Description I have had difficulty finding a value for the membrane potential of the sarcoplasmic reticulum (SR). Let's use the transport of calcium ions by the SR Ca++ ion pump to estimate the membrane potential of the SR.
Calculate the SR membrane potential.
Question 1)
Instructions Let's assume the pump can use 100% of the energy of ATP hydrolysis to establish the equilibrium ratios of Ca++ found in the cytoplasm and SR.
1) The free energy of hydrolysis of ATP under cellular conditions is -12 kcal/mol.
2) The cytoplasm has a calcium concentration of 0.1 uM and the SR has a calcium concentration of 1.5 mM.
3) Use 0.6 kcal mol-1 for RT
4) Use 0.023 kcal mol-1 mVolt-1
5) report your answer as the membrane potential (in millivolts) in the same direction as the Ca++ transport by this pump.
Question 2)
The Ca++ binding sites of the Ca++ ATPase contain 4 acidic amino acid side chains (3 Glu, 1 Asp). When Ca++ is bound, we can assume that these sites are 100% ionized. However, when the pump is in the conformation that has low affinity for Ca++, these side chains might be expected to be weakly protonated (assume the pH of the SR=7 and the side chains have pKa's of 4.5). This would occasionally result in in the translocation of a proton from the SR to the cytoplasm (on average about 1.2 protons for every 100 times the pump changes conformation). Under what conditions would this give us additional free energy that could be used to pump more Ca++?
If there is a difference in pH where the cytoplasmic pH is lower than that of the SR and the electrical potential across the membrane is more positive on the cytoplasmic side.
If there is a difference in pH where the cytoplasmic pH is higher than that of the SR and the electrical potential across the membrane is more negative on the cytoplasmic side.