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• pKa of the side chain R-groups: D (3.9), E (4.2), H (6.0)' C (8.3), Y (10.1), K (10.5), and R (12.5). •pka of amino acids a-amino group (9.6) and a-carboxyl group (2.1). • pKa of N-terminal oligopeptide a-amino group (7.4) and C-terminal a-carboxyl group (3.6).
5. A histidine residue (His146) in Hb subunit has a side chain pka of 6.2 in the lung. Due to a conformational shift of Hb in the muscle, an amino acid moves into the close proximity of this histidine, shifting this His146 pka to 7.7: • a) what is the net charge of this His146 in the lung at pH 7.6 (show all the calculations)? • b) what is the net charge of this His146 in the muscle at pH 7.2 (show all the calculations)? • c) what is the likely identity of the residue that causes the pKa shift for His146 residue? Explain why such residue causes the shift in the histidine pka. 6. Free heme is an effective oxygen-binding molecule, but it has two shortcomings: its iron is readily oxidized, and it forms a heme:O₂ complex that has low solubility. (a) How has the incorporation of heme into a protein corrected these heme flaws? (b) What are the mechanisms of affecting Hb (hemoglobin) affinity to O₂ .. by O₂, CO2, CO, H*? Please explain in detail. (c) What are the advantages (and disadvantages) of allosteric regulation?