monomeric helical protein that contains heme & delivers oxygen in muscle
a nonprotein group forming part of or combined with a protein
• heme in hemoglobin; can be found in a deep, non-polar pocket to sequester it from water (helps keep the Fe in the +2 state)
Why is heme is located in a central, nonpolar region of myoglobin?
• when exposed to water, Fe is oxidized to the 3+ state where it can't hold oxygen
• if sequestered in a nonpolar region of the protein, heme can maintain its oxygen-carrying Fe2+ state
What 2 things do myoglobin and hemoglobin have in common?
1. both proteins have high alpha-helical content
2. contain a heme prosthetic group, which is the oxygen carrier
REMEMBER THE DIFFERENCE BETWEEN OXYGENATION & OXIDATION
oxygenation → Hb Fe2+
oxidation → Hb: Fe3+
oxygenated heme is bright red
oxiDIZed heme is brownish
deoxygenated heme is more blue
If carbon monoxide (CO) binds to isolated heme 25,000x more tightly than O2, how can O2 be carried by heme at all?
• in isolated heme, CO binds to Fe at a 180° angle & O2 binds at 120° (bent)
• however in myoglobin histidine E7 forces a substrate to bind at 120°, therefore optimal CO binding is prevented, but O2
binding is ideal
* CO binds more weakly to heme in myoglobin than to free heme allowing oxygen binding in myoglobin*
What triggers myoglobin to change its structure a little when oxygen (O2) binds?
• when it binds to oxygen, the oxygenated ferrous ion (Fe2+
) moves into the plane of the porphyrin ring (straightens it out)
- • this pulls along some of the protein with it (mediated by histidine F8)
- - movement of F8 helps the protein sense whether or not O2 is bound to the heme group
• myoglobin is a monomer; the change in structure upon oxygenation has little consequence
What happens to the structure of myoglobin upon deoxygenation?
only a minor conformational change
The 2 Histidines in Myoglobin
• one forms a ligand to the iron that moves upon oxygenation
• the other reduces CO binding by requiring binding to occur at a 120° angle
• transports oxygen between the lungs & tissues
• is made up of 4 myoglobin-like subunits: 2 alpha & 2 beta subunits (a dimer of alpha/beta units)
• it’s globular
• has 4 hemes in total, 1 in each subunit
What is the relationship between the alpha & beta hemoglobin subunits?
each alpha subunit is in close contact with both beta subunits
• in contrast, there are FEW interactions between like subunits
THE STRUCTURES OF MYOGLOBIN AND INDIVIDUAL SUBUNITS OF HEMOGLOBIN ARE NEARLY THE SAME
Just know they're nearly identical
Myoglobin Oxygen Dissociation Curve
suggests that simple equilibrium processes are involved in binding & release
Hemoglobin Oxygen Dissociation Curve
Sigmoidal (sort of looks like an S)
indicates cooperativity among multiple binding sites (binding of oxygen to 1 site influences the oxygen affinity of the remaining sites on HbA)
pO2 (partial pressure of oxygen)
represents the level of dissolved oxygen in plasma, or whatever surroundings/environment being discussed
describes the effect ligands bound to a macromolecule have on binding a 2nd or additional ligand to that same macromolecule
- • n > 1 = positive cooperatively
- - binding of 1 ligand promotes the binding of another
• n = 1: no cooperativity
- • n < 1 negative cooperatively
- - binding 1 ligand inhibits binding of a second
What are the Hill coefficients of myoglobin & hemoglobin?
M = 1 (binding of 1 ligand has no effect on the binding of another)
Hb = 2.8 (binding of 1 ligand promotes the binding of another)
Taut (T) State
conformational state of DEoxyhemoglobin
• in the absence of oxygen, the T form is more stable than the R form because it contains more electrostatic & hydrogen bonds
• *has a greater affinity for protons
Relaxed (R) State
conformational state of OXYhemoglobin
• R state has the greatest affinity for oxygen
Taut → Relaxed
• initial trigger is the movement of the Fe2+ ion upon oxygenation
• leads to a conformational change in a subunit
• the alpha subunit is in contact with the beta subunit (& vice versa), causing the other subunits to take on the oxygen-binding conformation
How do the R & T states differ?
they differ in the ARRANGEMENT of the subunits although the subunits themselves BARELY change structure
The Bohr Effect
the oxygen binding affinity of hemoglobin is reduced by increased CO2 & H+ concentration (low pH)
• this allows hemoglobin to deliver more oxygen to actively metabolizing tissues
How do high CO2 & rising H+ levels (low pH) affect hemoglobins' affinity for oxygen?
CO2 & low pH promote the release of oxygen by lowering the oxygen affinity of hemoglobin
regulation of a protein by binding an effector molecule at a site other than the enzyme's active site
eg. H+ & CO2 are negative allosteric effectors
Where on hemoglobin does carbon dioxide bind?
Carbon dioxide binds to the N-terminal amino groups of the alpha subunits
• this is DIFFERENT from carbon monoxide, which binds to the oxygen binding site
How does the binding of carbon dioxide to hemoglobin reduces oxygen affinity?
• CO2 reacts with an N-terminal amino group of an alpha subunit & forms a Carbamate
• Carbamate stabilizes the Taut (deoxy) state of hemoglobin, lowering its oxygen affinity
another molecule that lowers hemoglobin's oxygen affinity by binding & stabilizing deoxyhemoglobin (Taut)
• BPG is negatively charged & binds in a positively charged cavity formed by the beta chains of hemoglobin
What are the subunits of fetal & adult hemoglobin?
• Fetal (HbF) is composed of 2 alpha & 2 gamma subunits
• Adult (HbA) is composed of 2 alpha & 2 beta subunits
Which has a higher higher oxygen affinity, fetal or adult hemoglobin?
• the gamma subunits in HbF have fewer positively charged amino acid residues in the BPG binding region than beta subunits in HbA
• HbF binds oxygen with a higher affinity because BPG binds less to fetal hemoglobin
Sickle Cell Anemia
- • 1st described genetic disease
- • characterized by sickled appearance of RBCs due to insoluble HbS (hemoglobin)
- • result = clogged capillaries & general organ damage causing pain (+ anemia, jaundice, frequent infections)
- • individuals can either be homo (more severely affected) or heterozygous (range of severity)
- • 1/12 african americans = heteros
- • 1/100 hispanic americans = heteros
- • 80,000 with disease in US
What is the molecular defect associated with Sickle Cell Anemia?
point mutation converting Glutamate → Valine on the exterior of the beta subunit
• results in an altered charge of the beta subunit
• Glutamate is very negatively charged; Valine is neutral; electrophoresis of the Hb can be used to diagnose (HbS won’t migrate as much as HbA toward the + Anode because it’s less negative)
Why do sickle cell anemia symptoms not become apparent until after birth?
• only then are the beta subunits of adult hemoglobin produced
• symptoms associated with sickle cell anemia correlate with decreased synthesis of gamma subunits & increased synthesis of beta subunits
What causes sickle cell hemoglobin (HbS) to polymerize?
• a hydrophobic hole is formed in both HbA & HbS in the deoxy (taut) form
• a hydrophobic knob is present on the beta subunit of HbS in both oxy & deoxy forms (R & T)
• in areas of low oxygen (eg. high altitude), HbS polymerizes (knobs fit into holes) & distorts red cells, leading to clogging of capillaries & blood vessels
• long term result = organ damage
Treatment of Sickle Cell Anemia
• Antibiotics: prevents 2ndary infections
• Hydroxyurea: stimulates production of HbF
- • Bone marrow transplantation: replaces HbS with HbA
- - cure, but have to worry about immunological consequences
• Gene therapy: works in mice
What is the oxidation number of the heme iron in methemoglobin?
this is an abnormal state that can't bind oxygen
How does methemoglobinemia present clinically?
- shortness of breath
- mental status changes (~50%)
• diagnosed by the absorption spectrum of the blood
List two causes of methemoglobinemia:
1. ingestion of large amounts of oxidizing agents such as nitrates (eg. fertilizer found in well water)
2. defects can also arise from the lack of maintenance of reducing agent in RBCs
3. mutations in the heme-binding pocket of Hb can promote HbM (mutation of Histidine F8 → Tyrosine)
How is methemoglobinemia treated?
by administering reducing agents like vitamin C or methylene blue
- • an imbalance of the synthesis of the alpha & beta subunits of hemoglobin
- • ~1000 cases in the US of severe disease
- • alpha thalessemias have defective levels of alpha subunits; beta, the beta subunits
- • treatment usually involved blood transfusion