
ii. Graph – sygmoidal curve  logs of affinity: 2 tangents – when 1st O2 bound, 4th O2 bound  demonstrates cooperativity – 2nd binds 9x faster, 3rd 36x, 4th 100x iii. Salt bridges of Hb –
1. charge-charge interactions between termini and other residues in deoxy state:
a. C terminus of 2 (146 His) with Asp of 2, helix C of 1 (Lys40) b. 1 and 2 have same interactions (due to symmetry)
c. N term of 1 with C term of 2; C term of 1 with N term of 2 d. C term of 1 with Asp126 of 2; inverse
e. Tyr140 of 1 h bonds to COOH of Val93 of 1; also in  2. When O2 binds, these interactions are disrupted and Hb relaxes, permitting sliding and rotating to assume oxy conformation
iv. Geometric explanation
1. Hemes are dome-shaped
2. O2 binds  pulls Fe down  dome becomes flat  pulls helix

C. Role of pH, CO2, and 2, 3 biphosphoglycerate
i. pH (The Bohr Effect)
1. deoxy Hb exchanges its protons and CO2 with O2
2. Hb(O2)n + nH+  Hb(H+ )n + nO2
3. Decreased pH (increased [H+])  decreased ability for Hb to hold O2 / increased ability to give up O2 = The Bohr Effect
ii. Effects of CO2
1. Hb carries CO2 from tissue to lungs
2. CO2 + H2O  HCO3- + H+
a. Protons facilitate Bohr effect
b. HCO3- can bind to the N-terminus groups of chains
to form carbamates  / interaction  deoxy conf.
 O2 released
iii. 2,3 biphosphoglycerate (BPG)
1. Cavity between  and  subunits has BPG inside
2. BPG binding to oxy Hb  conf shifts to deoxy  O2 released 3. Applications:
a. At high altitudes, body makes more BPG to achieve this effect (compensates for less O2 received from atmosphere)
i. takes 3-5 days to adjust to new altitude (produce BPG)
ii. reversible process
b. Stored blood has less BPG  needs to be reconstituted with 5mm BPG so blood will deliver O2, not just bind it
c. smokers have higher BPG –CO ties up some Hb  less O2 available
4. CO2 and BPG have additive effect  they bind at different sites on Hb D. Other factors
i. Fetal hemoglobin
1.  instead of  subunits  doesn’t bind BPG as well  quickly turns to oxy  fetal Hb has higher affinity so it can take O2 from placenta 2. In the  chain His is replaced by Ser – BPG binds more weakly ii. Toxicity of CO

1. Heme pocket can bind other small molecules besides O2
2. CO is approximately same size:
a. Blocks O2 from binding

b. Has greater affinity  oxy conformation  curve shifts left  harder to release O2  blocks respiration
IV. DEFECTS IN HEMOGLOBIN STRUCTURE AND DISEASE
A. Sickle Cell Disease
i. General comments:
1. Heterzygous: not serious; homozygous: very serious
2. Malarial parasite’s life cycle not continued in sickle cell malarial regions select for sickle cell allele
ii. Mechanism of sickling
1. Mutation in Hb globin gene  change in nucleotide sequence  6 Glu becomes Val  significant chemical change to protein
2. Hydrophobic regions of  subunits of two Hb molecules form dimer, then polymer (in deoxy form)
3. Polymers aggregate into elongated tube
4. Tube cannot flow easily through vessels, especially capillaries  blocks transport  more deoxy molecules  more sickling
5. Membrane also gets deformed – loses K+  lyses  Hb fibers spill out and get metabolized
*Can detect via electrophoresis: Glu to Val means loss of neg charge iii. Complications
1. Stroke – clotting of vessels in brain
2. Susceptibility to infectious diseases
3. Organ damage and infarction
4. Impaired growth
5. Infertility
6. Renal damage
iv. Treatments- need to know?
B. Other Hemoglobin variants
i. Hemoglobin S …….. β6 Glu  Val
ii. Hemoglobin C………. β6 Glu  Lys – very little consequence iii. Hemoglobin E ………. β26 Glu Lys – very little consequence iv. Hemoglobin Constant Spring (α-globin chain that is abnormally long) v. Hemoglobin H ………….(β4)- high affinity – doesn’t deliver O2 very well vi. Hemoglobin Barts………(γ4) – fetus usually dead before birth vii. Other variants in lecture notes?

ii.
Thalassemias
i. Causes
1. One or more genes coding for Hb chains are deleted –called  or  2. Genes present but mutation causes short polypeptide or frameshift (nonfunctional)
3. Genes present but mutation may affect transcription/mRA processing – missing/nonfunctional protein
ii. Subunits

1. Alpha always present
2.  shuts off slowly – 50% present at birth, finally completely shut off 3.  increases as  decreases
iii.  Thalassemias (normally 4 genes, 2 from each parent) 1. 1 deficient gene (still 3 copies) = silent carriers
2. 2 def =  thalassemia trait
3. 3 = Hem H disease: some 22, some 4, mild to moderately severe anemia (too much aggregation)
4. 4 = hydrops fetalis (blue baby) – death of fetus at or before birth a. Hb is all 4 (Hb Bart) or 4 (Hb H) – cannot deliver O2 (loss of cooperativity – binds like myoglobin)
iv.  Thalassemias (normally 2 genes, 1 from each parent)
1. Minor: 1 copy of gene, thalassemia traits
+
minor:  
0
2. Major: no copies of gene (or defective copies), must rely on fetal chain + 0
0 0
+ +
(22 Hb) – healthy at birth, severe anemia in first years of life 0 require major:  
 
regular transfusions
3.  thalassemia can also couple with sickling or other variants

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