Ch. 5 Protein Function Flashcards
Characteristics of myoglobin
- is a relatively small protein with a compact shape
- Myoglobin lacks a beta structure and all but 32 of its 153 amino acids
-Myoglobin contains a polypeptide chain and iron-containing porphyrin known as heme. - Hemoglobin is a tetrameric protein whose four subunits each resemble myoglobin
Heme Group
- Heme is an iron-containing porphyrin derivative and an organic compound that allows for proteins to carry out some function that the polypeptide alone can’t perform
- heme is a prosthetic group found in myoglobin and hemoglobin and is also planar and hydrophobic
- Since heme is not able to bond to O2 on its own due to the central Fe 2+ atom is easily oxidized by Fe 3+). so to fix this O2 binds reversibly to form a 6th coordination bond which allows the heme-containing proteins myoglobin and hemoglobin to become oxygen carriers/ Residue His E7 forms a hydrogen bond to the O2 molecule.
The equation describing how O2 binds to myoglobin and the meaning of Kd
-Mb +O2 —-> Mb-O2
-Kd is the dissociation constant and K=[Mb][O2]/[Mb-O2]
Oxygen binding curve of myoglobin and p50
- The O2 binding to myoglobin shows a hyperbolic curve
-p50 is 2.8 torr for human myoglobin - the equation of a hyperbolic curve is
Y= bound Mb/Total Mb
Y= [Mb* O2]/[Mb]+[Mb*O2]
Y= pO2/Kd +PO2 - Y is the fractional saturation or the proportion of myoglobin molecules that have bound O2
Differences between primary,secondary, and tertiary structure of myoglobin and alpha and beta subunits of hemoglobin.
- Myglobin and Hemoglobin subunits have similar secondary and thertiary structures and their primary subunits are only 18% identical
Interpretation of oxygen binding curve of hemoglobin and the meaning of p50
- Oxygen binds cooperatively to hemoglobin
- While hemoglobin binds to O2 reversibly a plot of Y versus pO2 for hemoglobin is sigmoidal shaped(S-shaped) rather than hyperbolic.
- Hemoglobin’s overall oxygen affinity is lower than myoglobin and is half saturated at O2 pressure of 26torr( p50 = 26).
Why is hemoglobins binding curve sigmoidal?
- At low O2 concentration, hemoglobin appears to be reluctant to bind the first O2, but as pO2 increases, O2 binging increases sharply until hemoglobin is almost fully saturated.
- At high O2 concentration, oxygenated hemoglobin is reluctant to give up its first O2 but as pO2 decreases, all the O2 molecules are easily given up. Which suggests that the binding of the first O2 increases the affinity of remaining O2 binding sites. (which is called cooperative binding)
From the use of binding curves why is hemoglobin a suitable oxygen transport
- Due to the difference in oxygen affinity between myoglobin and hemoglobin ensures that O2 bound to hemoglobin in the lungs is released to myoglobin in the muscles. This O2 system is efficient because the tissue pO2 corresponds to the part of the hemoglobin binding curve where the O2 affinity falls most sharply
What is the effect of O2 binding to the Fe atom of heme group and on the alpha helix that has proximal histidine, and overall quaternary structure of hemoglobin
-The heme Fe ion has five ligands and the porphyrin ring is dome shaped and Fe lies about 0.6A from the poryphin group which results in the heme group being bowed slightly towards his F8. When O2 binds to produce oxyhemoglobin Fe moves to the center of the porphyrin ring and the movement of Fe ion pulls His 8 further towards the heme group.
What are the two quaternary structures of hemoglobin
-Dexoxyhemoglobin(T) and Oxyhemoglobin(R)
- The two conformational states of hemoglobin are T for tense and R for relaxed
-Deoxyhemoglobin is reluctant to bind to the first O2 molecule because the protein is in the deoxy(T) conformation which is unfavorable for O2 binding, Once O2 has bound to the alpha chain in each alpha beta pair. The entire tetramer switches to the oxy(R) conformation as the Fe atom and the F helix move.
Allosteric protein
- The binding of a ligand to one binding site affects the ligand-binding affinity of other sites and binding is said to be cooperative(cooperative effect)
-Hemoglobin is called an honorary enzyme and is also an allosteric protein and has multiple binding sites/ subunits
Chemical nature of 2,3 bisphospoglycerate
- is also known as BPG
-BPG has a carboxylic acid at the terminal end and two phosphate groups - It also has 5 negative charges and binds to positively charged residues such as lysine and glycine
BPG binding site in hemoglobin and its role in hemoglobin function
- BPG binds only to the tense (deoxy) conformation of hemoglobin between 4 subunits
-BPG prevents oxygen binding to hemoglobin in vivo and decreases hemoglobin O2 affinity
Bohr effect
- Lower pH decreases the hemoglobin’s oxygen binding affinity and some of the groups of hemoglobin get protonated which also decreases O2 ability to bind.
- Tissues release CO2 as they consume O2 in respiration. The dissolved CO2 enters the red blood cells where it is then rapidly converted to HCO3- by the action of the enzyme carbonic anhydrase
Biochemical bases of sickle-cell anemia
-Hemoglobin S (Hb S) is a mutant form of the protein
- Glu-6 is drastically replaced by Val in Hb S and non-polar Val creates a hydrophobic patch on the protein surface of the beta subunit when the protein releases the O2 in the tissues.
-This leads to the aggregation of the protein.
- hemoglobin S is polymerized
