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<ol><li>The monomer-dimer equilibrium of CXCL12 is very sensitive to pH. At

physiological pH, 7.4, the monomer-dimer equilibrium is prevalent and at high CXCL12 protein concentrations the dimer can form. However, at low pH the monomer tends to predominate and dimer formation is unlikely even at high protein concentrations. Above is a figure showing a CXCL12 dimer with one subunit of the dimer in gray and the other in tan. The surface of the molecule is shown as well as a cartoon image of the polypeptide backbone. The dashed line highlights the dimer interface and the close proximity (3.8 Å) of positively charged Histidine 25 and Lysine 27 side chains from different subunits. Based these positive charges, your knowledge of amino acids and the Henderson-Hasselbalch equation, is the pH of the solution 6.0 or 7.4? The pK for the lysine side chain amine is 10.5. The pK for the Histidine side chain imidazole group is 6.8 (slightly different than the value listed in Table 1.4 of your text, but then this histidine is in a protein). Be sure that your answer includes the HendersonHasselbalch equation and identifies which amino acid's side chain, K24, H25 or K27, allows you to make your decision regarding the pH.</li><li> Please explain why the monomer-dimer equilibrium is sensitive to pH? i.e. why would the dimer be unlikely to form at the pH and amino acid charge states shown in the figure above? Why can dimer form at physiologic pH, 7.4 and high protein concentrations but monomer predominates at low pH (pH &lt; 6.8) even at high protein concentrations? </li><li> Cancer often relies heavily on glycolysis to generate ATP. One of the products of glycolysis in humans is pyruvate, which is converted to lactic acid. Lactic acid can acidify the local environment around a cancer cell, the so-called Warburg effect. Would you predict local environment near the surface of a cancer cell to promote the formation of monomeric CXCL12, the oligmeric state that promotes cancer cell migration, or dimeric CXCL12, the oligmeric state that halts migration? Why?</li><li> Is the Henderson-Hasselbalch equation finally cool?</li></ol>

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