What binds to Hb in addition to O 2?

Transcription

1 Reading: Ch5; , , Problems: Ch5 (text); 3,7,8,10 Ch5 (study guide-facts); 1,2,3,4,5,8 Ch5 (study guide-apply); 2,3 Remember Today at 5:30 in CAS-522 is the second chance for the MB lecture & quiz NEXT Reading: Ch4; Problems: Ch4 (text); 14, 16 Ch6 (text); 1, 4 Lecture 20 (11/1/17) ENZYMES: A. Enzyme Regulation: Hemoglobin 1. Structure-Function; Structural basis for physiology (R & T states) a. Structure of R & T b. Relationship to binding of effectors 2. Mechanism of Cooperativity a. Oxygen binding b. Bond breaking c. Conformational change 3. Other Hb a. Fetal b. High altitude c. Disease B. Protein Structure 1. Stability a. Two-state model b. Energetics c. Denaturation d. Methods to study 2. Protein Folding a. Evidence-Anfinsen b. Mechanism; in vitro vs. in vivo i. Kinetics ii. Thermodynamics 3. Prediction 4. Dynamics What binds to Hb in addition to O 2? Protons Carbon Dioxide 1

2 Carbon Dioxide binds via Carbamate formation Carbon dioxide binds better to the form of Hb that is not bound to oxygen; deoxy-hb Therefore, oxygen binding releases CO 2 Hb H + CO + O 2 Hb O 2 + CO H Consequences of carbamate: charge change contributes to acidity: when CO 2 increases, carbamate formation increases, which is conducive to the Bohr effect CO 2 Binds to Deoxyhemoglobin b 2 b 1 a 1 Arg-141 (of a1) Val-1 (of a2) Val-1 Arg-141 (of a2) a 2 (of a1) Val-1 & Arg-141 further away 28 weaker electrostatic bond So, BPG, protons, and CO 2 bind specifically to deoxy-hb. Do these stabilize the T-state? Lets look at these states more closely. 2

3 T-state and R-state of Hemoglobin a 2 b 1 b 2 a 1 T state Less active R state More active 29 R and T States of Hemoglobin (146) (40) Look at where this His-146 (HC3) is after binding of O 2 (R-state). 3

4 Oxygen triggers Hb to switch from its low affinity (T) state to its high affinity (R) state. What kind of allosteric effector is oxygen for Hb? What kind of allosteric effector is BPG for Hb? A. Heterotropic positive allosteric effector B. Homotropic negative allosteric effector C. Heterotropic negative allosteric effector D. Homotropic positive allosteric effector Influences on T R Transition BPG O 2 H O 2 HO2 H BPG + H + H 2CO 2 + H + H O 2 H 2 + H BPG CO 2 CO 2 BPG 2CO H O 2 b 2 b 1 O 2 a 2 a BPG O 2 O 2 O 2 CO 2CO 2 T-state [To] Recall: L ( [Ro] ) = 300,000 and [KR] C ( ) = 0.01 [KT] R-state 4

5 Effects of BPG & CO 2 on Hb s O 2 Dissociation Curve O 2 binds to Hb Protons Carbon Dioxide How are all these related to the cooperative binding mechanism? Lets look at binding of oxygen to the T-state, which its most likely to encounter (L=300,000). 5

6 Structural Basis of Cooperativity Helix F Deoxy Oxygen bound High-spin (paramagnetic)(larger) Deoxy Oxygen bound Low-spin (diamagnetic)(smaller with Fe-N bonds 0.1 Å shorter) 35 Conformational Change Is Triggered by Oxygen Binding 6

7 Structural Basis of Cooperativity What causes the conformational change? 37 Structural Basis of Cooperativity Changes in H-bonds when an oxygen binds to a single subunit b Y145 Changes within each subunit b V98 a V93 The change at the F- helix changes the C- term stability within each subunit a Y140 b D99 a Y42 b N102 a D94 Changes between dimers (a1 and b2) The change at the F-helix changes the C-term stability between subunits 38 7

8 Structural Basis of Cooperativity Changes in H-bonds when an oxygen binds to a single subunit b Y145 Changes within each subunit b V98 a V93 The change at the F- helix changes the C- term stability within each subunit a Y140 b D99 a Y42 b N102 a D94 Changes between dimers (a1 and b2) The change at the F-helix changes the C-term stability between subunits 39 Structural Basis of Cooperativity Asp94 Asp99 Asn102 Tyr42 a 2 b 1 a 2 b 1 b 2 a 1 b 2 a 1 Notice residue 97 (His) Human Deoxyhemoglobin & Human Oxyhemoglobin PDBids 2HHB & 1HHO 40 8

9 Structural Basis of Cooperativity Asp94 Asp99 Asn102 Tyr42 How do these slight changes due to movement of the F-helix upon O 2 binding destabilize the C- termini and lead to conformational change? Recall H-bonds broken ( a Y140 & b Y145) Human Deoxyhemoglobin & Human Oxyhemoglobin PDBids 2HHB & 1HHO 41 Carbamate formation favors the T-state Changes at C-term of a 1 Hb H + CO + O 2 Hb O 2 + CO H Recall H-bonds broken ( a Y140) Changes between dimers (Saltbridge interactions at C-term of a 1 ) Recall, this is the first thing we saw destabilized 9

10 Protonation favors the T state Changes at C-term of b 2 Hb H + + O 2 Hb O 2 + H + Recall H-bonds broken ( b Y145) Changes between dimers (Saltbridge interactions at C-term of b 2 ) Recall, this is the first thing we saw destabilized But, what actually causes the dimers to rotate the 15? The Bohr effect: when these interactions are lost, the proton is released (pk a drops) Oxygen-Binding affects Bonds to C-terminus Hemoglobin Dynamics at C-term of beta-subunit See: O 2 binds The salt-bridge between His-146 and Asp-94 on the same b-subunit breaks The salt-bridge between the C-term carboxylate of b-subunit loses contact with Lys-40 of a-subunit Anchor is lost and subunits move DON T See: Fe moving into plane of heme when O 2 binds Helix F and FG loop moving when His-91 (F8) on helix-f moves H-bond with Tyr-145 on and Val-98 (on FG loop) on b-subunit breaking The H-bond between the Asp-99 of b-subunit and Tyr-42 of a-subunit breaking NONE of the comparable changes at the C-term of the a-subunit The T- and R- states of Hb 10

11 b in gray O 2 binds The salt-bridge between His-146 and Asp-94 on the same b-subunit breaks a in blue The salt-bridge between the C-term carboxylate of b-subunit loses contact with Lys-40 of a-subunit Anchor is lost and subunits move Summary of changes in Hb T-state to R-state 11

12 Enzyme Regulation 47 Enzyme Regulation 48 12

13 Which of the following is true regarding the ability of Hb to bind oxygen? A. CO 2 promotes the release of oxygen B. Salt bridges stabilize the deoxy form of Hb. C. H + and BPG stabilize the deoxy form of Hb. D. B and C are true. E. All the above are true. Fetal Hemoglobins Hb is always a tetramer of a-type and b-type subunits (a-b) 2 The g-subunit of HbF does not bind BPG as well as the b-subunit This shifts the TßàR equilibrium to the right. This shifts the oxygen binding curve to the left such that at all po 2, HbF binds oxygen better than HbA 50 13

14 Which of the following lines represents the binding of oxygen to fetal Hb if the red line represents the binding of oxygen to maternal Hb? A. The red line B. The green line C. The blue line D. The dotted line High altitude adaptation [BPG] goes up to 8 mm This shifts the TßàR equilibrium to the left. This shifts the oxygen binding curve to the right such that at all po 2, Hb binds oxygen worse and compensates for the lower po 2 at high altitudes

15 Abnormal Hemoglobins Sickle-Cell Anemia 53 Hemoglobin S variant Sickle-cell hemoglobin fibers 54 15

16 Formation of Hb Strands in Sickle-Cell Anemia Structure of Deoxyhemoglobin S Fiber Why does hemoglobin S persist in the population? 56 16

17 Hydroxyurea Treatment for Sickle-Cell Anemia 17

18 Protein Stability, Folding, and Dynamics Two-state model: D 2 (denatured) N (native) What is favored in this equilibrium? What forces operate? Which forces are the most important? Protein Stability, Folding, and Dynamics What is the equilibrium? Lies to the right Therefore, DG is negative What forces operate? Non covalent: H-bonds? Ionic (salt-bridges)? van der Waals? Hydrophobic? Covalent: Disulfide bonds? Which force(s) are the most important? Hydrophobic! yes, definitely, but those with water in D-state yes, but not that many and non specific yes, but not a driving force until there is compaction YES, bury hydrophobic residues yes, but most proteins don t have any 1

19 Protein Stability, Folding, and Dynamics Hydrophobic effect drives protein folding 5 Protein Stability, Folding, and Dynamics Problem Which substitution would be more likely to disrupt a protein s structure? Val replaced by: Ala or Phe Lys replaced by: Asp or Arg Gln replaced by: Glu or Asn Pro replaced by: His or Gly 6 2