Myoglobin - Biochemistry - Lecture Slides, Slides of Biochemistry

Download Myoglobin - Biochemistry - Lecture Slides and more Slides Biochemistry in PDF only on Docsity! Myoglobin and Hemoglobin Quaternary structure and allosteric properties of proteins Fe Fe Fe Fe Fe Docsity.com The importance of quaternary structure in proteins is best illustrated by the binding of O2 to hemoglobin Philosophy: By comparing hemoglobin with myoglobin, (a single subunit protein), subunits can be seen to regulate the strength of binding and give rise to a property of proteins called “cooperativity” Cooperativity is the basis of another property of proteins called “allosterism” or “allostery” Docsity.com Derive an expression for q as a function of O2 We are asking: How do the sites filled vary with oxygen pressure? [MbO2 ] [Mb][O2] K = MbO2 = K[Mb][O2] = MbO2 Mb + MbO2 q Mb + O2 MbO2 Sites Filled Total Sites 71% filled q = 0.71 10 out of 14 Docsity.com Substitute the value for MbO2 into the equation q = K[Mb][O2] K[Mb][O2] [Mb] + Mb Mb 1 q = K[O2] 1 + K[O2] K K K 1 K = or Kd = O2 pressure that half fills sites = P50 When 1/K = [O2], q = 0.5 (also called Kd) Therefore: (P50 distinguishes the specific O2-binding protein) Docsity.com Redefining O2 = pO2 Charles Law: Quantity of gas absorbed by a liquid is proportional to the partial pressure of the gas above the liquid. q = pO2 pO2 P50 + Note: Mb is left out of the final equation. Note: Only connection to Mb is 1/K or P50 Docsity.com q = pO2 pO2 P50 + Assume P50 = 10 pO2 q 1 0.09 2 0.17 5 0.33 10 0.50 20 0.67 100 0.91 Docsity.com q = pO2 pO2 P50 + n n n = number of binding sites on the protein What about Hemoglobin? For Hb, n = 4 Docsity.com q 100 50 pO2 Hemoglobin P5 0 Region of Cooperativity Sigmoidal Curve Docsity.com Protein-Ligand Interactions Revisited P + L = PL PL = P + L Ka = [PL] [P] [L] Kd = [P] [L] [PL] Kd = 1/Ka = [L]0.5 or O = L + L 1/Ka O = L + L Kd Association Disassociation Docsity.com P + nL = PLn Hb + 4O2 = Hb(O2)4 Ka = [PLn] [P] [L]n Ka = [Hb(O2)4] [Hb] [O2] 4 O = Ln + Ln Kd O = pO2 4 + 1/Ka pO2 4 Any Ligand Hemoglobin n=4 O = Ln + Ln 1/Ka O = pO2 4 + Kd pO2 4 Where Kd = 1/Ka = [L] n 0.5 = P n 50 Where Kd = 1/Ka = [O2] 4 0.5 = P450 Multi-Ligand Interactions Docsity.com ALLOSTERIC BINDING Rearranging the Equation n log pO2 - nlog P50 = n = Hill Coefficient pO2 q 1 – q = P5 0 n n log q 1 – q O = pO2 n + P50 n pO2 n Docsity.com