Myoglobin Hemoglobin

Oxygen binding to myoglobin Mb + O2 à MbO2 dissociation constant for this process K is For Chap 6: K = [Mb] [O2]/ [MbO2] from which we can get You are responsible for definitions of primary secondary tertiary and quaternary structures. Peptide bond is planer (Figure 6.2.) and only two rotations C-N and C-C are allowed (Figure 6.4) that gives rise to α-helix (figure 6.7 pitch, residues per turn hydrogen boding etc are important to know) β-sheets (parallel and anti parallel and how they form hydrogen bond - Figure. 6.9) and β bend structures (figure 6,14) Keratin structure and collagen structure. Then skip several pages until you hit page 159. Protein denaturation. Anfinson’s experiment – two parts Figure 6.39 is a must for exam. Little bit on protein folding That is all you are responsible in this chapter!!!! -Chapter 7 protein function we will cover only myoglobin and hemoglobin page 176-195 rest are not going to be covered and won’t be in the exam. [MbO2] = [Mb] [O2]/ K - eq 1 Now fractional sites at which oxygen is bound on myoglobin or fractional saturation Is given by Y = O2 occupied sites on Mb / total available sites for oxygen binding on Mb Y = [MBO2] / [Mb] + [MbO2] We can substitute eq –1 in this and get Y= [[Mb] [O2]/ K] ---------------------------[Mb] + [Mb] [O2]/ K multiplying numerator and denominator by K/[Mb] simplifies this equation as Know the differences between myoglobin and hemoglobin Myoglobin Hemoglobin 153 aa 146 and 141 aa Globular 44 x 44 x 25 A globular 64 x 55 x 50 A Storage protein Transport protein Hyperbolic kinetics (M.M kinetics) Sigmoidal kinetics pH has no effect pH has dramatic effect CO2 has no effect CO2 has dramatic effect Bisphosphoglycerate – No effect Bisphosphoglycerate has dramatic effect Only one form Tight and relaxed form – deoxy and oxy forms Nitric oxide has no effect NO has drastic effect. K = 2.8 torr K = 26 torr (10 times weaker binding than Myo) 1.0 mole oxygen per mole of protein- Y 0 Since oxygen is a gas its concentration is given in terms of partial pressure of oxygen pO2 . Y = pO2 /K + pO2 So if we plot a graph of fractional saturation against oxygen concentration. We get a hyperbolic graph (because this equation describes a hyperbola) When the partial pressure of oxygen has the same value as K the equation simplifies to Y = pO2 / pO2 + pO2 which is = 0.5 In other words, K, the dissociation constant is equivalent to the concentration of oxygen at 50% saturation of myoglobin. -------- MYOGLOBIN HEMOGLOBIN [Oxygen] conc Y = [O2] /K + [O2] - pO2 Hemoglobin Only 18 residues are identical between myoglobin and α and β subunits of hemoglobin. So two proteins with different primary structures can have the same tertiary structure and the same biological activity!! Oxygen binding to hemoglobin is said to be cooperative. That is binding of one molecule of oxygen to the first subunit in hemoglobin will assist the binding of next oxygen molecule to the next subunit. This is called positive cooperativity. Myoglobin does not have any cooperativity in its binding. (Some proteins exhibit negative cooperativity also ) CO2 + H2O à H+ + HCO3- The partial pressure of oxygen in lungs is 100 torr In veins, it is only 30 torr. At these conditions, myoglobin is saturated with oxygen 95 % in the veins and 98% in the lungs. Whereas, hemoglobin is saturated with oxygen 55% in the veins and 95% in the lungs. This means hemoglobin can deliver (or release) almost 40% of the bound oxygen to the tissues by traveling from lungs to tissues. While myoglobin retain nearly all the oxygen. Thus it can effectively store oxygen. The CO2 and protons are transported back to lungs and released. See fig 7.12 CO2 also binds to hemoglobin reversibly at the N-terminal residue. Y for hemoglobin is defined as Bisphosphoglycerate binds to central cavity of the T form know the structure!!!!!!!! It does not bind to R form Fig 7.14 Y= [pO2 ]n -----------------Kn + [pO2 ]n n is defined as the Hill’s coefficient If n = 1 the equation simplifies to myoglobin. No cooperativity If n = 4 that is maximum cooperativity. It is the limit (four subunits for hemoglobin) In reality value of 2.8 to 3.0 is obtained for hemoglobin. If the value of n is < than 1 it is negative cooperativity. (This is possible and occurs with some other proteins not in the case of hemoglobin). --The cooperativity in hemoglobin is possible because it exists in two different forms (not referring to the subunits) The tight form T form has central cavity that is larger to accommodate a small molecule called bisphosphoglycerate. It does not bind to oxygen readily. So it is also called deoxy form. The relaxed form or the R form has a smaller central cavity that cannot accommodate bisphosphoglycerate. This form readily binds to oxygen so it is also called oxy form. Upon binding of one oxygen molecule to one subunit, The following events take place. 1. oxygen binding to heme pulls the Fe atom into the center of the porphyrin ring. 2. this pulling causes movement of peptide (F-8 His is pulled in) that is transmitted to other subunits as well. 3. the αβ pair that sits on another αβ pair rotates as much as 15o 4. In the deoxy form α-141 arg and β=146 His are bonded by ionic interaction with other residues. These are broken in the oxy form Bohr effect Figure 7-11 The oxygen binding affinity of hemoglobin increases with increase in pH Which means, at acidic pH hemoglobin can deliver more oxygen. In tissues CO2 is formed. That reacts with water and generates H+ More acidity, more oxygen release!! R-NH2 + CO2 à R-NH-COO- + H+ The T form binds to CO2 more. R does not. See Fig 7.13 Thus proton, CO2 and bisphosphoglycerate bind to T form and assist delivery of more oxygen. Only oxygen binds to R form. When it binds the other three ligands come out. --Fetus hemoglobin is α2γ2 - β chains 143 His is converted to Ser in γ chain This makes the Bisphosphoglycerate bind less tightly to T form. So more oxygen binds to fetus hemoglobin. Thus maternal hemoglobin is able to deliver oxygen to fetus. Soon the fetus is born, the fetus hemoglobin is replaced with adult hemoglobin, Some times this degradation is slow and the babies appear if they have jaundice. Heme degradation is slow and can be assisted by light. --High altitude When we go from sea level to high altitudes we have trouble breathing due to low oxygen conc. The next day we are fine. Since making more hemoglobin takes more time, simply by raisin the conc of Bisphosphoglycerate from 4 mM to 8 mM the available hemoglobin was made to deliver more oxygen to the tissue. Page 192 -sickle cell anemia A single amino acid mutation in hemoglobin - glu à Val 6 in β chain makes this protein sticky. 14 strands are formed ( fig 7,18) . This makes the hemoglobin to form fibrils that change the shape of red blood cells. Malarial parasite that attacks red blood cells do not stay in such sickle cells successfully to cause the disease. So sickle cell anemia gives resistance to malaria. P. 196-197